PERF: Avoiding some unnecessary allocations/boxing in metadata emit

 (changeset 1211723)

Src/Compilers/CSharp/Source/Binder/Binder_Attributes.cs

@@ -226,7 +226,7 @@ protected virtual bool IsAttributeConditionallyOmitted(NamedTypeSymbol attribute
 
             if (attributeType.IsConditional)
             {
-                IEnumerable<string> conditionalSymbols = attributeType.GetAppliedConditionalSymbols();
+                ImmutableArray<string> conditionalSymbols = attributeType.GetAppliedConditionalSymbols();
                 Debug.Assert(conditionalSymbols != null);
                 if (syntaxTree.IsAnyPreprocessorSymbolDefined(conditionalSymbols))
                 {

Src/Compilers/CSharp/Source/CSharpExtensions.cs

@@ -334,7 +334,7 @@ internal static bool HasReferenceDirectives(this SyntaxTree tree)
             return csharpTree != null && csharpTree.HasReferenceDirectives;
         }
 
-        internal static bool IsAnyPreprocessorSymbolDefined(this SyntaxTree tree, IEnumerable<string> conditionalSymbols)
+        internal static bool IsAnyPreprocessorSymbolDefined(this SyntaxTree tree, ImmutableArray<string> conditionalSymbols)
         {
             var csharpTree = tree as CSharpSyntaxTree;
             return csharpTree != null && csharpTree.IsAnyPreprocessorSymbolDefined(conditionalSymbols);

Src/Compilers/CSharp/Source/Emitter/Model/AttributeDataAdapter.cs

@@ -15,14 +15,21 @@ namespace Microsoft.CodeAnalysis.CSharp.Symbols
 {
     internal abstract partial class CSharpAttributeData : Cci.ICustomAttribute
     {
-        IEnumerable<Cci.IMetadataExpression> Cci.ICustomAttribute.GetArguments(Microsoft.CodeAnalysis.Emit.Context context)
+        ImmutableArray<Cci.IMetadataExpression> Cci.ICustomAttribute.GetArguments(Microsoft.CodeAnalysis.Emit.Context context)
         {
-            foreach (var argument in this.CommonConstructorArguments)
+            var commonArgs = this.CommonConstructorArguments;
+            if(commonArgs.IsEmpty)
             {
-                Debug.Assert(argument.Kind != TypedConstantKind.Error);
+                return ImmutableArray<Cci.IMetadataExpression>.Empty;
+            }
 
-                yield return CreateMetadataExpression(argument, context);
+            var builder = ArrayBuilder<Cci.IMetadataExpression>.GetInstance();
+            foreach (var argument in commonArgs)
+            {
+                Debug.Assert(argument.Kind != TypedConstantKind.Error);
+                builder.Add(CreateMetadataExpression(argument, context));
             }
+            return builder.ToImmutableAndFree();
         }
 
         Cci.IMethodReference Cci.ICustomAttribute.Constructor(Microsoft.CodeAnalysis.Emit.Context context)
@@ -31,12 +38,20 @@ Cci.IMethodReference Cci.ICustomAttribute.Constructor(Microsoft.CodeAnalysis.Emi
             return (Cci.IMethodReference)moduleBeingBuilt.Translate(this.AttributeConstructor, (CSharpSyntaxNode)context.SyntaxNodeOpt, context.Diagnostics);
         }
 
-        IEnumerable<Cci.IMetadataNamedArgument> Cci.ICustomAttribute.GetNamedArguments(Microsoft.CodeAnalysis.Emit.Context context)
+        ImmutableArray<Cci.IMetadataNamedArgument> Cci.ICustomAttribute.GetNamedArguments(Microsoft.CodeAnalysis.Emit.Context context)
         {
-            foreach (var namedArgument in this.CommonNamedArguments)
+            var commonArgs = this.CommonNamedArguments;
+            if (commonArgs.IsEmpty)
+            {
+                return ImmutableArray<Cci.IMetadataNamedArgument>.Empty;
+            }
+
+            var builder = ArrayBuilder<Cci.IMetadataNamedArgument>.GetInstance();
+            foreach (var namedArgument in commonArgs)
             {
-                yield return CreateMetadataNamedArgument(namedArgument.Key, namedArgument.Value, context);
+                builder.Add(CreateMetadataNamedArgument(namedArgument.Key, namedArgument.Value, context));
             }
+            return builder.ToImmutableAndFree();
         }
 
         int Cci.ICustomAttribute.ArgumentCount

Src/Compilers/CSharp/Source/Emitter/Model/ExpandedVarargsMethodReference.cs

@@ -51,7 +51,7 @@ Cci.IMethodDefinition Cci.IMethodReference.GetResolvedMethod(Microsoft.CodeAnaly
             return underlyingMethod.GetResolvedMethod(context);
         }
 
-        IEnumerable<Cci.IParameterTypeInformation> Cci.IMethodReference.ExtraParameters
+        ImmutableArray<Cci.IParameterTypeInformation> Cci.IMethodReference.ExtraParameters
         {
             get
             {
@@ -97,7 +97,7 @@ ushort Cci.ISignature.ParameterCount
             get { return underlyingMethod.ParameterCount; }
         }
 
-        IEnumerable<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
+        ImmutableArray<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
         {
             return underlyingMethod.GetParameters(context);
         }

Src/Compilers/CSharp/Source/Emitter/Model/MethodReference.cs

@@ -6,6 +6,7 @@
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
 using System.Diagnostics;
+using System.Collections.Immutable;
 
 namespace Microsoft.CodeAnalysis.CSharp.Emit
 {
@@ -65,11 +66,11 @@ Microsoft.Cci.IMethodDefinition Microsoft.Cci.IMethodReference.GetResolvedMethod
             return null;
         }
 
-        IEnumerable<Microsoft.Cci.IParameterTypeInformation> Microsoft.Cci.IMethodReference.ExtraParameters
+        ImmutableArray<Microsoft.Cci.IParameterTypeInformation> Microsoft.Cci.IMethodReference.ExtraParameters
         {
             get
             {
-                return SpecializedCollections.EmptyEnumerable<Microsoft.Cci.IParameterTypeInformation>();
+                return ImmutableArray<Microsoft.Cci.IParameterTypeInformation>.Empty;
             }
         }
 
@@ -81,7 +82,7 @@ Microsoft.Cci.CallingConvention Microsoft.Cci.ISignature.CallingConvention
             }
         }
 
-        IEnumerable<Microsoft.Cci.IParameterTypeInformation> Microsoft.Cci.ISignature.GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
+        ImmutableArray<Microsoft.Cci.IParameterTypeInformation> Microsoft.Cci.ISignature.GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
         {
             PEModuleBuilder moduleBeingBuilt = (PEModuleBuilder)context.Module;
             return moduleBeingBuilt.Translate(UnderlyingMethod.Parameters);

Src/Compilers/CSharp/Source/Emitter/Model/MethodSymbolAdapter.cs

@@ -171,11 +171,11 @@ private Cci.IMethodDefinition ResolvedMethodImpl(Microsoft.CodeAnalysis.Emit.Con
             return null;
         }
 
-        IEnumerable<Cci.IParameterTypeInformation> Cci.IMethodReference.ExtraParameters
+        ImmutableArray<Cci.IParameterTypeInformation> Cci.IMethodReference.ExtraParameters
         {
             get
             {
-                return SpecializedCollections.EmptyEnumerable<Cci.IParameterTypeInformation>();
+                return ImmutableArray<Cci.IParameterTypeInformation>.Empty;
             }
         }
 
@@ -187,14 +187,14 @@ Cci.CallingConvention Cci.ISignature.CallingConvention
             }
         }
 
