diff --git a/csharp/src/Google.Protobuf.Test/ByteStringTest.cs b/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
new file mode 100644
index 0000000..685e130
--- /dev/null
+++ b/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
@@ -0,0 +1,171 @@
+#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.Text;
+using NUnit.Framework;
+
+namespace Google.Protobuf
+{
+    public class ByteStringTest
+    {
+        [Test]
+        public void Equality()
+        {
+            ByteString b1 = ByteString.CopyFrom(1, 2, 3);
+            ByteString b2 = ByteString.CopyFrom(1, 2, 3);
+            ByteString b3 = ByteString.CopyFrom(1, 2, 4);
+            ByteString b4 = ByteString.CopyFrom(1, 2, 3, 4);
+            EqualityTester.AssertEquality(b1, b1);
+            EqualityTester.AssertEquality(b1, b2);
+            EqualityTester.AssertInequality(b1, b3);
+            EqualityTester.AssertInequality(b1, b4);
+            EqualityTester.AssertInequality(b1, null);
+#pragma warning disable 1718 // Deliberately calling ==(b1, b1) and !=(b1, b1)
+            Assert.IsTrue(b1 == b1);
+            Assert.IsTrue(b1 == b2);
+            Assert.IsFalse(b1 == b3);
+            Assert.IsFalse(b1 == b4);
+            Assert.IsFalse(b1 == null);
+            Assert.IsTrue((ByteString) null == null);
+            Assert.IsFalse(b1 != b1);
+            Assert.IsFalse(b1 != b2);
+#pragma warning disable 1718
+            Assert.IsTrue(b1 != b3);
+            Assert.IsTrue(b1 != b4);
+            Assert.IsTrue(b1 != null);
+            Assert.IsFalse((ByteString) null != null);
+        }
+
+        [Test]
+        public void EmptyByteStringHasZeroSize()
+        {
+            Assert.AreEqual(0, ByteString.Empty.Length);
+        }
+
+        [Test]
+        public void CopyFromStringWithExplicitEncoding()
+        {
+            ByteString bs = ByteString.CopyFrom("AB", Encoding.Unicode);
+            Assert.AreEqual(4, bs.Length);
+            Assert.AreEqual(65, bs[0]);
+            Assert.AreEqual(0, bs[1]);
+            Assert.AreEqual(66, bs[2]);
+            Assert.AreEqual(0, bs[3]);
+        }
+
+        [Test]
+        public void IsEmptyWhenEmpty()
+        {
+            Assert.IsTrue(ByteString.CopyFromUtf8("").IsEmpty);
+        }
+
+        [Test]
+        public void IsEmptyWhenNotEmpty()
+        {
+            Assert.IsFalse(ByteString.CopyFromUtf8("X").IsEmpty);
+        }
+
+        [Test]
+        public void CopyFromByteArrayCopiesContents()
+        {
+            byte[] data = new byte[1];
+            data[0] = 10;
+            ByteString bs = ByteString.CopyFrom(data);
+            Assert.AreEqual(10, bs[0]);
+            data[0] = 5;
+            Assert.AreEqual(10, bs[0]);
+        }
+
+        [Test]
+        public void ToByteArrayCopiesContents()
+        {
+            ByteString bs = ByteString.CopyFromUtf8("Hello");
+            byte[] data = bs.ToByteArray();
+            Assert.AreEqual((byte)'H', data[0]);
+            Assert.AreEqual((byte)'H', bs[0]);
+            data[0] = 0;
+            Assert.AreEqual(0, data[0]);
+            Assert.AreEqual((byte)'H', bs[0]);
+        }
+
+        [Test]
+        public void CopyFromUtf8UsesUtf8()
+        {
+            ByteString bs = ByteString.CopyFromUtf8("\u20ac");
+            Assert.AreEqual(3, bs.Length);
+            Assert.AreEqual(0xe2, bs[0]);
+            Assert.AreEqual(0x82, bs[1]);
+            Assert.AreEqual(0xac, bs[2]);
+        }
+
+        [Test]
+        public void CopyFromPortion()
+        {
+            byte[] data = new byte[] {0, 1, 2, 3, 4, 5, 6};
+            ByteString bs = ByteString.CopyFrom(data, 2, 3);
+            Assert.AreEqual(3, bs.Length);
+            Assert.AreEqual(2, bs[0]);
+            Assert.AreEqual(3, bs[1]);
+        }
+
+        [Test]
+        public void ToStringUtf8()
+        {
+            ByteString bs = ByteString.CopyFromUtf8("\u20ac");
+            Assert.AreEqual("\u20ac", bs.ToStringUtf8());
+        }
+
+        [Test]
+        public void ToStringWithExplicitEncoding()
+        {
+            ByteString bs = ByteString.CopyFrom("\u20ac", Encoding.Unicode);
+            Assert.AreEqual("\u20ac", bs.ToString(Encoding.Unicode));
+        }
+
+        [Test]
+        public void FromBase64_WithText()
+        {
+            byte[] data = new byte[] {0, 1, 2, 3, 4, 5, 6};
+            string base64 = Convert.ToBase64String(data);
+            ByteString bs = ByteString.FromBase64(base64);
+            Assert.AreEqual(data, bs.ToByteArray());
+        }
+
+        [Test]
+        public void FromBase64_Empty()
+        {
+            // Optimization which also fixes issue 61.
+            Assert.AreSame(ByteString.Empty, ByteString.FromBase64(""));
+        }
+    }
+}
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