No need to convert between Immutable and mutable arrays.

src/EditorFeatures/Core/SymbolSearch/SymbolSearchUpdateEngine.cs

@@ -175,10 +175,11 @@ public SymbolSearchUpdateEngine(ISymbolSearchLogService logService)
                     // Only look at reference assembly results.
                     if (type.PackageName.ToString() == MicrosoftAssemblyReferencesName)
                     {
-                        var nameParts = new List<string>();
+                        var nameParts = ArrayBuilder<string>.GetInstance();
                         GetFullName(nameParts, type.FullName.Parent);
                         var result = new ReferenceAssemblyWithTypeResult(
-                            type.AssemblyName.ToString(), type.Name.ToString(), containingNamespaceNames: nameParts);
+                            type.AssemblyName.ToString(), type.Name.ToString(), 
+                            containingNamespaceNames: nameParts.ToImmutableAndFree());
                         results.Add(result);
                     }
                 }
@@ -200,7 +201,7 @@ where this.IsType(symbol) && !this.IsType(symbol.Parent())
 
         private PackageWithTypeResult CreateResult(AddReferenceDatabase database, Symbol type)
         {
-            var nameParts = new List<string>();
+            var nameParts = ArrayBuilder<string>.GetInstance();
             GetFullName(nameParts, type.FullName.Parent);
 
             var packageName = type.PackageName.ToString();
@@ -212,7 +213,7 @@ private PackageWithTypeResult CreateResult(AddReferenceDatabase database, Symbol
                 typeName: type.Name.ToString(), 
                 version: version,
                 rank: GetRank(type),
-                containingNamespaceNames: nameParts);
+                containingNamespaceNames: nameParts.ToImmutableAndFree());
         }
 
         private int GetRank(Symbol symbol)
@@ -260,7 +261,7 @@ private bool IsType(Symbol symbol)
             return symbol.Type.IsType();
         }
 
-        private void GetFullName(List<string> nameParts, Path8 path)
+        private void GetFullName(ArrayBuilder<string> nameParts, Path8 path)
         {
             if (!path.IsEmpty)
             {

src/Workspaces/Core/Portable/SymbolSearch/ISymbolSearchService.cs

@@ -64,7 +64,7 @@ protected PackageResult(string packageName, int rank)
 
     internal class PackageWithTypeResult : PackageResult
     {
-        public readonly IReadOnlyList<string> ContainingNamespaceNames;
+        public readonly ImmutableArray<string> ContainingNamespaceNames;
         public readonly string TypeName;
         public readonly string Version;
 
@@ -73,7 +73,7 @@ internal class PackageWithTypeResult : PackageResult
             string typeName,
             string version,
             int rank,
-            IReadOnlyList<string> containingNamespaceNames)
+            ImmutableArray<string> containingNamespaceNames)
             : base(packageName, rank)
         {
             TypeName = typeName;
@@ -118,14 +118,14 @@ public int CompareTo(PackageWithAssemblyResult other)
 
     internal class ReferenceAssemblyWithTypeResult
     {
-        public readonly IReadOnlyList<string> ContainingNamespaceNames;
+        public readonly ImmutableArray<string> ContainingNamespaceNames;
         public readonly string AssemblyName;
         public readonly string TypeName;
 
         public ReferenceAssemblyWithTypeResult(
             string assemblyName,
             string typeName,
-            IReadOnlyList<string> containingNamespaceNames)
+            ImmutableArray<string> containingNamespaceNames)
         {
             AssemblyName = assemblyName;
             TypeName = typeName;

src/Workspaces/Remote/ServiceHub/Shared/RoslynJsonConverter.RoslynOnly.cs

@@ -161,7 +161,7 @@ protected override PackageWithTypeResult ReadValue(JsonReader reader, JsonSerial
                 var typeName = ReadProperty<string>(reader);
                 var version = ReadProperty<string>(reader);
                 var rank = (int)ReadProperty<long>(reader);
-                var containingNamespaceNames = ReadProperty<string[]>(serializer, reader);
+                var containingNamespaceNames = ReadProperty<ImmutableArray<string>>(serializer, reader);
 
                 Contract.ThrowIfFalse(reader.Read());
                 Contract.ThrowIfFalse(reader.TokenType == JsonToken.EndObject);
@@ -186,7 +186,7 @@ protected override void WriteValue(JsonWriter writer, PackageWithTypeResult sour
                 writer.WriteValue(source.Rank);
 
                 writer.WritePropertyName(nameof(PackageWithTypeResult.ContainingNamespaceNames));
-                writer.WriteValue(source.ContainingNamespaceNames.ToArray());
+                serializer.Serialize(writer, source.ContainingNamespaceNames);
 
                 writer.WriteEndObject();
             }
@@ -233,7 +233,7 @@ protected override ReferenceAssemblyWithTypeResult ReadValue(JsonReader reader,
 
                 var assemblyName = ReadProperty<string>(reader);
                 var typeName = ReadProperty<string>(reader);
-                var containingNamespaceNames = ReadProperty<string[]>(serializer, reader);
+                var containingNamespaceNames = ReadProperty<ImmutableArray<string>>(serializer, reader);
 
                 Contract.ThrowIfFalse(reader.Read());
                 Contract.ThrowIfFalse(reader.TokenType == JsonToken.EndObject);
@@ -252,7 +252,7 @@ protected override void WriteValue(JsonWriter writer, ReferenceAssemblyWithTypeR
                 writer.WriteValue(source.TypeName);
 
                 writer.WritePropertyName(nameof(ReferenceAssemblyWithTypeResult.ContainingNamespaceNames));
-                writer.WriteValue(source.ContainingNamespaceNames.ToArray());
+                serializer.Serialize(writer, source.ContainingNamespaceNames);
 
                 writer.WriteEndObject();
             }