-        IEnumerable<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
+        ImmutableArray<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
         {
             Debug.Assert(this.IsDefinitionOrDistinct());
 
             PEModuleBuilder moduleBeingBuilt = (PEModuleBuilder)context.Module;
             if (this.IsDefinition && this.ContainingModule == moduleBeingBuilt.SourceModule)
             {
-                return EnumerateDefinitionParameters();
+                return StaticCast<Cci.IParameterTypeInformation>.From(this.EnumerateDefinitionParameters());
             }
             else
             {
@@ -202,19 +202,11 @@ IEnumerable<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Microsof
             }
         }
 
-        private IEnumerable<Cci.IParameterDefinition> EnumerateDefinitionParameters()
+        private ImmutableArray<Cci.IParameterDefinition> EnumerateDefinitionParameters()
         {
             Debug.Assert(this.Parameters.All(p => p.IsDefinition));
 
-            var parameters = this.Parameters;
-            if (parameters.Length != 0)
-            {
-                return this.Parameters;
-            }
-            else
-            {
-                return SpecializedCollections.EmptyEnumerable<Cci.IParameterDefinition>();
-            }
+            return StaticCast<Cci.IParameterDefinition>.From(this.Parameters);
         }
 
         IEnumerable<Cci.ICustomModifier> Cci.ISignature.ReturnValueCustomModifiers
@@ -570,7 +562,7 @@ bool Cci.IMethodDefinition.IsVirtual
         /// </summary>
         internal abstract bool IsMetadataFinal();
 
-        IEnumerable<Cci.IParameterDefinition> Cci.IMethodDefinition.Parameters
+        ImmutableArray<Cci.IParameterDefinition> Cci.IMethodDefinition.Parameters
         {
             get
             {

Src/Compilers/CSharp/Source/Emitter/Model/PEModuleBuilder.cs

@@ -1148,22 +1148,16 @@ public static Cci.TypeMemberVisibility MemberVisibility(Symbol symbol)
             return methodRef;
         }
 
-        internal IEnumerable<Cci.IParameterTypeInformation> Translate(ImmutableArray<ParameterSymbol> @params)
+        internal ImmutableArray<Cci.IParameterTypeInformation> Translate(ImmutableArray<ParameterSymbol> @params)
         {
             Debug.Assert(@params.All(p => p.IsDefinitionOrDistinct()));
-            if (@params.Length == 0)
-            {
-                return SpecializedCollections.EmptyEnumerable<Cci.IParameterTypeInformation>();
-            }
-
-            var firstParam = @params.First();
 
-            bool mustBeTranslated = MustBeWrapped(firstParam);
+            bool mustBeTranslated = @params.Any() && MustBeWrapped(@params.First());
             Debug.Assert(@params.All(p => mustBeTranslated == MustBeWrapped(p)), "either all or no parameters need translating");
 
             if (!mustBeTranslated)
             {
-                return @params;
+                return StaticCast<Cci.IParameterTypeInformation>.From(@params);
             }
 
             return TranslateAll(@params);
@@ -1189,12 +1183,9 @@ private static bool MustBeWrapped(ParameterSymbol param)
             return false;
         }
 
-        private IEnumerable<Cci.IParameterTypeInformation> TranslateAll(ImmutableArray<ParameterSymbol> @params)
+        private ImmutableArray<Cci.IParameterTypeInformation> TranslateAll(ImmutableArray<ParameterSymbol> @params)
         {
-            foreach (var param in @params)
-            {
-                yield return CreateParameterTypeInformationWrapper(param);
-            }
+            return @params.SelectAsArray(param => CreateParameterTypeInformationWrapper(param));
         }
 
         private Cci.IParameterTypeInformation CreateParameterTypeInformationWrapper(ParameterSymbol param)

Src/Compilers/CSharp/Source/Emitter/Model/PropertySymbolAdapter.cs

@@ -8,6 +8,7 @@
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
 using Cci = Microsoft.Cci;
+using System.Collections.Immutable;
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols
 {
@@ -106,12 +107,12 @@ bool Cci.IPropertyDefinition.IsSpecialName
             }
         }
 
-        IEnumerable<Cci.IParameterDefinition> Cci.IPropertyDefinition.Parameters
+        ImmutableArray<Cci.IParameterDefinition> Cci.IPropertyDefinition.Parameters
         {
             get
             {
                 CheckDefinitionInvariant();
-                return this.Parameters;
+                return StaticCast<Cci.IParameterDefinition>.From(this.Parameters);
             }
         }
 
@@ -152,10 +153,10 @@ ushort Cci.ISignature.ParameterCount
             }
         }
 
-        IEnumerable<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
+        ImmutableArray<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
         {
             CheckDefinitionInvariant();
-            return this.Parameters;
+            return StaticCast<Cci.IParameterTypeInformation>.From(this.Parameters);
         }
 
         IEnumerable<Cci.ICustomModifier> Cci.ISignature.ReturnValueCustomModifiers

Src/Compilers/CSharp/Source/FlowAnalysis/PreciseAbstractFlowPass.cs

@@ -448,8 +448,8 @@ protected virtual ImmutableArray<PendingBranch> RemoveReturns()
                 // track exception states.
                 result = this.pendingBranches
                         .Where(b => b.Branch != null)
-                        .ToArray()
                         .AsImmutableOrNull();
+
                 var oldExceptions = this.pendingBranches[0];
                 Debug.Assert(oldExceptions.Branch == null);
                 this.pendingBranches.Clear();

Src/Compilers/CSharp/Source/Symbols/AnonymousTypes/PublicSymbols/AnonymousType.TypePublicSymbol.cs

@@ -311,9 +311,9 @@ internal override IEnumerable<Microsoft.Cci.SecurityAttribute> GetSecurityInform
                 throw ExceptionUtilities.Unreachable;
             }
 
-            internal override IEnumerable<string> GetAppliedConditionalSymbols()
+            internal override ImmutableArray<string> GetAppliedConditionalSymbols()
             {
-                return SpecializedCollections.EmptyEnumerable<string>();
+                return ImmutableArray<string>.Empty;
             }
 
             internal override AttributeUsageInfo GetAttributeUsageInfo()

Src/Compilers/CSharp/Source/Symbols/AnonymousTypes/SynthesizedSymbols/AnonymousType.TemplateSymbol.cs

@@ -423,9 +423,9 @@ internal override IEnumerable<Microsoft.Cci.SecurityAttribute> GetSecurityInform
                 throw ExceptionUtilities.Unreachable;
             }
 
-            internal override IEnumerable<string> GetAppliedConditionalSymbols()
+            internal override ImmutableArray<string> GetAppliedConditionalSymbols()
             {
-                return SpecializedCollections.EmptyEnumerable<string>();
+                return ImmutableArray<string>.Empty;
             }
 
             internal override AttributeUsageInfo GetAttributeUsageInfo()

Src/Compilers/CSharp/Source/Symbols/ErrorTypeSymbol.cs

@@ -484,9 +484,9 @@ internal sealed override IEnumerable<Microsoft.Cci.SecurityAttribute> GetSecurit
             throw ExceptionUtilities.Unreachable;
         }
 
-        internal sealed override IEnumerable<string> GetAppliedConditionalSymbols()
+        internal sealed override ImmutableArray<string> GetAppliedConditionalSymbols()
         {
-            return SpecializedCollections.EmptyEnumerable<string>();
+            return ImmutableArray<string>.Empty;
         }
 
         internal override AttributeUsageInfo GetAttributeUsageInfo()

Src/Compilers/CSharp/Source/Symbols/Metadata/PE/PENamedTypeSymbol.cs

@@ -1896,12 +1896,12 @@ private NamedTypeSymbol MakeComImportCoClassType()
             return null;
         }
 
