Address PR feedback and use singleton for null instance

src/Compilers/CSharp/Portable/Operations/CSharpOperationFactory.cs

@@ -18,6 +18,9 @@ internal sealed partial class CSharpOperationFactory
         private readonly ConcurrentDictionary<BoundNode, IOperation> _cache =
             new ConcurrentDictionary<BoundNode, IOperation>(concurrencyLevel: 2, capacity: 10);
 
+        private readonly Lazy<IOperation> _lazyNullOperation =
+            new Lazy<IOperation>(() => null);
+
         private readonly SemanticModel _semanticModel;
 
         public CSharpOperationFactory(SemanticModel semanticModel)
@@ -479,7 +482,7 @@ private IAnonymousObjectCreationOperation CreateBoundAnonymousObjectCreationExpr
         private IPropertyReferenceOperation CreateBoundAnonymousPropertyDeclarationOperation(BoundAnonymousPropertyDeclaration boundAnonymousPropertyDeclaration)
         {
             PropertySymbol property = boundAnonymousPropertyDeclaration.Property;
-            Lazy<IOperation> instance = new Lazy<IOperation>(() => null);
+            Lazy<IOperation> instance = _lazyNullOperation;
             Lazy<ImmutableArray<IArgumentOperation>> arguments = new Lazy<ImmutableArray<IArgumentOperation>>(() => ImmutableArray<IArgumentOperation>.Empty);
             SyntaxNode syntax = boundAnonymousPropertyDeclaration.Syntax;
             ITypeSymbol type = boundAnonymousPropertyDeclaration.Type;
@@ -1309,7 +1312,7 @@ private IWhileLoopOperation CreateBoundWhileStatementOperation(BoundWhileStateme
         {
             Lazy<IOperation> condition = new Lazy<IOperation>(() => Create(boundWhileStatement.Condition));
             Lazy<IOperation> body = new Lazy<IOperation>(() => Create(boundWhileStatement.Body));
-            Lazy<IOperation> ignoredCondition = new Lazy<IOperation>(() => null);
+            Lazy<IOperation> ignoredCondition = _lazyNullOperation;
             ImmutableArray<ILocalSymbol> locals = boundWhileStatement.Locals.As<ILocalSymbol>();
             bool conditionIsTop = true;
             bool conditionIsUntil = false;
@@ -1324,7 +1327,7 @@ private IWhileLoopOperation CreateBoundDoStatementOperation(BoundDoStatement bou
         {
             Lazy<IOperation> condition = new Lazy<IOperation>(() => Create(boundDoStatement.Condition));
             Lazy<IOperation> body = new Lazy<IOperation>(() => Create(boundDoStatement.Body));
-            Lazy<IOperation> ignoredCondition = new Lazy<IOperation>(() => null);
+            Lazy<IOperation> ignoredCondition = _lazyNullOperation;
             bool conditionIsTop = false;
             bool conditionIsUntil = false;
             ImmutableArray<ILocalSymbol> locals = boundDoStatement.Locals.As<ILocalSymbol>();
@@ -1366,7 +1369,7 @@ private IForEachLoopOperation CreateBoundForEachStatementOperation(BoundForEachS
             }
             else
             {
-                loopControlVariable = new Lazy<IOperation>(() => null);
+                loopControlVariable = _lazyNullOperation;
             }
 
             Lazy<IOperation> collection = new Lazy<IOperation>(() => Create(boundForEachStatement.Expression));

src/Compilers/VisualBasic/Portable/Operations/VisualBasicOperationFactory.vb

@@ -13,6 +13,9 @@ Namespace Microsoft.CodeAnalysis.Operations
         Private ReadOnly _cache As ConcurrentDictionary(Of BoundNode, IOperation) =
             New ConcurrentDictionary(Of BoundNode, IOperation)(concurrencyLevel:=2, capacity:=10)
 
+        Private ReadOnly _lazyNothingOperation As Lazy(Of IOperation) =
+            New Lazy(Of IOperation)(Function() Nothing)
+
         Private ReadOnly _semanticModel As SemanticModel
 
         Public Sub New(semanticModel As SemanticModel)
@@ -1215,7 +1218,7 @@ Namespace Microsoft.CodeAnalysis.Operations
         Private Function CreateBoundWhileStatementOperation(boundWhileStatement As BoundWhileStatement) As IWhileLoopOperation
             Dim condition As Lazy(Of IOperation) = New Lazy(Of IOperation)(Function() Create(boundWhileStatement.Condition))
             Dim body As Lazy(Of IOperation) = New Lazy(Of IOperation)(Function() Create(boundWhileStatement.Body))
-            Dim ignoredCondition As Lazy(Of IOperation) = New Lazy(Of IOperation)(Function() Nothing)
+            Dim ignoredCondition As Lazy(Of IOperation) = _lazyNothingOperation
             Dim locals As ImmutableArray(Of ILocalSymbol) = ImmutableArray(Of ILocalSymbol).Empty
             Dim conditionIsTop As Boolean = True
             Dim conditionIsUntil As Boolean = False
@@ -1246,7 +1249,7 @@ Namespace Microsoft.CodeAnalysis.Operations
 
         Private Function CreateBoundLabelStatementOperation(boundLabelStatement As BoundLabelStatement) As ILabeledOperation
             Dim label As ILabelSymbol = boundLabelStatement.Label
-            Dim statement As Lazy(Of IOperation) = New Lazy(Of IOperation)(Function() Nothing)
+            Dim statement As Lazy(Of IOperation) = _lazyNothingOperation
             Dim syntax As SyntaxNode = boundLabelStatement.Syntax
             Dim type As ITypeSymbol = Nothing
             Dim constantValue As [Optional](Of Object) = New [Optional](Of Object)()
@@ -1493,7 +1496,7 @@ Namespace Microsoft.CodeAnalysis.Operations
         End Function
 
         Private Function CreateBoundAnonymousTypePropertyAccessOperation(boundAnonymousTypePropertyAccess As BoundAnonymousTypePropertyAccess) As IPropertyReferenceOperation
-            Dim instance As Lazy(Of IOperation) = New Lazy(Of IOperation)(Function() Nothing)
+            Dim instance As Lazy(Of IOperation) = _lazyNothingOperation
             Dim [property] As IPropertySymbol = DirectCast(boundAnonymousTypePropertyAccess.ExpressionSymbol, IPropertySymbol)
             Dim arguments As Lazy(Of ImmutableArray(Of IArgumentOperation)) = New Lazy(Of ImmutableArray(Of IArgumentOperation))(Function() ImmutableArray(Of IArgumentOperation).Empty)
             Dim syntax As SyntaxNode = boundAnonymousTypePropertyAccess.Syntax