diff --git a/csharp/src/Google.Protobuf.Test/Proto3OptionalTest.cs b/csharp/src/Google.Protobuf.Test/Proto3OptionalTest.cs
new file mode 100644
index 000000000..20c1846da
--- /dev/null
+++ b/csharp/src/Google.Protobuf.Test/Proto3OptionalTest.cs
@@ -0,0 +1,143 @@
+#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 NUnit.Framework;
+using ProtobufUnittest;
+using System;
+using System.IO;
+
+namespace Google.Protobuf.Test
+{
+    class Proto3OptionalTest
+    {
+        [Test]
+        public void OptionalInt32FieldLifecycle()
+        {
+            var message = new TestProto3Optional();
+            Assert.IsFalse(message.HasOptionalInt32);
+            Assert.AreEqual(0, message.OptionalInt32);
+
+            message.OptionalInt32 = 5;
+            Assert.IsTrue(message.HasOptionalInt32);
+            Assert.AreEqual(5, message.OptionalInt32);
+
+            message.OptionalInt32 = 0;
+            Assert.IsTrue(message.HasOptionalInt32);
+            Assert.AreEqual(0, message.OptionalInt32);
+
+            message.ClearOptionalInt32();
+            Assert.IsFalse(message.HasOptionalInt32);
+            Assert.AreEqual(0, message.OptionalInt32);
+        }
+
+        [Test]
+        public void OptionalStringFieldLifecycle()
+        {
+            var message = new TestProto3Optional();
+            Assert.IsFalse(message.HasOptionalString);
+            Assert.AreEqual("", message.OptionalString);
+
+            message.OptionalString = "x";
+            Assert.IsTrue(message.HasOptionalString);
+            Assert.AreEqual("x", message.OptionalString);
+
+            message.OptionalString = "";
+            Assert.IsTrue(message.HasOptionalString);
+            Assert.AreEqual("", message.OptionalString);
+
+            message.ClearOptionalString();
+            Assert.IsFalse(message.HasOptionalString);
+            Assert.AreEqual("", message.OptionalString);
+
+            Assert.Throws<ArgumentNullException>(() => message.OptionalString = null);
+        }
+
+        [Test]
+        public void Clone()
+        {
+            var original = new TestProto3Optional { OptionalInt64 = 0L };
+
+            var clone = original.Clone();
+            Assert.False(clone.HasOptionalInt32);
+            Assert.AreEqual(0, clone.OptionalInt32);
+            Assert.True(clone.HasOptionalInt64);
+            Assert.AreEqual(0L, clone.OptionalInt64);
+        }
+
+        [Test]
+        public void Serialization_NotSet()
+        {
+            var stream = new MemoryStream();
+            var message = new TestProto3Optional();
+            message.WriteTo(stream);
+            Assert.AreEqual(0, stream.Length);
+        }
+
+        [Test]
+        public void Serialization_SetToDefault()
+        {
+            var stream = new MemoryStream();
+            var message = new TestProto3Optional { OptionalInt32 = 0 };
+            message.WriteTo(stream);
+            Assert.AreEqual(2, stream.Length); // Tag and value
+        }
+
+        [Test]
+        public void Serialization_Roundtrip()
+        {
+            var original = new TestProto3Optional { OptionalInt64 = 0L, OptionalFixed32 = 5U };
+            var stream = new MemoryStream();
+            original.WriteTo(stream);
+            stream.Position = 0;
+            var deserialized = TestProto3Optional.Parser.ParseFrom(stream);
+
+            Assert.AreEqual(0, deserialized.OptionalInt32);
+            Assert.IsFalse(deserialized.HasOptionalInt32);
+
+            Assert.AreEqual(0L, deserialized.OptionalInt64);
+            Assert.IsTrue(deserialized.HasOptionalInt64);
+
+            Assert.AreEqual(5U, deserialized.OptionalFixed32);
+            Assert.IsTrue(deserialized.HasOptionalFixed32);
+        }
+
+        [Test]
+        public void Equality_IgnoresPresence()
+        {
+            var message1 = new TestProto3Optional { OptionalInt32 = 0 };
+            var message2 = new TestProto3Optional();
+
+            Assert.IsTrue(message1.Equals(message2));
+            message1.ClearOptionalInt32();
+        }
+    }
+}