diff --git a/csharp/src/Google.Protobuf.Test/UnknownFieldSetTest.cs b/csharp/src/Google.Protobuf.Test/UnknownFieldSetTest.cs
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+++ b/csharp/src/Google.Protobuf.Test/UnknownFieldSetTest.cs
@@ -0,0 +1,176 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using System;
+using System.IO;
+using Google.Protobuf.TestProtos;
+using NUnit.Framework;
+
+namespace Google.Protobuf
+{
+    public class UnknownFieldSetTest
+    {
+        [Test]
+        public void EmptyUnknownFieldSet()
+        {
+            UnknownFieldSet unknownFields = new UnknownFieldSet();
+            Assert.AreEqual(0, unknownFields.CalculateSize());
+        }
+
+        [Test]
+        public void MergeUnknownFieldSet()
+        {
+            UnknownFieldSet unknownFields = new UnknownFieldSet();
+            UnknownField field = new UnknownField();
+            field.AddFixed32(123);
+            unknownFields.AddOrReplaceField(1, field);
+            UnknownFieldSet otherUnknownFields = new UnknownFieldSet();
+            Assert.IsFalse(otherUnknownFields.HasField(1));
+            UnknownFieldSet.MergeFrom(otherUnknownFields, unknownFields);
+            Assert.IsTrue(otherUnknownFields.HasField(1));
+        }
+
+        [Test]
+        public void TestMergeCodedInput()
+        {
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var emptyMessage = new TestEmptyMessage();
+            emptyMessage.MergeFrom(message.ToByteArray());
+            Assert.AreEqual(message.CalculateSize(), emptyMessage.CalculateSize());
+            Assert.AreEqual(message.ToByteArray(), emptyMessage.ToByteArray());
+
+            var newMessage = new TestAllTypes();
+            newMessage.MergeFrom(emptyMessage.ToByteArray());
+            Assert.AreEqual(message, newMessage);
+            Assert.AreEqual(message.CalculateSize(), newMessage.CalculateSize());
+        }
+
+        [Test]
+        public void TestMergeMessage()
+        {
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var emptyMessage = new TestEmptyMessage();
+            var otherEmptyMessage = new TestEmptyMessage();
+            emptyMessage.MergeFrom(message.ToByteArray());
+            otherEmptyMessage.MergeFrom(emptyMessage);
+
+            Assert.AreEqual(message.CalculateSize(), otherEmptyMessage.CalculateSize());
+            Assert.AreEqual(message.ToByteArray(), otherEmptyMessage.ToByteArray());
+        }
+
+        [Test]
+        public void TestEquals()
+        {
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var emptyMessage = new TestEmptyMessage();
+            var otherEmptyMessage = new TestEmptyMessage();
+            Assert.AreEqual(emptyMessage, otherEmptyMessage);
+            emptyMessage.MergeFrom(message.ToByteArray());
+            Assert.AreNotEqual(emptyMessage.CalculateSize(),
+                               otherEmptyMessage.CalculateSize());
+            Assert.AreNotEqual(emptyMessage, otherEmptyMessage);
+        }
+
+        [Test]
+        public void TestHashCode()
+        {
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var emptyMessage = new TestEmptyMessage();
+            int hashCode = emptyMessage.GetHashCode();
+            emptyMessage.MergeFrom(message.ToByteArray());
+            Assert.AreNotEqual(hashCode, emptyMessage.GetHashCode());
+        }
+
+        [Test]
+        public void TestClone()
+        {
+            var emptyMessage = new TestEmptyMessage();
+            var otherEmptyMessage = new TestEmptyMessage();
+            otherEmptyMessage = emptyMessage.Clone();
+            Assert.AreEqual(emptyMessage.CalculateSize(), otherEmptyMessage.CalculateSize());
+            Assert.AreEqual(emptyMessage.ToByteArray(), otherEmptyMessage.ToByteArray());
+
+            var message = SampleMessages.CreateFullTestAllTypes();
+            emptyMessage.MergeFrom(message.ToByteArray());
+            otherEmptyMessage = emptyMessage.Clone();
+            Assert.AreEqual(message.CalculateSize(), otherEmptyMessage.CalculateSize());
+            Assert.AreEqual(message.ToByteArray(), otherEmptyMessage.ToByteArray());
+        }
+
+        [Test]
+        public void TestDiscardUnknownFields()
+        {
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var goldenEmptyMessage = new TestEmptyMessage();
+            byte[] data = message.ToByteArray();
+            int fullSize = message.CalculateSize();
+
+            Action<IMessage> assertEmpty = msg =>
+            {
+                Assert.AreEqual(0, msg.CalculateSize());
+                Assert.AreEqual(goldenEmptyMessage, msg);
+            };
+
+            Action<IMessage> assertFull = msg => Assert.AreEqual(fullSize, msg.CalculateSize());
+
+            // Test the behavior of the parsers with and without discarding, both generic and non-generic.
+            MessageParser<TestEmptyMessage> retainingParser1 = TestEmptyMessage.Parser;
+            MessageParser retainingParser2 = retainingParser1;
+            MessageParser<TestEmptyMessage> discardingParser1 = retainingParser1.WithDiscardUnknownFields(true);
+            MessageParser discardingParser2 = retainingParser2.WithDiscardUnknownFields(true);
+
+            // Test parse from byte[]
+            assertFull(retainingParser1.ParseFrom(data));
+            assertFull(retainingParser2.ParseFrom(data));
+            assertEmpty(discardingParser1.ParseFrom(data));
+            assertEmpty(discardingParser2.ParseFrom(data));
+
+            // Test parse from byte[] with offset
+            assertFull(retainingParser1.ParseFrom(data, 0, data.Length));
+            assertFull(retainingParser2.ParseFrom(data, 0, data.Length));
+            assertEmpty(discardingParser1.ParseFrom(data, 0, data.Length));
+            assertEmpty(discardingParser2.ParseFrom(data, 0, data.Length));
+
+            // Test parse from CodedInputStream
+            assertFull(retainingParser1.ParseFrom(new CodedInputStream(data)));
+            assertFull(retainingParser2.ParseFrom(new CodedInputStream(data)));
+            assertEmpty(discardingParser1.ParseFrom(new CodedInputStream(data)));
+            assertEmpty(discardingParser2.ParseFrom(new CodedInputStream(data)));
+
+            // Test parse from Stream
+            assertFull(retainingParser1.ParseFrom(new MemoryStream(data)));
+            assertFull(retainingParser2.ParseFrom(new MemoryStream(data)));
+            assertEmpty(discardingParser1.ParseFrom(new MemoryStream(data)));
+            assertEmpty(discardingParser2.ParseFrom(new MemoryStream(data)));
+        }
+    }
+}