-        internal override IEnumerable<string> GetAppliedConditionalSymbols()
+        internal override ImmutableArray<string> GetAppliedConditionalSymbols()
         {
             var uncommon = GetUncommonProperties();
             if (uncommon == noUncommonProperties)
             {
-                return SpecializedCollections.EmptyEnumerable<string>();
+                return ImmutableArray<string>.Empty;
             }
 
             if (uncommon.lazyConditionalAttributeSymbols.IsDefault)

Src/Compilers/CSharp/Source/Symbols/MethodSymbol.cs

@@ -382,7 +382,7 @@ private bool CallsAreConditionallyOmitted(SyntaxTree syntaxTree)
         {
             if (this.IsConditional)
             {
-                IEnumerable<string> conditionalSymbols = this.GetAppliedConditionalSymbols();
+                ImmutableArray<string> conditionalSymbols = this.GetAppliedConditionalSymbols();
                 Debug.Assert(conditionalSymbols != null);
                 if (syntaxTree.IsAnyPreprocessorSymbolDefined(conditionalSymbols))
                 {

Src/Compilers/CSharp/Source/Symbols/NamedTypeSymbol.cs

@@ -1066,7 +1066,7 @@ protected CharSet DefaultMarshallingCharSet
         /// <summary>
         /// Returns a sequence of preprocessor symbols specified in <see cref="T:ConditionalAttribute"/> applied on this symbol, or null if there are none.
         /// </summary>
-        internal abstract IEnumerable<string> GetAppliedConditionalSymbols();
+        internal abstract ImmutableArray<string> GetAppliedConditionalSymbols();
 
         /// <summary>
         /// If CoClassAttribute was applied to the type and the attribute argument is a valid named type argument, i.e. accessible class type, then it returns the type symbol for the argument.

Src/Compilers/CSharp/Source/Symbols/Retargeting/RetargetingNamedTypeSymbol.cs

@@ -552,7 +552,7 @@ internal override IEnumerable<Microsoft.Cci.SecurityAttribute> GetSecurityInform
             return this.underlyingType.GetSecurityInformation();
         }
 
-        internal override IEnumerable<string> GetAppliedConditionalSymbols()
+        internal override ImmutableArray<string> GetAppliedConditionalSymbols()
         {
             return this.underlyingType.GetAppliedConditionalSymbols();
         }

Src/Compilers/CSharp/Source/Symbols/Source/ImplicitNamedTypeSymbol.cs

@@ -145,9 +145,9 @@ internal sealed override IEnumerable<Microsoft.Cci.SecurityAttribute> GetSecurit
             throw ExceptionUtilities.Unreachable;
         }
 
-        internal override IEnumerable<string> GetAppliedConditionalSymbols()
+        internal override ImmutableArray<string> GetAppliedConditionalSymbols()
         {
-            return SpecializedCollections.EmptyEnumerable<string>();
+            return ImmutableArray<string>.Empty;
         }
 
         internal override ObsoleteAttributeData ObsoleteAttributeData

Src/Compilers/CSharp/Source/Symbols/Source/SourceNamedTypeSymbol.cs

@@ -883,10 +883,10 @@ internal sealed override IEnumerable<Microsoft.Cci.SecurityAttribute> GetSecurit
             return null;
         }
 
-        internal override IEnumerable<string> GetAppliedConditionalSymbols()
+        internal override ImmutableArray<string> GetAppliedConditionalSymbols()
         {
             var data = GetEarlyDecodedWellKnownAttributeData();
-            return data != null ? data.ConditionalSymbols : SpecializedCollections.EmptyEnumerable<string>();
+            return data != null ? data.ConditionalSymbols : ImmutableArray<string>.Empty;
         }
 
         internal override void PostDecodeWellKnownAttributes(ImmutableArray<CSharpAttributeData> boundAttributes, ImmutableArray<AttributeSyntax> allAttributeSyntaxNodes, DiagnosticBag diagnostics, AttributeLocation symbolPart, WellKnownAttributeData decodedData)

Src/Compilers/CSharp/Source/Symbols/SubstitutedNamedTypeSymbol.cs

@@ -469,7 +469,7 @@ internal sealed override IEnumerable<Microsoft.Cci.SecurityAttribute> GetSecurit
             return originalDefinition.GetSecurityInformation();
         }
 
-        internal sealed override IEnumerable<string> GetAppliedConditionalSymbols()
+        internal sealed override ImmutableArray<string> GetAppliedConditionalSymbols()
         {
             return originalDefinition.GetAppliedConditionalSymbols();
         }

Src/Compilers/CSharp/Source/Symbols/Synthesized/SynthesizedContainer.cs

@@ -272,9 +272,9 @@ internal sealed override ObsoleteAttributeData ObsoleteAttributeData
             get { return null; }
         }
 
-        internal sealed override IEnumerable<string> GetAppliedConditionalSymbols()
+        internal sealed override ImmutableArray<string> GetAppliedConditionalSymbols()
         {
-            return SpecializedCollections.EmptyEnumerable<string>();
+            return ImmutableArray<string>.Empty;
         }
 
         internal override bool HasDeclarativeSecurity

Src/Compilers/CSharp/Source/Syntax/CSharpSyntaxTree.cs

@@ -153,7 +153,7 @@ private Syntax.InternalSyntax.DirectiveStack GetDirectives()
             return this.directives;
         }
 
-        internal bool IsAnyPreprocessorSymbolDefined(IEnumerable<string> conditionalSymbols)
+        internal bool IsAnyPreprocessorSymbolDefined(ImmutableArray<string> conditionalSymbols)
         {
             System.Diagnostics.Debug.Assert(conditionalSymbols != null);
 

Src/Compilers/CSharp/Test/Symbol/Symbols/MockNamedTypeSymbol.cs

@@ -286,9 +286,9 @@ internal override IEnumerable<Microsoft.Cci.SecurityAttribute> GetSecurityInform
             return null;
         }
 
-        internal override IEnumerable<string> GetAppliedConditionalSymbols()
+        internal override ImmutableArray<string> GetAppliedConditionalSymbols()
         {
-            return SpecializedCollections.EmptyEnumerable<string>();
+            return ImmutableArray<string>.Empty;
         }
 
         internal override AttributeUsageInfo GetAttributeUsageInfo()

Src/Compilers/Core/Source/CodeGen/ArrayMembers.cs

@@ -5,6 +5,7 @@
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
 using Cci = Microsoft.Cci;
+using System.Collections.Immutable;
 
 
 // Contains support for pseudo-methods on multidimensional arrays.
@@ -221,20 +222,19 @@ public override Cci.ITypeReference GetType(Microsoft.CodeAnalysis.Emit.Context c
                 return context.Module.GetPlatformType(Cci.PlatformType.SystemVoid, context);
             }
 
-            protected override ArrayMethodParameterInfo[] MakeParameters()
+            protected override ImmutableArray<ArrayMethodParameterInfo> MakeParameters()
             {
-                var indexNum = (ushort)arrayType.Rank;
-                var parameters = new ArrayMethodParameterInfo[indexNum + 1];
+                int rank = (int)arrayType.Rank;
+                var parameters = ArrayBuilder<ArrayMethodParameterInfo>.GetInstance(rank + 1);
 
-                for (ushort i = 0; i < indexNum; i++)
+                for (int i = 0; i < rank; i++)
                 {
-                    parameters[i] = ArrayMethodParameterInfo.GetIndexParameter(i);
+                    parameters.Add(ArrayMethodParameterInfo.GetIndexParameter((ushort)i));
                 }
 
