diff --git a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
index b613cc8037..aa977db141 100644
--- a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
+++ b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
@@ -1,6 +1,9 @@
 using Google.Protobuf.TestProtos.Proto2;
+using Proto2 = Google.Protobuf.TestProtos.Proto2;
 using NUnit.Framework;
 
+using static Google.Protobuf.TestProtos.Proto2.UnittestExtensions;
+
 namespace Google.Protobuf
 {
     /// <summary>
@@ -24,7 +27,7 @@ namespace Google.Protobuf
             Assert.IsNull(message.OptionalImportMessage);
             Assert.AreEqual(0, message.OptionalInt32);
             Assert.AreEqual(0L, message.OptionalInt64);
-            Assert.AreEqual(TestAllTypes.Types.NestedEnum.Foo, message.OptionalNestedEnum);
+            Assert.AreEqual(Proto2.TestAllTypes.Types.NestedEnum.Foo, message.OptionalNestedEnum);
             Assert.IsNull(message.OptionalNestedMessage);
             Assert.IsNull(message.OptionalPublicImportMessage);
             Assert.AreEqual(0, message.OptionalSfixed32);
@@ -57,7 +60,7 @@ namespace Google.Protobuf
             Assert.AreEqual(0, message.RepeatedUint64.Count);
 
             // Oneof fields
-            Assert.AreEqual(TestAllTypes.OneofFieldOneofCase.None, message.OneofFieldCase);
+            Assert.AreEqual(Proto2.TestAllTypes.OneofFieldOneofCase.None, message.OneofFieldCase);
             Assert.AreEqual(0, message.OneofUint32);
             Assert.AreEqual("", message.OneofString);
             Assert.AreEqual(ByteString.Empty, message.OneofBytes);
@@ -74,7 +77,7 @@ namespace Google.Protobuf
             Assert.AreEqual(ImportEnum.ImportBar, message.DefaultImportEnum);
             Assert.AreEqual(41, message.DefaultInt32);
             Assert.AreEqual(42, message.DefaultInt64);
-            Assert.AreEqual(TestAllTypes.Types.NestedEnum.Bar, message.DefaultNestedEnum);
+            Assert.AreEqual(Proto2.TestAllTypes.Types.NestedEnum.Bar, message.DefaultNestedEnum);
             Assert.AreEqual(49, message.DefaultSfixed32);
             Assert.AreEqual(-50, message.DefaultSfixed64);
             Assert.AreEqual(-45, message.DefaultSint32);
@@ -106,6 +109,103 @@ namespace Google.Protobuf
             Assert.False(message.HasDefaultUint64);
         }
 
