Testing
diff --git a/src/ProtocolBuffersLite.Test/ExtendableMessageLiteTest.cs b/src/ProtocolBuffersLite.Test/ExtendableMessageLiteTest.cs
index 0b12d25..86b8f11 100644
--- a/src/ProtocolBuffersLite.Test/ExtendableMessageLiteTest.cs
+++ b/src/ProtocolBuffersLite.Test/ExtendableMessageLiteTest.cs
@@ -43,6 +43,52 @@
   [TestFixture]
   public class ExtendableMessageLiteTest {
 
+    [Test, Ignore("Not implemented, no assertion made"), ExpectedException(typeof(ArgumentException))]
+    public void ExtensionWriterInvalidExtension() {
+      TestPackedExtensionsLite.CreateBuilder()[UnitTestLiteProtoFile.OptionalForeignMessageExtensionLite.Descriptor] = 
+        ForeignMessageLite.DefaultInstance;
+    }
+
+    [Test]
+    public void ExtensionWriterTestMessages() {
+      TestAllExtensionsLite.Builder b = TestAllExtensionsLite.CreateBuilder().SetExtension(
+        UnitTestLiteProtoFile.OptionalForeignMessageExtensionLite, ForeignMessageLite.CreateBuilder().SetC(123).Build());
+      TestAllExtensionsLite copy, msg = b.Build();
+
+      ExtensionRegistry registry = ExtensionRegistry.CreateInstance();
+      UnitTestLiteProtoFile.RegisterAllExtensions(registry);
+
+      copy = TestAllExtensionsLite.ParseFrom(msg.ToByteArray(), registry);
+      Assert.AreEqual(msg.ToByteArray(), copy.ToByteArray());
+    }
+
+    [Test]
+    public void ExtensionWriterIsInitialized() {
+      Assert.IsTrue(ForeignMessageLite.DefaultInstance.IsInitialized);
+      Assert.IsTrue(TestPackedExtensionsLite.CreateBuilder().IsInitialized);
+      Assert.IsTrue(TestAllExtensionsLite.CreateBuilder().SetExtension(
+        UnitTestLiteProtoFile.OptionalForeignMessageExtensionLite, ForeignMessageLite.DefaultInstance)
+        .IsInitialized);
+    }
+
+    [Test]
+    public void ExtensionWriterTestSetExtensionLists() {
+      TestAllExtensionsLite msg, copy;
+      TestAllExtensionsLite.Builder builder = TestAllExtensionsLite.CreateBuilder()
+        .SetExtension(UnitTestLiteProtoFile.RepeatedBoolExtensionLite, new[] { true, false })
+        .SetExtension(UnitTestLiteProtoFile.RepeatedCordExtensionLite, new[] { "123", "456" })
+        .SetExtension(UnitTestLiteProtoFile.RepeatedForeignEnumExtensionLite, new[] { ForeignEnumLite.FOREIGN_LITE_BAZ, ForeignEnumLite.FOREIGN_LITE_FOO })
+        ;
+
+      msg = builder.Build();
+      ExtensionRegistry registry = ExtensionRegistry.CreateInstance();
+      UnitTestLiteProtoFile.RegisterAllExtensions(registry);
+
+      copy = TestAllExtensionsLite.ParseFrom(msg.ToByteArray(), registry);
+      Assert.AreEqual(msg.ToByteArray(), copy.ToByteArray());
+
+      Assert.AreEqual(ForeignEnumLite.FOREIGN_LITE_FOO, copy.GetExtension(UnitTestLiteProtoFile.RepeatedForeignEnumExtensionLite, 1));
+    }
 
     [Test]
     public void ExtensionWriterTest() {
@@ -184,121 +230,75 @@
       Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.RepeatedUint64ExtensionLite, 0));
     }
 
