Use immutable arrays in more places.

src/Workspaces/Core/Portable/FindSymbols/FindReferences/DependentTypeFinder.cs

@@ -38,7 +38,7 @@ internal static class DependentTypeFinder
         /// <summary>
         /// Used for implementing the Inherited-By relation for progression.
         /// </summary>
-        internal static Task<IEnumerable<SymbolAndProjectId<INamedTypeSymbol>>> FindImmediatelyDerivedClassesAsync(
+        internal static Task<ImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>> FindImmediatelyDerivedClassesAsync(
             INamedTypeSymbol type,
             Solution solution,
             CancellationToken cancellationToken)
@@ -51,7 +51,7 @@ internal static class DependentTypeFinder
         /// <summary>
         /// This is an internal implementation of <see cref="SymbolFinder.FindDerivedClassesAsync(SymbolAndProjectId{INamedTypeSymbol}, Solution, IImmutableSet{Project}, CancellationToken)"/>, which is a publically callable method.
         /// </summary>
-        public static Task<IEnumerable<SymbolAndProjectId<INamedTypeSymbol>>> FindTransitivelyDerivedClassesAsync(
+        public static Task<ImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>> FindTransitivelyDerivedClassesAsync(
             INamedTypeSymbol type,
             Solution solution,
             IImmutableSet<Project> projects,
@@ -62,7 +62,7 @@ internal static class DependentTypeFinder
                 transitive: true, cancellationToken: cancellationToken);
         }
 
-        private static Task<IEnumerable<SymbolAndProjectId<INamedTypeSymbol>>> FindDerivedClassesAsync(
+        private static Task<ImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>> FindDerivedClassesAsync(
             SymbolAndProjectId<INamedTypeSymbol> type,
             Solution solution,
             IImmutableSet<Project> projects, 
@@ -85,7 +85,7 @@ internal static class DependentTypeFinder
                     cancellationToken: cancellationToken);
             }
 
-            return SpecializedTasks.EmptyEnumerable<SymbolAndProjectId<INamedTypeSymbol>>();
+            return SpecializedTasks.EmptyImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>();
         }
 
         /// <summary>
@@ -109,7 +109,7 @@ internal static class DependentTypeFinder
         /// <summary>
         /// Used for implementing the Inherited-By relation for progression.
         /// </summary>
-        internal static Task<IEnumerable<SymbolAndProjectId<INamedTypeSymbol>>> FindImmediatelyDerivedAndImplementingTypesAsync(
+        internal static Task<ImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>> FindImmediatelyDerivedAndImplementingTypesAsync(
             INamedTypeSymbol type,
             Solution solution,
             CancellationToken cancellationToken)
@@ -119,7 +119,7 @@ internal static class DependentTypeFinder
                 transitive: false, cancellationToken: cancellationToken);
         }
 
-        private static Task<IEnumerable<SymbolAndProjectId<INamedTypeSymbol>>> FindDerivedAndImplementingTypesAsync(
+        private static Task<ImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>> FindDerivedAndImplementingTypesAsync(
             SymbolAndProjectId<INamedTypeSymbol> type,
             Solution solution,
             IImmutableSet<Project> projects,
@@ -141,10 +141,10 @@ internal static class DependentTypeFinder
                     cancellationToken: cancellationToken);
            }
 
-            return SpecializedTasks.EmptyEnumerable<SymbolAndProjectId<INamedTypeSymbol>>();
+            return SpecializedTasks.EmptyImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>();
         }
 
-        private static async Task<IEnumerable<SymbolAndProjectId<INamedTypeSymbol>>> FindTypesAsync(
+        private static async Task<ImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>> FindTypesAsync(
             SymbolAndProjectId<INamedTypeSymbol> type,
             Solution solution,
             IImmutableSet<Project> projects,
@@ -214,7 +214,7 @@ internal static class DependentTypeFinder
                     transitive, cancellationToken).ConfigureAwait(false);
             }
 
-            return result;
+            return result.ToImmutableArray();
         }
 
         private static async Task FindTypesInProjectAsync(

src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/AbstractMethodOrPropertyOrEventSymbolReferenceFinder.cs