+        [Test]
+        public void DefaultExtensionValues()
+        {
+            var message = new TestAllExtensions();
+            Assert.AreEqual(false, message.GetExtension(OptionalBoolExtension));
+            Assert.AreEqual(ByteString.Empty, message.GetExtension(OptionalBytesExtension));
+            Assert.AreEqual(0.0, message.GetExtension(OptionalDoubleExtension));
+            Assert.AreEqual(0, message.GetExtension(OptionalFixed32Extension));
+            Assert.AreEqual(0L, message.GetExtension(OptionalFixed64Extension));
+            Assert.AreEqual(0.0f, message.GetExtension(OptionalFloatExtension));
+            Assert.AreEqual(ForeignEnum.ForeignFoo, message.GetExtension(OptionalForeignEnumExtension));
+            Assert.IsNull(message.GetExtension(OptionalForeignMessageExtension));
+            Assert.AreEqual(ImportEnum.ImportFoo, message.GetExtension(OptionalImportEnumExtension));
+            Assert.IsNull(message.GetExtension(OptionalImportMessageExtension));
+            Assert.AreEqual(0, message.GetExtension(OptionalInt32Extension));
+            Assert.AreEqual(0L, message.GetExtension(OptionalInt64Extension));
+            Assert.AreEqual(Proto2.TestAllTypes.Types.NestedEnum.Foo, message.GetExtension(OptionalNestedEnumExtension));
+            Assert.IsNull(message.GetExtension(OptionalNestedMessageExtension));
+            Assert.IsNull(message.GetExtension(OptionalPublicImportMessageExtension));
+            Assert.AreEqual(0, message.GetExtension(OptionalSfixed32Extension));
+            Assert.AreEqual(0L, message.GetExtension(OptionalSfixed64Extension));
+            Assert.AreEqual(0, message.GetExtension(OptionalSint32Extension));
+            Assert.AreEqual(0L, message.GetExtension(OptionalSint64Extension));
+            Assert.AreEqual("", message.GetExtension(OptionalStringExtension));
+            Assert.AreEqual(0U, message.GetExtension(OptionalUint32Extension));
+            Assert.AreEqual(0UL, message.GetExtension(OptionalUint64Extension));
+
+            // Repeated fields
+            Assert.IsNull(message.GetExtension(RepeatedBoolExtension));
+            Assert.IsNull(message.GetExtension(RepeatedBytesExtension));
+            Assert.IsNull(message.GetExtension(RepeatedDoubleExtension));
+            Assert.IsNull(message.GetExtension(RepeatedFixed32Extension));
+            Assert.IsNull(message.GetExtension(RepeatedFixed64Extension));
+            Assert.IsNull(message.GetExtension(RepeatedFloatExtension));
+            Assert.IsNull(message.GetExtension(RepeatedForeignEnumExtension));
+            Assert.IsNull(message.GetExtension(RepeatedForeignMessageExtension));
+            Assert.IsNull(message.GetExtension(RepeatedImportEnumExtension));
+            Assert.IsNull(message.GetExtension(RepeatedImportMessageExtension));
+            Assert.IsNull(message.GetExtension(RepeatedNestedEnumExtension));
+            Assert.IsNull(message.GetExtension(RepeatedNestedMessageExtension));
+            Assert.IsNull(message.GetExtension(RepeatedSfixed32Extension));
+            Assert.IsNull(message.GetExtension(RepeatedSfixed64Extension));
+            Assert.IsNull(message.GetExtension(RepeatedSint32Extension));
+            Assert.IsNull(message.GetExtension(RepeatedSint64Extension));
+            Assert.IsNull(message.GetExtension(RepeatedStringExtension));
+            Assert.IsNull(message.GetExtension(RepeatedUint32Extension));
+            Assert.IsNull(message.GetExtension(RepeatedUint64Extension));
+
+            // Oneof fields
+            Assert.AreEqual(0, message.GetExtension(OneofUint32Extension));
+            Assert.AreEqual("", message.GetExtension(OneofStringExtension));
+            Assert.AreEqual(ByteString.Empty, message.GetExtension(OneofBytesExtension));
+            Assert.IsNull(message.GetExtension(OneofNestedMessageExtension));
+
+            Assert.AreEqual(true, message.GetExtension(DefaultBoolExtension));
+            Assert.AreEqual(ByteString.CopyFromUtf8("world"), message.GetExtension(DefaultBytesExtension));
+            Assert.AreEqual("123", message.GetExtension(DefaultCordExtension));
+            Assert.AreEqual(52e3, message.GetExtension(DefaultDoubleExtension));
+            Assert.AreEqual(47, message.GetExtension(DefaultFixed32Extension));
+            Assert.AreEqual(48, message.GetExtension(DefaultFixed64Extension));
+            Assert.AreEqual(51.5, message.GetExtension(DefaultFloatExtension));
+            Assert.AreEqual(ForeignEnum.ForeignBar, message.GetExtension(DefaultForeignEnumExtension));
+            Assert.AreEqual(ImportEnum.ImportBar, message.GetExtension(DefaultImportEnumExtension));
+            Assert.AreEqual(41, message.GetExtension(DefaultInt32Extension));
+            Assert.AreEqual(42, message.GetExtension(DefaultInt64Extension));
+            Assert.AreEqual(Proto2.TestAllTypes.Types.NestedEnum.Bar, message.GetExtension(DefaultNestedEnumExtension));
+            Assert.AreEqual(49, message.GetExtension(DefaultSfixed32Extension));
+            Assert.AreEqual(-50, message.GetExtension(DefaultSfixed64Extension));
+            Assert.AreEqual(-45, message.GetExtension(DefaultSint32Extension));
+            Assert.AreEqual(46, message.GetExtension(DefaultSint64Extension));
+            Assert.AreEqual("hello", message.GetExtension(DefaultStringExtension));
+            Assert.AreEqual("abc", message.GetExtension(DefaultStringPieceExtension));
+            Assert.AreEqual(43, message.GetExtension(DefaultUint32Extension));
+            Assert.AreEqual(44, message.GetExtension(DefaultUint64Extension));
+
+            Assert.False(message.HasExtension(DefaultBoolExtension));
+            Assert.False(message.HasExtension(DefaultBytesExtension));
+            Assert.False(message.HasExtension(DefaultCordExtension));
+            Assert.False(message.HasExtension(DefaultDoubleExtension));
+            Assert.False(message.HasExtension(DefaultFixed32Extension));
+            Assert.False(message.HasExtension(DefaultFixed64Extension));
+            Assert.False(message.HasExtension(DefaultFloatExtension));
+            Assert.False(message.HasExtension(DefaultForeignEnumExtension));
+            Assert.False(message.HasExtension(DefaultImportEnumExtension));
+            Assert.False(message.HasExtension(DefaultInt32Extension));
+            Assert.False(message.HasExtension(DefaultInt64Extension));
+            Assert.False(message.HasExtension(DefaultNestedEnumExtension));
+            Assert.False(message.HasExtension(DefaultSfixed32Extension));
+            Assert.False(message.HasExtension(DefaultSfixed64Extension));
+            Assert.False(message.HasExtension(DefaultSint32Extension));
+            Assert.False(message.HasExtension(DefaultSint64Extension));
+            Assert.False(message.HasExtension(DefaultStringExtension));
+            Assert.False(message.HasExtension(DefaultStringPieceExtension));
+            Assert.False(message.HasExtension(DefaultUint32Extension));
+            Assert.False(message.HasExtension(DefaultUint64Extension));
+        }
+
         [Test]
         public void FieldPresence()
         {
@@ -233,7 +333,7 @@ namespace Google.Protobuf
             };
 