-    
-    
-    /*
-    
     [Test]
-    public void ExtensionWriterTest() {
-      TestAllExtensionsLite.Builder builder = TestAllExtensionsLite.CreateBuilder()
-          .SetExtension(UnitTestLiteProtoFile.DefaultBoolExtensionLite, true)
-          .SetExtension(UnitTestLiteProtoFile.DefaultBytesExtensionLite, ByteString.CopyFromUtf8("123"))
-          .SetExtension(UnitTestLiteProtoFile.DefaultCordExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.DefaultDoubleExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultFixed32ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.DefaultFixed64ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.DefaultFloatExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultForeignEnumExtensionLite, ForeignEnumLite.FOREIGN_LITE_FOO)
-          .SetExtension(UnitTestLiteProtoFile.DefaultImportEnumExtensionLite, ImportEnumLite.IMPORT_LITE_BAZ)
-          .SetExtension(UnitTestLiteProtoFile.DefaultInt32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultInt64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultNestedEnumExtensionLite, TestAllTypesLite.Types.NestedEnum.FOO)
-          .SetExtension(UnitTestLiteProtoFile.DefaultSfixed32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultSfixed64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultSint32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultSint64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultStringExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.DefaultStringPieceExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.DefaultUint32ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.DefaultUint64ExtensionLite, 123u)
-          //Optional
-          .SetExtension(UnitTestLiteProtoFile.OptionalBoolExtensionLite, true)
-          .SetExtension(UnitTestLiteProtoFile.OptionalBytesExtensionLite, ByteString.CopyFromUtf8("123"))
-          .SetExtension(UnitTestLiteProtoFile.OptionalCordExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.OptionalDoubleExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.OptionalFixed32ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.OptionalFixed64ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.OptionalFloatExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.OptionalForeignEnumExtensionLite, ForeignEnumLite.FOREIGN_LITE_FOO)
-          .SetExtension(UnitTestLiteProtoFile.OptionalImportEnumExtensionLite, ImportEnumLite.IMPORT_LITE_BAZ)
-          .SetExtension(UnitTestLiteProtoFile.OptionalInt32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.OptionalInt64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.OptionalNestedEnumExtensionLite, TestAllTypesLite.Types.NestedEnum.FOO)
-          .SetExtension(UnitTestLiteProtoFile.OptionalSfixed32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.OptionalSfixed64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.OptionalSint32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.OptionalSint64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.OptionalStringExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.OptionalStringPieceExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.OptionalUint32ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.OptionalUint64ExtensionLite, 123u)
-          //Repeated
-          .AddExtension(UnitTestLiteProtoFile.RepeatedBoolExtensionLite, true)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedBytesExtensionLite, ByteString.CopyFromUtf8("123"))
-          .AddExtension(UnitTestLiteProtoFile.RepeatedCordExtensionLite, "123")
-          .AddExtension(UnitTestLiteProtoFile.RepeatedDoubleExtensionLite, 123)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedFixed32ExtensionLite, 123u)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedFixed64ExtensionLite, 123u)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedFloatExtensionLite, 123)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedForeignEnumExtensionLite, ForeignEnumLite.FOREIGN_LITE_FOO)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedImportEnumExtensionLite, ImportEnumLite.IMPORT_LITE_BAZ)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedInt32ExtensionLite, 123)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedInt64ExtensionLite, 123)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedNestedEnumExtensionLite, TestAllTypesLite.Types.NestedEnum.FOO)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedSfixed32ExtensionLite, 123)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedSfixed64ExtensionLite, 123)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedSint32ExtensionLite, 123)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedSint64ExtensionLite, 123)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedStringExtensionLite, "123")
-          .AddExtension(UnitTestLiteProtoFile.RepeatedStringPieceExtensionLite, "123")
-          .AddExtension(UnitTestLiteProtoFile.RepeatedUint32ExtensionLite, 123u)
-          .AddExtension(UnitTestLiteProtoFile.RepeatedUint64ExtensionLite, 123u)
-        ;
-      builder = TestAllExtensionsLite.CreateBuilder()
-          .SetExtension(UnitTestLiteProtoFile.DefaultBoolExtensionLite, true)
-          .SetExtension(UnitTestLiteProtoFile.DefaultBytesExtensionLite, ByteString.CopyFromUtf8("123"))
-          .SetExtension(UnitTestLiteProtoFile.DefaultCordExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.DefaultDoubleExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultFixed32ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.DefaultFixed64ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.DefaultFloatExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultForeignEnumExtensionLite, ForeignEnumLite.FOREIGN_LITE_FOO)
-          .SetExtension(UnitTestLiteProtoFile.DefaultImportEnumExtensionLite, ImportEnumLite.IMPORT_LITE_BAZ)
-          .SetExtension(UnitTestLiteProtoFile.DefaultInt32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultInt64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultNestedEnumExtensionLite, TestAllTypesLite.Types.NestedEnum.FOO)
-          .SetExtension(UnitTestLiteProtoFile.DefaultSfixed32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultSfixed64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultSint32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultSint64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.DefaultStringExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.DefaultStringPieceExtensionLite, "123")
-          .SetExtension(UnitTestLiteProtoFile.DefaultUint32ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.DefaultUint64ExtensionLite, 123u)
-        ;
+    public void ExtensionWriterTestPacked() {
 