-                parameters[indexNum] = new ArraySetValueParameterInfo(indexNum, arrayType);
-
-                return parameters;
-            }
+                parameters.Add(new ArraySetValueParameterInfo((ushort)rank, arrayType));
+                return parameters.ToImmutableAndFree();
+           }
         }
     }
 
@@ -333,7 +333,7 @@ public override Cci.ITypeReference GetType(Microsoft.CodeAnalysis.Emit.Context c
     /// </summary>
     internal abstract class ArrayMethod : Cci.IMethodReference
     {
-        private readonly ArrayMethodParameterInfo[] parameters;
+        private readonly ImmutableArray<ArrayMethodParameterInfo> parameters;
         protected readonly Cci.IArrayTypeReference arrayType;
 
         protected ArrayMethod(Cci.IArrayTypeReference arrayType)
@@ -352,21 +352,22 @@ public virtual bool ReturnValueIsByRef
         }
 
         // Set overrides this to include "value" parameter.
-        protected virtual ArrayMethodParameterInfo[] MakeParameters()
+        protected virtual ImmutableArray<ArrayMethodParameterInfo> MakeParameters()
         {
-            var parameters = new ArrayMethodParameterInfo[arrayType.Rank];
+            int rank = (int)arrayType.Rank;
+            var parameters = ArrayBuilder<ArrayMethodParameterInfo>.GetInstance(rank);
 
-            for (ushort i = 0; i < parameters.Length; i++)
+            for (int i = 0; i < rank; i++)
             {
-                parameters[i] = ArrayMethodParameterInfo.GetIndexParameter(i);
+                parameters.Add(ArrayMethodParameterInfo.GetIndexParameter((ushort)i));
             }
 
-            return parameters;
+            return parameters.ToImmutableAndFree();
         }
 
-        public IEnumerable<Cci.IParameterTypeInformation> GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
+        public ImmutableArray<Cci.IParameterTypeInformation> GetParameters(Microsoft.CodeAnalysis.Emit.Context context)
         {
-            return parameters;
+            return StaticCast<Cci.IParameterTypeInformation>.From(parameters);
         }
 
         public bool AcceptsExtraArguments
@@ -389,9 +390,9 @@ public Cci.IMethodDefinition GetResolvedMethod(Microsoft.CodeAnalysis.Emit.Conte
             return null;
         }
 
-        public IEnumerable<Cci.IParameterTypeInformation> ExtraParameters
+        public ImmutableArray<Cci.IParameterTypeInformation> ExtraParameters
         {
-            get { return SpecializedCollections.EmptyEnumerable<Cci.IParameterTypeInformation>(); }
+            get { return ImmutableArray<Cci.IParameterTypeInformation>.Empty; }
         }
 
         public Cci.IGenericMethodInstanceReference AsGenericMethodInstanceReference

Src/Compilers/Core/Source/CodeGen/MethodBody.cs

@@ -68,7 +68,7 @@ void Cci.IMethodBody.Dispatch(Cci.MetadataVisitor visitor)
             throw ExceptionUtilities.Unreachable;
         }
 
-        IEnumerable<Cci.ExceptionHandlerRegion> Cci.IMethodBody.ExceptionRegions
+        ImmutableArray<Cci.ExceptionHandlerRegion> Cci.IMethodBody.ExceptionRegions
         {
             get { return this.exceptionHandlers; }
         }
@@ -78,9 +78,9 @@ bool Cci.IMethodBody.LocalsAreZeroed
             get { return true; }
         }
 
-        IEnumerable<Cci.ILocalDefinition> Cci.IMethodBody.LocalVariables
+        ImmutableArray<Cci.ILocalDefinition> Cci.IMethodBody.LocalVariables
         {
-            get { return this.locals; }
+            get { return StaticCast<Cci.ILocalDefinition>.From(this.locals); }
         }
 
         Cci.IMethodDefinition Cci.IMethodBody.MethodDefinition

Src/Compilers/Core/Source/CodeGen/PermissionSetAttribute.cs

@@ -10,6 +10,7 @@
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
 using Cci = Microsoft.Cci;
+using System.Collections.Immutable;
 
 namespace Microsoft.CodeAnalysis.CodeGen
 {
@@ -44,7 +45,7 @@ public PermissionSetAttributeWithFileReference(Cci.ICustomAttribute sourceAttrib
         /// <summary>
         /// Zero or more positional arguments for the attribute constructor.
         /// </summary>
-        public IEnumerable<Cci.IMetadataExpression> GetArguments(Microsoft.CodeAnalysis.Emit.Context context)
+        public ImmutableArray<Cci.IMetadataExpression> GetArguments(Microsoft.CodeAnalysis.Emit.Context context)
         {
             return this.sourceAttribute.GetArguments(context);
         }
@@ -60,7 +61,7 @@ public Cci.IMethodReference Constructor(Microsoft.CodeAnalysis.Emit.Context cont
         /// <summary>
         /// Zero or more named arguments that specify values for fields and properties of the attribute.
         /// </summary>
-        public IEnumerable<Cci.IMetadataNamedArgument> GetNamedArguments(Microsoft.CodeAnalysis.Emit.Context context)
+        public ImmutableArray<Cci.IMetadataNamedArgument> GetNamedArguments(Microsoft.CodeAnalysis.Emit.Context context)
         {
             // Perform fixup 
             Cci.ITypeReference stringType = context.Module.GetPlatformType(Cci.PlatformType.SystemString, context);
@@ -108,7 +109,7 @@ public IEnumerable<Cci.IMetadataNamedArgument> GetNamedArguments(Microsoft.CodeA
             }
 
             // Synthesize a named attribute argument "Hex = hexFileContent".
-            yield return new HexPropertyMetadataNamedArgument(stringType, new MetadataConstant(stringType, hexFileContent));
+            return ImmutableArray.Create<Cci.IMetadataNamedArgument>(new HexPropertyMetadataNamedArgument(stringType, new MetadataConstant(stringType, hexFileContent)));
         }
 
         // internal for testing purposes.

Src/Compilers/Core/Source/CodeGen/ReferenceDependencyWalker.cs

@@ -7,6 +7,7 @@
 using System.Threading;
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
+using System.Collections.Immutable;
 
 namespace Microsoft.CodeAnalysis.CodeGen
 {
@@ -143,7 +144,7 @@ private static void VisitMethodReference(Microsoft.Cci.IMethodReference methodRe
             }
         }
 
-        private static void VisitParameters(IEnumerable<Microsoft.Cci.IParameterTypeInformation> parameters, Microsoft.CodeAnalysis.Emit.Context context)
+        private static void VisitParameters(ImmutableArray<Microsoft.Cci.IParameterTypeInformation> parameters, Microsoft.CodeAnalysis.Emit.Context context)
         {
             foreach (var param in parameters)
             {

Src/Compilers/Core/Source/Collections/ImmutableArrayExtensions.cs

@@ -135,9 +135,9 @@ public static ImmutableArray<byte> ToImmutable(this MemoryStream stream)
             }
 
             var builder = ArrayBuilder<TResult>.GetInstance(items.Length);
-            foreach (var e in items)
+            for (int i = 0; i < items.Length; i++)
             {
-                builder.Add(map(e));
+                builder.Add(map(items[i]));
             }
 
             return builder.ToImmutableAndFree();

Src/Compilers/Core/Source/Emit/NoPia/CommonEmbeddedMethod.cs

@@ -120,9 +120,9 @@ void Cci.IMethodBody.Dispatch(Cci.MetadataVisitor visitor)
                     visitor.Visit((Cci.IMethodBody)this);
                 }
 