             byte[] bytes = message.ToByteArray();
-            TestAllTypes parsed = TestAllTypes.Parser.ParseFrom(bytes);
+            TestAllTypes parsed = Proto2.TestAllTypes.Parser.ParseFrom(bytes);
             Assert.AreEqual(message, parsed);
         }
 
diff --git a/csharp/src/Google.Protobuf/FieldCodec.cs b/csharp/src/Google.Protobuf/FieldCodec.cs
index c2c4b57c39..4beb998f6b 100644
--- a/csharp/src/Google.Protobuf/FieldCodec.cs
+++ b/csharp/src/Google.Protobuf/FieldCodec.cs
@@ -218,7 +218,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<string> ForString(uint tag, string defaultValue)
         {
-            return new FieldCodec<string>(input => input.ReadString(), (output, value) => output.WriteString(value), CodedOutputStream.ComputeStringSize, tag);
+            return new FieldCodec<string>(input => input.ReadString(), (output, value) => output.WriteString(value), CodedOutputStream.ComputeStringSize, tag, defaultValue);
         }
 
         /// <summary>
@@ -229,7 +229,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<ByteString> ForBytes(uint tag, ByteString defaultValue)
         {
-            return new FieldCodec<ByteString>(input => input.ReadBytes(), (output, value) => output.WriteBytes(value), CodedOutputStream.ComputeBytesSize, tag);
+            return new FieldCodec<ByteString>(input => input.ReadBytes(), (output, value) => output.WriteBytes(value), CodedOutputStream.ComputeBytesSize, tag, defaultValue);
         }
 