-      TestAllExtensionsLite msg = builder.Build();
-      byte[] data = msg.ToByteArray();
-      TestAllExtensionsLite.Builder copyBuilder = TestAllExtensionsLite.CreateBuilder().MergeFrom(data);
-      TestAllExtensionsLite copy = copyBuilder.Build();
+      TestPackedExtensionsLite.Builder builder = TestPackedExtensionsLite.CreateBuilder()
+        .AddExtension(UnitTestLiteProtoFile.PackedBoolExtensionLite, true)
+        .AddExtension(UnitTestLiteProtoFile.PackedDoubleExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedFixed32ExtensionLite, 123u)
+        .AddExtension(UnitTestLiteProtoFile.PackedFixed64ExtensionLite, 123u)
+        .AddExtension(UnitTestLiteProtoFile.PackedFloatExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedInt32ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedInt64ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedSfixed32ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedSfixed64ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedSint32ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedSint64ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedUint32ExtensionLite, 123u)
+        .AddExtension(UnitTestLiteProtoFile.PackedUint64ExtensionLite, 123u)
+        .AddExtension(UnitTestLiteProtoFile.PackedBoolExtensionLite, true)
+        .AddExtension(UnitTestLiteProtoFile.PackedDoubleExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedFixed32ExtensionLite, 123u)
+        .AddExtension(UnitTestLiteProtoFile.PackedFixed64ExtensionLite, 123u)
+        .AddExtension(UnitTestLiteProtoFile.PackedFloatExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedInt32ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedInt64ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedSfixed32ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedSfixed64ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedSint32ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedSint64ExtensionLite, 123)
+        .AddExtension(UnitTestLiteProtoFile.PackedUint32ExtensionLite, 123u)
+        .AddExtension(UnitTestLiteProtoFile.PackedUint64ExtensionLite, 123u);
+
+      TestPackedExtensionsLite msg = builder.Build();
+
+      ExtensionRegistry registry = ExtensionRegistry.CreateInstance();
+      UnitTestLiteProtoFile.RegisterAllExtensions(registry);
+
+      TestPackedExtensionsLite.Builder copyBuilder = TestPackedExtensionsLite.CreateBuilder().MergeFrom(msg.ToByteArray(), registry);
+      TestPackedExtensionsLite copy = copyBuilder.Build();
 
       Assert.AreEqual(msg.ToByteArray(), copy.ToByteArray());
+
+      Assert.AreEqual(true, copy.GetExtension(UnitTestLiteProtoFile.PackedBoolExtensionLite, 0));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedDoubleExtensionLite, 0));
+      Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.PackedFixed32ExtensionLite, 0));
+      Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.PackedFixed64ExtensionLite, 0));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedFloatExtensionLite, 0));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedInt32ExtensionLite, 0));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedInt64ExtensionLite, 0));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedSfixed32ExtensionLite, 0));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedSfixed64ExtensionLite, 0));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedSint32ExtensionLite, 0));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedSint64ExtensionLite, 0));
+      Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.PackedUint32ExtensionLite, 0));
+      Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.PackedUint64ExtensionLite, 0));
+
+      Assert.AreEqual(true, copy.GetExtension(UnitTestLiteProtoFile.PackedBoolExtensionLite, 1));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedDoubleExtensionLite, 1));
+      Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.PackedFixed32ExtensionLite, 1));
+      Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.PackedFixed64ExtensionLite, 1));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedFloatExtensionLite, 1));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedInt32ExtensionLite, 1));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedInt64ExtensionLite, 1));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedSfixed32ExtensionLite, 1));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedSfixed64ExtensionLite, 1));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedSint32ExtensionLite, 1));
+      Assert.AreEqual(123, copy.GetExtension(UnitTestLiteProtoFile.PackedSint64ExtensionLite, 1));
+      Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.PackedUint32ExtensionLite, 1));
+      Assert.AreEqual(123u, copy.GetExtension(UnitTestLiteProtoFile.PackedUint64ExtensionLite, 1));
+
     }
-     * 
-          //Packed
-          .SetExtension(UnitTestLiteProtoFile.PackedBoolExtensionLite, true)
-          .SetExtension(UnitTestLiteProtoFile.PackedDoubleExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.PackedFixed32ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.PackedFixed64ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.PackedFloatExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.PackedInt32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.PackedInt64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.PackedSfixed32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.PackedSfixed64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.PackedSint32ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.PackedSint64ExtensionLite, 123)
-          .SetExtension(UnitTestLiteProtoFile.PackedUint32ExtensionLite, 123u)
-          .SetExtension(UnitTestLiteProtoFile.PackedUint64ExtensionLite, 123u)
-     */
   }
 }