-                IEnumerable<Cci.ExceptionHandlerRegion> Cci.IMethodBody.ExceptionRegions
+                ImmutableArray<Cci.ExceptionHandlerRegion> Cci.IMethodBody.ExceptionRegions
                 {
-                    get { return SpecializedCollections.EmptyEnumerable<Cci.ExceptionHandlerRegion>(); }
+                    get { return ImmutableArray<Cci.ExceptionHandlerRegion>.Empty; }
                 }
 
                 bool Cci.IMethodBody.LocalsAreZeroed
@@ -130,9 +130,9 @@ bool Cci.IMethodBody.LocalsAreZeroed
                     get { return false; }
                 }
 
-                IEnumerable<Cci.ILocalDefinition> Cci.IMethodBody.LocalVariables
+                ImmutableArray<Cci.ILocalDefinition> Cci.IMethodBody.LocalVariables
                 {
-                    get { return SpecializedCollections.EmptyEnumerable<Cci.ILocalDefinition>(); }
+                    get { return ImmutableArray<Cci.ILocalDefinition>.Empty; }
                 }
 
                 Cci.IMethodDefinition Cci.IMethodBody.MethodDefinition
@@ -331,11 +331,11 @@ System.Reflection.MethodImplAttributes Cci.IMethodDefinition.GetImplementationAt
                 return GetImplementationAttributes(context);
             }
 
-            IEnumerable<Cci.IParameterDefinition> Cci.IMethodDefinition.Parameters
+            ImmutableArray<Cci.IParameterDefinition> Cci.IMethodDefinition.Parameters
             {
                 get
                 {
-                    return parameters;
+                    return StaticCast<Cci.IParameterDefinition>.From(parameters);
                 }
             }
 
@@ -453,12 +453,12 @@ Cci.IMethodDefinition Cci.IMethodReference.GetResolvedMethod(Context context)
                 return this;
             }
 
-            IEnumerable<Cci.IParameterTypeInformation> Cci.IMethodReference.ExtraParameters
+            ImmutableArray<Cci.IParameterTypeInformation> Cci.IMethodReference.ExtraParameters
             {
                 get
                 {
                     // This is a definition, no information about extra parameters 
-                    return SpecializedCollections.EmptyEnumerable<Cci.IParameterTypeInformation>();
+                    return ImmutableArray<Cci.IParameterTypeInformation>.Empty;
                 }
             }
 
@@ -494,9 +494,9 @@ ushort Cci.ISignature.ParameterCount
                 }
             }
 
-            IEnumerable<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Context context)
+            ImmutableArray<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Context context)
             {
-                return parameters;
+                return StaticCast<Cci.IParameterTypeInformation>.From(parameters);
             }
 
             IEnumerable<Cci.ICustomModifier> Cci.ISignature.ReturnValueCustomModifiers

Src/Compilers/Core/Source/Emit/NoPia/CommonEmbeddedProperty.cs

@@ -122,9 +122,9 @@ bool Cci.IPropertyDefinition.IsSpecialName
                 }
             }
 
-            IEnumerable<Cci.IParameterDefinition> Cci.IPropertyDefinition.Parameters
+            ImmutableArray<Cci.IParameterDefinition> Cci.IPropertyDefinition.Parameters
             {
-                get { return parameters; }
+                get { return StaticCast<Cci.IParameterDefinition>.From(parameters); }
             }
 
             Cci.CallingConvention Cci.ISignature.CallingConvention
@@ -140,9 +140,9 @@ ushort Cci.ISignature.ParameterCount
                 get { return (ushort)parameters.Length; }
             }
 
-            IEnumerable<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Context context)
+            ImmutableArray<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Context context)
             {
-                return parameters;
+                return StaticCast<Cci.IParameterTypeInformation>.From(parameters);
             }
 
             bool Cci.ISignature.ReturnValueIsModified

Src/Compilers/Core/Source/Emit/NoPia/VtblGap.cs

@@ -98,9 +98,9 @@ System.Reflection.MethodImplAttributes Cci.IMethodDefinition.GetImplementationAt
             return System.Reflection.MethodImplAttributes.Managed | System.Reflection.MethodImplAttributes.Runtime;
         }
 
-        IEnumerable<Cci.IParameterDefinition> Cci.IMethodDefinition.Parameters
+        ImmutableArray<Cci.IParameterDefinition> Cci.IMethodDefinition.Parameters
         {
-            get { return SpecializedCollections.EmptyEnumerable<Cci.IParameterDefinition>(); }
+            get { return ImmutableArray<Cci.IParameterDefinition>.Empty; }
         }
 
         Cci.IPlatformInvokeInformation Cci.IMethodDefinition.PlatformInvokeData
@@ -193,9 +193,9 @@ Cci.IMethodDefinition Cci.IMethodReference.GetResolvedMethod(Context context)
             return this;
         }
 
-        IEnumerable<Cci.IParameterTypeInformation> Cci.IMethodReference.ExtraParameters
+        ImmutableArray<Cci.IParameterTypeInformation> Cci.IMethodReference.ExtraParameters
         {
-            get { return SpecializedCollections.EmptyEnumerable<Cci.IParameterTypeInformation>(); }
+            get { return ImmutableArray<Cci.IParameterTypeInformation>.Empty; }
         }
 
         Cci.IGenericMethodInstanceReference Cci.IMethodReference.AsGenericMethodInstanceReference
@@ -218,9 +218,9 @@ ushort Cci.ISignature.ParameterCount
             get { return 0; }
         }
 
-        IEnumerable<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Context context)
+        ImmutableArray<Cci.IParameterTypeInformation> Cci.ISignature.GetParameters(Context context)
         {
-            return SpecializedCollections.EmptyEnumerable<Cci.IParameterTypeInformation>();
+            return ImmutableArray<Cci.IParameterTypeInformation>.Empty;
         }
 
         IEnumerable<Cci.ICustomModifier> Cci.ISignature.ReturnValueCustomModifiers

Src/Compilers/Core/Source/PEWriter/ICustomAttribute.cs

 // Copyright (c) Microsoft Open Technologies, Inc.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System.Collections.Generic;
+using System.Collections.Immutable;
 
 namespace Microsoft.Cci
 {
@@ -12,7 +13,7 @@ internal interface ICustomAttribute
         /// <summary>
         /// Zero or more positional arguments for the attribute constructor.
         /// </summary>
-        IEnumerable<IMetadataExpression> GetArguments(Microsoft.CodeAnalysis.Emit.Context context);
+        ImmutableArray<IMetadataExpression> GetArguments(Microsoft.CodeAnalysis.Emit.Context context);
 
         /// <summary>
         /// A reference to the constructor that will be used to instantiate this custom attribute during execution (if the attribute is inspected via Reflection).
@@ -22,7 +23,7 @@ internal interface ICustomAttribute
         /// <summary>
         /// Zero or more named arguments that specify values for fields and properties of the attribute.
         /// </summary>
-        IEnumerable<IMetadataNamedArgument> GetNamedArguments(Microsoft.CodeAnalysis.Emit.Context context);
+        ImmutableArray<IMetadataNamedArgument> GetNamedArguments(Microsoft.CodeAnalysis.Emit.Context context);
 
         /// <summary>
         /// The number of positional arguments.