@@ -44,9 +44,8 @@ protected AbstractMethodOrPropertyOrEventSymbolReferenceFinder()
                     // Finally, methods can cascade through virtual/override inheritance.  NOTE(cyrusn):
                     // We only need to go up or down one level.  Then, when we're finding references on
                     // those members, we'll end up traversing the entire hierarchy.
-                    var overridesStream = await SymbolFinder.FindOverridesAsync(
+                    var overrides = await SymbolFinder.FindOverridesAsync(
                         symbolAndProjectId, solution, projects, cancellationToken).ConfigureAwait(false);
-                    var overrides = overridesStream.ToImmutableArray();
 
                     var overriddenMember = symbolAndProjectId.WithSymbol(symbol.OverriddenMember());
                     if (overriddenMember.Symbol == null)

src/Workspaces/Core/Portable/FindSymbols/SymbolFinder_Hierarchy.cs

@@ -26,10 +26,10 @@ public static partial class SymbolFinder
                 SymbolAndProjectId.Create(symbol, projectId: null),
                 solution, projects, cancellationToken).ConfigureAwait(false);
 
-            return result.Select(s => s.Symbol).ToList();
+            return result.SelectAsArray(s => s.Symbol);
         }
 
-        internal static async Task<IEnumerable<SymbolAndProjectId>> FindOverridesAsync(
+        internal static async Task<ImmutableArray<SymbolAndProjectId>> FindOverridesAsync(
             SymbolAndProjectId symbolAndProjectId, Solution solution, IImmutableSet<Project> projects = null, CancellationToken cancellationToken = default(CancellationToken))
         {
             // Method can only have overrides if its a virtual, abstract or override and is not
@@ -45,7 +45,7 @@ public static partial class SymbolFinder
                     symbolAndProjectId.WithSymbol(containingType),
                     solution, projects, cancellationToken).ConfigureAwait(false);
 
-                List<SymbolAndProjectId> results = null;
+                var results = ArrayBuilder<SymbolAndProjectId>.GetInstance();
                 foreach (var type in derivedTypes)
                 {
                     foreach (var m in GetMembers(type, symbol.Name))
@@ -58,19 +58,15 @@ public static partial class SymbolFinder
                             member.OverriddenMember() != null &&
                             OriginalSymbolsMatch(member.OverriddenMember().OriginalDefinition, symbol.OriginalDefinition, solution, cancellationToken))
                         {
-                            results = results ?? new List<SymbolAndProjectId>();
                             results.Add(bestMember);
                         }
                     }
                 }
 
-                if (results != null)
-                {
-                    return results;
-                }
+                return results.ToImmutableAndFree();
             }
 
-            return SpecializedCollections.EmptyEnumerable<SymbolAndProjectId>();
+            return ImmutableArray<SymbolAndProjectId>.Empty;
         }
 
         /// <summary>
@@ -186,10 +182,10 @@ public static partial class SymbolFinder
             var result = await FindDerivedClassesAsync(
                 SymbolAndProjectId.Create(type, projectId: null),
                 solution, projects, cancellationToken).ConfigureAwait(false);
-            return result.Select(s => s.Symbol).ToList();
+            return result.SelectAsArray(s => s.Symbol);
         }
 
-        internal static Task<IEnumerable<SymbolAndProjectId<INamedTypeSymbol>>> FindDerivedClassesAsync(
+        internal static Task<ImmutableArray<SymbolAndProjectId<INamedTypeSymbol>>> FindDerivedClassesAsync(
             SymbolAndProjectId<INamedTypeSymbol> typeAndProjectId, Solution solution, IImmutableSet<Project> projects = null, CancellationToken cancellationToken = default(CancellationToken))
         {
             var type = typeAndProjectId.Symbol;
@@ -215,7 +211,7 @@ public static partial class SymbolFinder
             var result = await FindImplementationsAsync(
                 SymbolAndProjectId.Create(symbol, projectId: null),
                 solution, projects, cancellationToken).ConfigureAwait(false);
-            return result.Select(s => s.Symbol).ToList();
+            return result.SelectAsArray(s => s.Symbol);
         }
 
         internal static async Task<ImmutableArray<SymbolAndProjectId>> FindImplementationsAsync(
@@ -299,7 +295,7 @@ public static async Task<IEnumerable<SymbolCallerInfo>> FindCallersAsync(ISymbol
             var directReferences = callReferences.Where(
                 r => SymbolEquivalenceComparer.Instance.Equals(symbol, r.Definition)).FirstOrDefault();
 
-            var indirectReferences = callReferences.Where(r => r != directReferences).ToList();
+            var indirectReferences = callReferences.WhereAsArray(r => r != directReferences);
 
             List<SymbolCallerInfo> results = null;