         /// <summary>
@@ -240,7 +240,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<bool> ForBool(uint tag, bool defaultValue)
         {
-            return new FieldCodec<bool>(input => input.ReadBool(), (output, value) => output.WriteBool(value), CodedOutputStream.BoolSize, tag);
+            return new FieldCodec<bool>(input => input.ReadBool(), (output, value) => output.WriteBool(value), CodedOutputStream.BoolSize, tag, defaultValue);
         }
 
         /// <summary>
@@ -251,7 +251,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<int> ForInt32(uint tag, int defaultValue)
         {
-            return new FieldCodec<int>(input => input.ReadInt32(), (output, value) => output.WriteInt32(value), CodedOutputStream.ComputeInt32Size, tag);
+            return new FieldCodec<int>(input => input.ReadInt32(), (output, value) => output.WriteInt32(value), CodedOutputStream.ComputeInt32Size, tag, defaultValue);
         }
 
         /// <summary>
@@ -262,7 +262,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<int> ForSInt32(uint tag, int defaultValue)
         {
-            return new FieldCodec<int>(input => input.ReadSInt32(), (output, value) => output.WriteSInt32(value), CodedOutputStream.ComputeSInt32Size, tag);
+            return new FieldCodec<int>(input => input.ReadSInt32(), (output, value) => output.WriteSInt32(value), CodedOutputStream.ComputeSInt32Size, tag, defaultValue);
         }
 
         /// <summary>
@@ -273,7 +273,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<uint> ForFixed32(uint tag, uint defaultValue)
         {
-            return new FieldCodec<uint>(input => input.ReadFixed32(), (output, value) => output.WriteFixed32(value), 4, tag);
+            return new FieldCodec<uint>(input => input.ReadFixed32(), (output, value) => output.WriteFixed32(value), 4, tag, defaultValue);
         }
 
         /// <summary>
@@ -284,7 +284,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<int> ForSFixed32(uint tag, int defaultValue)
         {
-            return new FieldCodec<int>(input => input.ReadSFixed32(), (output, value) => output.WriteSFixed32(value), 4, tag);
+            return new FieldCodec<int>(input => input.ReadSFixed32(), (output, value) => output.WriteSFixed32(value), 4, tag, defaultValue);
         }
 
         /// <summary>
@@ -295,7 +295,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<uint> ForUInt32(uint tag, uint defaultValue)
         {
-            return new FieldCodec<uint>(input => input.ReadUInt32(), (output, value) => output.WriteUInt32(value), CodedOutputStream.ComputeUInt32Size, tag);
+            return new FieldCodec<uint>(input => input.ReadUInt32(), (output, value) => output.WriteUInt32(value), CodedOutputStream.ComputeUInt32Size, tag, defaultValue);
         }
 
         /// <summary>
@@ -306,7 +306,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<long> ForInt64(uint tag, long defaultValue)
         {
-            return new FieldCodec<long>(input => input.ReadInt64(), (output, value) => output.WriteInt64(value), CodedOutputStream.ComputeInt64Size, tag);
+            return new FieldCodec<long>(input => input.ReadInt64(), (output, value) => output.WriteInt64(value), CodedOutputStream.ComputeInt64Size, tag, defaultValue);
         }
 
         /// <summary>
@@ -317,7 +317,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<long> ForSInt64(uint tag, long defaultValue)
         {
-            return new FieldCodec<long>(input => input.ReadSInt64(), (output, value) => output.WriteSInt64(value), CodedOutputStream.ComputeSInt64Size, tag);
+            return new FieldCodec<long>(input => input.ReadSInt64(), (output, value) => output.WriteSInt64(value), CodedOutputStream.ComputeSInt64Size, tag, defaultValue);
         }
 