Src/Compilers/Core/Source/PEWriter/Members.cs

@@ -367,7 +367,7 @@ internal interface IMethodBody
         /// <summary>
         /// A list exception data within the method body IL.
         /// </summary>
-        IEnumerable<ExceptionHandlerRegion> ExceptionRegions
+        ImmutableArray<ExceptionHandlerRegion> ExceptionRegions
         {
             get;
 
@@ -382,7 +382,7 @@ IEnumerable<ExceptionHandlerRegion> ExceptionRegions
         /// <summary>
         /// The local variables of the method.
         /// </summary>
-        IEnumerable<ILocalDefinition> LocalVariables { get; }
+        ImmutableArray<ILocalDefinition> LocalVariables { get; }
 
         /// <summary>
         /// The definition of the method whose body this is.
@@ -567,7 +567,7 @@ bool IsVirtual
         /// <summary>
         /// The parameters forming part of this signature.
         /// </summary>
-        IEnumerable<IParameterDefinition> Parameters { get; }
+        ImmutableArray<IParameterDefinition> Parameters { get; }
 
         /// <summary>
         /// Detailed information about the PInvoke stub. Identifies which method to call, which module has the method and the calling convention among other things.
@@ -727,7 +727,7 @@ IMetadataConstant DefaultValue
         /// <summary>
         /// The parameters forming part of this signature.
         /// </summary>
-        IEnumerable<IParameterDefinition> Parameters { get; }
+        ImmutableArray<IParameterDefinition> Parameters { get; }
 
         /// <summary>
         /// The method used to set the value of this property. May be absent (null).
@@ -754,7 +754,7 @@ internal interface ISignature
         /// <summary>
         /// The parameters forming part of this signature.
         /// </summary>
-        IEnumerable<IParameterTypeInformation> GetParameters(Microsoft.CodeAnalysis.Emit.Context context);
+        ImmutableArray<IParameterTypeInformation> GetParameters(Microsoft.CodeAnalysis.Emit.Context context);
 
         /// <summary>
         /// Returns the list of custom modifiers, if any, associated with the returned value. Evaluate this property only if ReturnValueIsModified is true.
@@ -906,7 +906,7 @@ ushort GenericParameterCount
         /// <summary>
         /// Information about this types of the extra arguments supplied at the call sites that references the method with this object.
         /// </summary>
-        IEnumerable<IParameterTypeInformation> ExtraParameters { get; }
+        ImmutableArray<IParameterTypeInformation> ExtraParameters { get; }
 
         IGenericMethodInstanceReference AsGenericMethodInstanceReference { get; }
         ISpecializedMethodReference AsSpecializedMethodReference { get; }

Src/Compilers/Core/Source/PEWriter/MetadataVisitor.cs

@@ -5,6 +5,7 @@
 using System.Runtime.InteropServices;
 using Roslyn.Utilities;
 using Cci = Microsoft.Cci;
+using System.Collections.Immutable;
 
 namespace Microsoft.Cci
 {
@@ -332,7 +333,7 @@ public virtual void Visit(IGlobalMethodDefinition globalMethodDefinition)
         /// Visits the specified local definitions.
         /// </summary>
         /// <param name="localDefinitions">The local definitions.</param>
-        public void Visit(IEnumerable<ILocalDefinition> localDefinitions)
+        public void Visit(ImmutableArray<ILocalDefinition> localDefinitions)
         {
             foreach (ILocalDefinition localDefinition in localDefinitions)
             {
@@ -643,7 +644,7 @@ public virtual void Visit(INestedTypeReference nestedTypeReference)
         /// <summary>
         /// Visits the specified operation exception informations.
         /// </summary>
-        public void Visit(IEnumerable<ExceptionHandlerRegion> exceptionRegions)
+        public void Visit(ImmutableArray<ExceptionHandlerRegion> exceptionRegions)
         {
             foreach (ExceptionHandlerRegion region in exceptionRegions)
             {
@@ -667,7 +668,7 @@ public virtual void Visit(ExceptionHandlerRegion exceptionRegion)
         /// Visits the specified parameters.
         /// </summary>
         /// <param name="parameters">The parameters.</param>
-        public void Visit(IEnumerable<IParameterDefinition> parameters)
+        public void Visit(ImmutableArray<IParameterDefinition> parameters)
         {
             foreach (IParameterDefinition parameter in parameters)
             {
@@ -712,7 +713,7 @@ public virtual void Visit(IParameterDefinition parameterDefinition)
         /// Visits the specified parameter type informations.
         /// </summary>
         /// <param name="parameterTypeInformations">The parameter type informations.</param>
-        public void Visit(IEnumerable<IParameterTypeInformation> parameterTypeInformations)
+        public void Visit(ImmutableArray<IParameterTypeInformation> parameterTypeInformations)
         {
             foreach (IParameterTypeInformation parameterTypeInformation in parameterTypeInformations)
             {

Src/Compilers/Core/Source/PEWriter/Miscellaneous.cs

@@ -13,15 +13,6 @@ namespace Microsoft.Cci
     /// </summary>
     internal static class IteratorHelper
     {
-        /// <summary>
-        /// Returns an enumerable containing no objects.
-        /// </summary>
-        /// <returns></returns>
-        public static IEnumerable<T> GetEmptyEnumerable<T>()
-        {
-            return SpecializedCollections.EmptyEnumerable<T>();
-        }
-
         /// <summary>
         /// True if the given enumerable is not null and contains at least one element.
         /// </summary>

Src/Compilers/Core/Source/PEWriter/PeWriter.cs

@@ -2204,7 +2204,7 @@ private uint GetPropertySignatureIndex(IPropertyDefinition propertyDef)
 
             MemoryStream sig = MemoryStream.GetInstance();
             BinaryWriter writer = new BinaryWriter(sig);
-            this.SerializeSignature(propertyDef, 0, IteratorHelper.GetEmptyEnumerable<IParameterTypeInformation>(), writer);
+            this.SerializeSignature(propertyDef, 0, ImmutableArray<IParameterTypeInformation>.Empty, writer);
             result = this.GetBlobIndex(sig.ToArray());
             this.signatureIndex.Add(propertyDef, result);
             sig.Free();
@@ -3005,7 +3005,8 @@ private uint SerializeLocalVariableSignatureAndReturnToken(IMethodBody methodBod
         {
             Debug.Assert(!this.tableIndicesAreComplete);
             this.localDefIndex.Clear();
-            ushort numLocals = (ushort)IteratorHelper.EnumerableCount(methodBody.LocalVariables);
+            var locals = methodBody.LocalVariables;
+            ushort numLocals = (ushort)locals.Length;
             if (numLocals == 0)
             {
                 return 0;
@@ -5163,7 +5164,7 @@ private void SerializeMethodBody(IMethodBody methodBody, BinaryWriter writer)
 
             byte[] il = this.SerializeMethodBodyIL(methodBody);
 
-            uint numberOfExceptionHandlers = IteratorHelper.EnumerableCount(methodBody.ExceptionRegions);
+            uint numberOfExceptionHandlers = (uint)methodBody.ExceptionRegions.Length;
             if (il.Length < 64 && methodBody.MaxStack <= 8 && localVariableSignatureToken == 0 && numberOfExceptionHandlers == 0)
             {
                 //If 'possiblyDuplicateMethodBodies' is not null, check if an identical
@@ -5308,7 +5309,7 @@ private void SerializeDebugInfo(IMethodBody methodBody, uint ilLength)
 
         private void SerializeReferenceToMethodWithModuleInfo()
         {
-            MemoryStream customMetadata = new MemoryStream();
+            MemoryStream customMetadata = new MemoryStream(12);
             BinaryWriter cmw = new BinaryWriter(customMetadata);
             cmw.WriteByte(4); // version
             cmw.WriteByte(2); // kind: ForwardToModuleInfo
@@ -5320,7 +5321,7 @@ private void SerializeReferenceToMethodWithModuleInfo()
 
         private void SerializeReferenceToPreviousMethodWithUsingInfo()
         {
-            MemoryStream customMetadata = new MemoryStream();
+            MemoryStream customMetadata = new MemoryStream(12);
             BinaryWriter cmw = new BinaryWriter(customMetadata);
             cmw.WriteByte(4); // version
             cmw.WriteByte(1); // kind: ForwardInfo
@@ -5823,7 +5824,8 @@ private void DefineScopeLocals(ILocalScope currentScope)
 
         private void SerializeMethodBodyExceptionHandlerTable(IMethodBody methodBody, uint numberOfExceptionHandlers, BinaryWriter writer)
         {
-            bool useSmallExceptionHeaders = MayUseSmallExceptionHeaders(numberOfExceptionHandlers, methodBody.ExceptionRegions);
+            var regions = methodBody.ExceptionRegions;
+            bool useSmallExceptionHeaders = MayUseSmallExceptionHeaders(numberOfExceptionHandlers, regions);
             writer.Align(4);
             if (useSmallExceptionHeaders)
             {
@@ -5840,7 +5842,7 @@ private void SerializeMethodBodyExceptionHandlerTable(IMethodBody methodBody, ui
                 writer.WriteUshort((ushort)((dataSize >> 8) & 0xffff));
             }
 
