Serialize without unsafe code.

src/Compilers/Core/CodeAnalysisTest/ObjectSerializationTests.cs

@@ -1036,6 +1036,9 @@ private void TestRoundTripChar(Char ch)
         public void TestRoundTripGuid()
         {
             TestRoundTripGuid(Guid.Empty);
+            TestRoundTripGuid(new Guid(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1));
+            TestRoundTripGuid(new Guid(0b10000000000000000000000000000000, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1));
+            TestRoundTripGuid(new Guid(0b10000000000000000000000000000000, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0));
             for (int i = 0; i < 10; i++)
             {
                 TestRoundTripGuid(Guid.NewGuid());

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

@@ -126,15 +126,13 @@ public void Dispose()
 
         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();
+            var accessor = new ObjectWriter.GuidAccessor
+            {
+                Low64 = ReadInt64(),
+                High64 = ReadInt64()
+            };
 
-            return guid;
+            return accessor.Guid;
         }
 
         public object ReadValue()

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

@@ -119,13 +119,32 @@ public void Dispose()
         public void WriteUInt16(ushort value) => _writer.Write(value);
         public void WriteString(string value) => WriteStringValue(value);
 
-        public unsafe void WriteGuid(Guid guid)
+        /// <summary>
+        /// Used so we can easily grab the low/high 64bits of a guid for serialization.
+        /// </summary>
+        [StructLayout(LayoutKind.Explicit)]
+        internal struct GuidAccessor
+        {
+            unsafe static GuidAccessor()
+            {
+                Debug.Assert(sizeof(Guid) == 16);
+                Debug.Assert(sizeof(long) == 8);
+            }
+
+            [FieldOffset(0)]
+            public Guid Guid;
+
+            [FieldOffset(0)]
+            public long Low64;
+            [FieldOffset(8)]
+            public long High64;
+        }
+
+        public void WriteGuid(Guid guid)
         {
-            Debug.Assert(sizeof(Guid) == 16);
-            Debug.Assert(sizeof(long) == 8);
-            long* pGuid = (long*)&guid;
-            WriteInt64(pGuid[0]);
-            WriteInt64(pGuid[1]);
+            var accessor = new GuidAccessor { Guid = guid };
+            WriteInt64(accessor.Low64);
+            WriteInt64(accessor.High64);
         }
 
         public void WriteValue(object value)