Serialize a guid without an intermediary array allocation.

src/Compilers/Core/CodeAnalysisTest/ObjectSerializationTests.cs

@@ -1032,6 +1032,21 @@ private void TestRoundTripChar(Char ch)
             TestRoundTrip(ch, (w, v) => w.WriteChar(v), r => r.ReadChar());
         }
 
+        [Fact]
+        public void TestRoundTripGuid()
+        {
+            TestRoundTripGuid(Guid.Empty);
+            for (int i = 0; i < 10; i++)
+            {
+                TestRoundTripGuid(Guid.NewGuid());
+            }
+        }
+
+        private void TestRoundTripGuid(Guid guid)
+        {
+            TestRoundTrip(guid, (w, v) => w.WriteGuid(v), r => r.ReadGuid());
+        }
+
         [Fact]
         public void TestRoundTripStringCharacters()
         {

src/Compilers/Core/Portable/Serialization/ObjectReader.cs

@@ -124,6 +124,19 @@ public void Dispose()
         public ushort ReadUInt16() => _reader.ReadUInt16();
         public string ReadString() => ReadStringValue();
 
+        public unsafe Guid ReadGuid()
+        {
+            Debug.Assert(sizeof(Guid) == 16);
+            Debug.Assert(sizeof(long) == 8);
+
+            Guid guid;
+            long* pGuid = (long*)&guid;
+            pGuid[0] = ReadInt64();
+            pGuid[1] = ReadInt64();
+
+            return guid;
+        }
+
         public object ReadValue()
         {
             var oldDepth = _recursionDepth;

src/Compilers/Core/Portable/Serialization/ObjectWriter.cs

@@ -119,6 +119,15 @@ public void Dispose()
         public void WriteUInt16(ushort value) => _writer.Write(value);
         public void WriteString(string value) => WriteStringValue(value);
 
+        public unsafe void WriteGuid(Guid guid)
+        {
+            Debug.Assert(sizeof(Guid) == 16);
+            Debug.Assert(sizeof(long) == 8);
+            long* pGuid = (long*)&guid;
+            WriteInt64(pGuid[0]);
+            WriteInt64(pGuid[1]);
+        }
+
         public void WriteValue(object value)
         {
             Debug.Assert(value == null || !value.GetType().GetTypeInfo().IsEnum, "Enum should not be written with WriteValue.  Write them as ints instead.");

src/Workspaces/Core/Portable/Execution/AbstractReferenceSerializationService.cs

@@ -234,7 +234,7 @@ private void WriteAnalyzerFileReferenceMvid(AnalyzerFileReference reference, Obj
                     var mvidHandle = metadataReader.GetModuleDefinition().Mvid;
                     var guid = metadataReader.GetGuid(mvidHandle);
 
-                    writer.WriteValue(guid.ToByteArray());
+                    writer.WriteGuid(guid);
                 }
             }
             catch
@@ -312,7 +312,7 @@ private void WriteMvidTo(ModuleMetadata metadata, ObjectWriter writer, Cancellat
             var mvidHandle = metadataReader.GetModuleDefinition().Mvid;
             var guid = metadataReader.GetGuid(mvidHandle);
 
-            writer.WriteValue(guid.ToByteArray());
+            writer.WriteGuid(guid);
         }
 
         private void WritePortableExecutableReferenceTo(

src/Workspaces/Core/Portable/Workspace/Solution/DocumentId.cs

@@ -102,7 +102,7 @@ void IObjectWritable.WriteTo(ObjectWriter writer)
         {
             ProjectId.WriteTo(writer);
 
-            writer.WriteValue(Id.ToByteArray());
+            writer.WriteGuid(Id);
             writer.WriteString(DebugName);
         }
 
@@ -110,7 +110,7 @@ internal static DocumentId ReadFrom(ObjectReader reader)
         {
             var projectId = ProjectId.ReadFrom(reader);
 
-            var guid = new Guid((byte[])reader.ReadValue());
+            var guid = reader.ReadGuid();
             var debugName = reader.ReadString();
 
             return CreateFromSerialized(projectId, guid, debugName);

src/Workspaces/Core/Portable/Workspace/Solution/ProjectId.cs

@@ -86,13 +86,13 @@ public override int GetHashCode()
 
         void IObjectWritable.WriteTo(ObjectWriter writer)
         {
-            writer.WriteValue(Id.ToByteArray());
+            writer.WriteGuid(Id);
             writer.WriteString(DebugName);
         }
 
         internal static ProjectId ReadFrom(ObjectReader reader)
         {
-            var guid = new Guid((byte[])reader.ReadValue());
+            var guid = reader.ReadGuid();
             var debugName = reader.ReadString();
 
             return CreateFromSerialized(guid, debugName);

src/Workspaces/Core/Portable/Workspace/Solution/SolutionId.cs

@@ -83,13 +83,13 @@ public override int GetHashCode()
 
         void IObjectWritable.WriteTo(ObjectWriter writer)
         {
-            writer.WriteValue(Id.ToByteArray());
+            writer.WriteGuid(Id);
             writer.WriteString(DebugName);
         }
 
         internal static SolutionId ReadFrom(ObjectReader reader)
         {
-            var guid = new Guid((byte[])reader.ReadValue());
+            var guid = reader.ReadGuid();
             var debugName = reader.ReadString();
 
             return CreateFromSerialized(guid, debugName);