-            foreach (var region in methodBody.ExceptionRegions)
+            foreach (var region in regions)
             {
                 this.SerializeExceptionRegion(region, useSmallExceptionHeaders, writer);
             }
@@ -5876,7 +5878,7 @@ private void SerializeExceptionRegion(ExceptionHandlerRegion region, bool useSma
             }
         }
 
-        private static bool MayUseSmallExceptionHeaders(uint numberOfExceptionHandlers, IEnumerable<ExceptionHandlerRegion> exceptionRegions)
+        private static bool MayUseSmallExceptionHeaders(uint numberOfExceptionHandlers, ImmutableArray<ExceptionHandlerRegion> exceptionRegions)
         {
             if (numberOfExceptionHandlers * 12 + 4 > 0xff)
             {
@@ -6444,7 +6446,7 @@ private void SerializePermissionSet(IEnumerable<ICustomAttribute> permissionSet,
             // TODO: xml for older platforms
         }
 
-        private void SerializeSignature(ISignature signature, ushort genericParameterCount, IEnumerable<IParameterTypeInformation> extraArgumentTypes, BinaryWriter writer)
+        private void SerializeSignature(ISignature signature, ushort genericParameterCount, ImmutableArray<IParameterTypeInformation> extraArgumentTypes, BinaryWriter writer)
         {
             byte header = (byte)signature.CallingConvention;
             if (signature is IPropertyDefinition)
@@ -6458,8 +6460,9 @@ private void SerializeSignature(ISignature signature, ushort genericParameterCou
                 writer.WriteCompressedUInt(genericParameterCount);
             }
 
-            uint numberOfRequiredParameters = IteratorHelper.EnumerableCount(signature.GetParameters(Context));
-            uint numberOfOptionalParameters = IteratorHelper.EnumerableCount(extraArgumentTypes);
+            var @params = signature.GetParameters(Context);
+            uint numberOfRequiredParameters = (uint)@params.Length;
+            uint numberOfOptionalParameters = (uint)extraArgumentTypes.Length;
             writer.WriteCompressedUInt(numberOfRequiredParameters + numberOfOptionalParameters);
             if (signature.ReturnValueIsModified)
             {
@@ -6475,7 +6478,7 @@ private void SerializeSignature(ISignature signature, ushort genericParameterCou
             }
 
             this.SerializeTypeReference(signature.GetType(Context), writer, false, true);
-            foreach (IParameterTypeInformation parameterTypeInformation in signature.GetParameters(Context))
+            foreach (IParameterTypeInformation parameterTypeInformation in @params)
             {
                 this.SerializeParameterInformation(parameterTypeInformation, writer);
             }

Src/Compilers/Core/Source/PEWriter/Types.cs

 // Copyright (c) Microsoft Open Technologies, Inc.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Reflection.Metadata;
 using System.Runtime.InteropServices;
 
@@ -147,7 +148,7 @@ internal interface IFunctionPointerTypeReference : ITypeReference, ISignature
         /// <summary>
         /// The types and modifiers of extra arguments that the caller will pass to the methods that are pointed to by this pointer.
         /// </summary>
-        IEnumerable<IParameterTypeInformation> ExtraArgumentTypes { get; }
+        ImmutableArray<IParameterTypeInformation> ExtraArgumentTypes { get; }
     }
 
     /// <summary>

Src/Compilers/VisualBasic/Source/Emit/AttributeDataAdapter.vb

@@ -17,8 +17,8 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
     Partial Class VisualBasicAttributeData
         Implements Cci.ICustomAttribute
 
-        Private Function GetArguments1(context As Microsoft.CodeAnalysis.Emit.Context) As IEnumerable(Of Cci.IMetadataExpression) Implements Cci.ICustomAttribute.GetArguments
-            Return From arg In CommonConstructorArguments Select CreateMetadataExpression(arg, context)
+        Private Function GetArguments1(context As Microsoft.CodeAnalysis.Emit.Context) As ImmutableArray(Of Cci.IMetadataExpression) Implements Cci.ICustomAttribute.GetArguments
+            Return CommonConstructorArguments.SelectAsArray(Function(arg) CreateMetadataExpression(arg, context))
         End Function
 
         Private Function Constructor1(context As Microsoft.CodeAnalysis.Emit.Context) As Cci.IMethodReference Implements Cci.ICustomAttribute.Constructor
@@ -27,8 +27,8 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
                                               syntaxNodeOpt:=DirectCast(context.SyntaxNodeOpt, VisualBasicSyntaxNode), Diagnostics:=context.Diagnostics)
         End Function
 
-        Private Function GetNamedArguments1(context As Microsoft.CodeAnalysis.Emit.Context) As IEnumerable(Of Cci.IMetadataNamedArgument) Implements Cci.ICustomAttribute.GetNamedArguments
-            Return From namedArgument In CommonNamedArguments Select CreateMetadataNamedArgument(namedArgument.Key, namedArgument.Value, context)
+        Private Function GetNamedArguments1(context As Microsoft.CodeAnalysis.Emit.Context) As ImmutableArray(Of Cci.IMetadataNamedArgument) Implements Cci.ICustomAttribute.GetNamedArguments
+            Return CommonNamedArguments.SelectAsArray(Function(namedArgument) CreateMetadataNamedArgument(namedArgument.Key, namedArgument.Value, context))
         End Function
 
         Private ReadOnly Property ArgumentCount As Integer Implements Cci.ICustomAttribute.ArgumentCount

Src/Compilers/VisualBasic/Source/Emit/MethodReference.vb

@@ -2,6 +2,7 @@
 
 Imports System
 Imports System.Collections.Generic
+Imports System.Collections.Immutable
 Imports System.Linq
 Imports System.Text
 Imports Microsoft.CodeAnalysis.Text
@@ -53,9 +54,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Emit
             Return Nothing
         End Function
 
-        Private ReadOnly Property IMethodReferenceExtraParameters As IEnumerable(Of Microsoft.Cci.IParameterTypeInformation) Implements Microsoft.Cci.IMethodReference.ExtraParameters
+        Private ReadOnly Property IMethodReferenceExtraParameters As ImmutableArray(Of Microsoft.Cci.IParameterTypeInformation) Implements Microsoft.Cci.IMethodReference.ExtraParameters
             Get
-                Return SpecializedCollections.EmptyEnumerable(Of Microsoft.Cci.IParameterTypeInformation)()
+                Return ImmutableArray(Of Microsoft.Cci.IParameterTypeInformation).Empty
             End Get
         End Property
 
@@ -65,7 +66,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Emit
             End Get
         End Property
 