         /// <summary>
@@ -328,7 +328,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<ulong> ForFixed64(uint tag, ulong defaultValue)
         {
-            return new FieldCodec<ulong>(input => input.ReadFixed64(), (output, value) => output.WriteFixed64(value), 8, tag);
+            return new FieldCodec<ulong>(input => input.ReadFixed64(), (output, value) => output.WriteFixed64(value), 8, tag, defaultValue);
         }
 
         /// <summary>
@@ -339,7 +339,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<long> ForSFixed64(uint tag, long defaultValue)
         {
-            return new FieldCodec<long>(input => input.ReadSFixed64(), (output, value) => output.WriteSFixed64(value), 8, tag);
+            return new FieldCodec<long>(input => input.ReadSFixed64(), (output, value) => output.WriteSFixed64(value), 8, tag, defaultValue);
         }
 
         /// <summary>
@@ -350,7 +350,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<ulong> ForUInt64(uint tag, ulong defaultValue)
         {
-            return new FieldCodec<ulong>(input => input.ReadUInt64(), (output, value) => output.WriteUInt64(value), CodedOutputStream.ComputeUInt64Size, tag);
+            return new FieldCodec<ulong>(input => input.ReadUInt64(), (output, value) => output.WriteUInt64(value), CodedOutputStream.ComputeUInt64Size, tag, defaultValue);
         }
 
         /// <summary>
@@ -361,7 +361,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<float> ForFloat(uint tag, float defaultValue)
         {
-            return new FieldCodec<float>(input => input.ReadFloat(), (output, value) => output.WriteFloat(value), CodedOutputStream.FloatSize, tag);
+            return new FieldCodec<float>(input => input.ReadFloat(), (output, value) => output.WriteFloat(value), CodedOutputStream.FloatSize, tag, defaultValue);
         }
 
         /// <summary>
@@ -372,7 +372,7 @@ namespace Google.Protobuf
         /// <returns>A codec for the given tag.</returns>
         public static FieldCodec<double> ForDouble(uint tag, double defaultValue)
         {
-            return new FieldCodec<double>(input => input.ReadDouble(), (output, value) => output.WriteDouble(value), CodedOutputStream.DoubleSize, tag);
+            return new FieldCodec<double>(input => input.ReadDouble(), (output, value) => output.WriteDouble(value), CodedOutputStream.DoubleSize, tag, defaultValue);
         }
 
         // Enums are tricky. We can probably use expression trees to build these delegates automatically,
@@ -391,7 +391,7 @@ namespace Google.Protobuf
             return new FieldCodec<T>(input => fromInt32(
                 input.ReadEnum()),
                 (output, value) => output.WriteEnum(toInt32(value)),
-                value => CodedOutputStream.ComputeEnumSize(toInt32(value)), tag);
+                value => CodedOutputStream.ComputeEnumSize(toInt32(value)), tag, defaultValue);
         }
 
         /// <summary>
@@ -689,7 +689,8 @@ namespace Google.Protobuf
                 Func<CodedInputStream, T> reader,
                 Action<CodedOutputStream, T> writer,
                 int fixedSize,
-                uint tag) : this(reader, writer, _ => fixedSize, tag)
+                uint tag,
+                T defaultValue) : this(reader, writer, _ => fixedSize, tag, defaultValue)
         {
             FixedSize = fixedSize;
         }
@@ -699,7 +700,7 @@ namespace Google.Protobuf
             Action<CodedOutputStream, T> writer,
             Func<T, int> sizeCalculator,
             uint tag,
-            uint endTag = 0) : this(reader, writer, (CodedInputStream i, ref T v) => v = reader(i), (ref T v, T v2) => { v = v2; return true; }, sizeCalculator, tag, endTag, DefaultDefault)
+            T defaultValue) : this(reader, writer, (CodedInputStream i, ref T v) => v = reader(i), (ref T v, T v2) => { v = v2; return true; }, sizeCalculator, tag, 0, defaultValue)
         {
         }