-        Private Function ISignatureGetParameters(context As Microsoft.CodeAnalysis.Emit.Context) As IEnumerable(Of Microsoft.Cci.IParameterTypeInformation) Implements Microsoft.Cci.ISignature.GetParameters
+        Private Function ISignatureGetParameters(context As Microsoft.CodeAnalysis.Emit.Context) As ImmutableArray(Of Microsoft.Cci.IParameterTypeInformation) Implements Microsoft.Cci.ISignature.GetParameters
             Dim moduleBeingBuilt As PEModuleBuilder = DirectCast(context.Module, PEModuleBuilder)
 
             Return moduleBeingBuilt.Translate(m_UnderlyingMethod.Parameters)

Src/Compilers/VisualBasic/Source/Emit/MethodSymbolAdapter.vb

@@ -135,9 +135,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
             Return Nothing
         End Function
 
-        Private ReadOnly Property IMethodReferenceExtraParameters As IEnumerable(Of IParameterTypeInformation) Implements IMethodReference.ExtraParameters
+        Private ReadOnly Property IMethodReferenceExtraParameters As ImmutableArray(Of IParameterTypeInformation) Implements IMethodReference.ExtraParameters
             Get
-                Return SpecializedCollections.EmptyEnumerable(Of IParameterTypeInformation)()
+                Return ImmutableArray(Of IParameterTypeInformation).Empty
             End Get
         End Property
 
@@ -147,7 +147,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
             End Get
         End Property
 
-        Private Function ISignatureGetParameters(context As Microsoft.CodeAnalysis.Emit.Context) As IEnumerable(Of IParameterTypeInformation) Implements ISignature.GetParameters
+        Private Function ISignatureGetParameters(context As Microsoft.CodeAnalysis.Emit.Context) As ImmutableArray(Of IParameterTypeInformation) Implements ISignature.GetParameters
             Dim moduleBeingBuilt As PEModuleBuilder = DirectCast(context.Module, PEModuleBuilder)
 
             Debug.Assert(Me.IsDefinitionOrDistinct())
@@ -158,16 +158,16 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
 #End If
 
             If Me.IsDefinition AndAlso Me.ContainingModule = moduleBeingBuilt.SourceModule Then
-                Return EnumerateDefinitionParameters
+                Return EnumerateDefinitionParameters()
             Else
                 Return moduleBeingBuilt.Translate(Me.Parameters)
             End If
         End Function
 
-        Private Function EnumerateDefinitionParameters() As IEnumerable(Of Cci.IParameterTypeInformation)
+        Private Function EnumerateDefinitionParameters() As ImmutableArray(Of Cci.IParameterTypeInformation)
             Debug.Assert(Me.Parameters.All(Function(p) p.IsDefinition))
 
-            Return Me.Parameters
+            Return StaticCast(Of IParameterTypeInformation).From(Me.Parameters)
         End Function
 
         Private ReadOnly Property ISignatureReturnValueCustomModifiers As IEnumerable(Of Microsoft.Cci.ICustomModifier) Implements ISignature.ReturnValueCustomModifiers
@@ -435,7 +435,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
             End Get
         End Property
 
-        Private ReadOnly Property IMethodDefinitionParameters As IEnumerable(Of IParameterDefinition) Implements IMethodDefinition.Parameters
+        Private ReadOnly Property IMethodDefinitionParameters As ImmutableArray(Of IParameterDefinition) Implements IMethodDefinition.Parameters
             Get
                 CheckDefinitionInvariant()
 
@@ -444,7 +444,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
                     Debug.Assert(p Is p.OriginalDefinition)
                 Next
 #End If
-                Return Me.Parameters
+                Return StaticCast(Of IParameterDefinition).From(Me.Parameters)
             End Get
         End Property
 

Src/Compilers/VisualBasic/Source/Emit/PropertySymbolAdapter.vb

@@ -2,6 +2,7 @@
 
 Imports System
 Imports System.Collections.Generic
+Imports System.Collections.Immutable
 Imports Microsoft.Cci
 Imports Microsoft.CodeAnalysis.Text
 Imports Microsoft.CodeAnalysis.VisualBasic.Emit
@@ -71,10 +72,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
             End Get
         End Property
 
-        Private ReadOnly Property IPropertyDefinitionParameters As IEnumerable(Of IParameterDefinition) Implements IPropertyDefinition.Parameters
+        Private ReadOnly Property IPropertyDefinitionParameters As ImmutableArray(Of IParameterDefinition) Implements IPropertyDefinition.Parameters
             Get
                 CheckDefinitionInvariant()
-                Return Me.GetParameters()
+                Return StaticCast(Of IParameterDefinition).From(Me.Parameters)
             End Get
         End Property
 
@@ -99,9 +100,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
             End Get
         End Property
 
-        Private Function ISignatureGetParameters(context As Microsoft.CodeAnalysis.Emit.Context) As IEnumerable(Of IParameterTypeInformation) Implements ISignature.GetParameters
+        Private Function ISignatureGetParameters(context As Microsoft.CodeAnalysis.Emit.Context) As ImmutableArray(Of IParameterTypeInformation) Implements ISignature.GetParameters
             CheckDefinitionInvariant()
-            Return Me.GetParameters()
+            Return StaticCast(Of IParameterTypeInformation).From(Me.Parameters)
         End Function
 
         Private ReadOnly Property ISignatureReturnValueCustomModifiers As IEnumerable(Of Microsoft.Cci.ICustomModifier) Implements ISignature.ReturnValueCustomModifiers
@@ -165,12 +166,5 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
                 Return Me.MetadataName
             End Get
         End Property
-
-        Private Function GetParameters() As IEnumerable(Of ParameterSymbol)
-            CheckDefinitionInvariant()
-            Return Me.Parameters
-        End Function
-
     End Class
-
 End Namespace

Src/Compilers/VisualBasic/Source/Emit/SymbolTranslator.vb

@@ -402,19 +402,14 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Emit
             Return methodRef
         End Function
 
-        Friend Overloads Function Translate(params As ImmutableArray(Of ParameterSymbol)) As IEnumerable(Of Microsoft.Cci.IParameterTypeInformation)
+        Friend Overloads Function Translate(params As ImmutableArray(Of ParameterSymbol)) As ImmutableArray(Of Microsoft.Cci.IParameterTypeInformation)
             Debug.Assert(params.All(Function(p) p.IsDefinitionOrDistinct()))
-            If (params.Length = 0) Then
-                Return SpecializedCollections.EmptyEnumerable(Of Cci.IParameterTypeInformation)()
-            End If
-
-            Dim firstParam = params.First()
 
-            Dim mustBeTranslated = MustBeWrapped(firstParam)
+            Dim mustBeTranslated = params.Any AndAlso MustBeWrapped(params.First())
             Debug.Assert(params.All(Function(p) mustBeTranslated = MustBeWrapped(p)), "either all or no parameters need translating")
 
             If (Not mustBeTranslated) Then
-                Return params
+                Return StaticCast(Of Microsoft.Cci.IParameterTypeInformation).From(params)
             End If
 
             Return TranslateAll(params)
@@ -437,10 +432,8 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Emit
             Return False
         End Function
 
-        Private Iterator Function TranslateAll(params As ImmutableArray(Of ParameterSymbol)) As IEnumerable(Of Microsoft.Cci.IParameterTypeInformation)
-            For Each param In params
-                Yield CreateParameterTypeInformationWrapper(param)
-            Next
+        Private Function TranslateAll(params As ImmutableArray(Of ParameterSymbol)) As ImmutableArray(Of Microsoft.Cci.IParameterTypeInformation)
+            Return params.SelectAsArray(Function(param) CreateParameterTypeInformationWrapper(param))
         End Function
 
         Private Function CreateParameterTypeInformationWrapper(param As ParameterSymbol) As Cci.IParameterTypeInformation