diff --git a/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs b/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs
new file mode 100644
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+++ b/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs
@@ -0,0 +1,267 @@
+﻿#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2017 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 Google.Protobuf.Reflection;
+using Google.Protobuf.WellKnownTypes;
+using NUnit.Framework;
+using System.IO;
+using System.Linq;
+using UnitTest.Issues.TestProtos;
+using static Google.Protobuf.WireFormat;
+using static UnitTest.Issues.TestProtos.ComplexOptionType2.Types;
+using static UnitTest.Issues.TestProtos.DummyMessageContainingEnum.Types;
+using static Google.Protobuf.Test.Reflection.CustomOptionNumber;
+
+namespace Google.Protobuf.Test.Reflection
+{
+    // Internal enum to allow us to use "using static" for convenience.
+    // These are the options defined in unittest_custom_options_proto3.proto
+    internal enum CustomOptionNumber
+    {
+        FileOpt1 = 7736974,
+        MessageOpt1 = 7739036,
+        FieldOpt1 = 7740936,
+        EnumOpt1 = 7753576,
+        EnumValueOpt1 = 1560678,
+        ServiceOpt1 = 7887650,
+        MethodOpt1 = 7890860,
+
+        // All message options...
+        BoolOpt = 7706090,
+        Int32Opt = 7705709,
+        Int64Opt = 7705542,
+        UInt32Opt = 7704880,
+        UInt64Opt = 7702367,
+        SInt32Opt = 7701568,
+        SInt64Opt = 7700863,
+        Fixed32Opt = 7700307,
+        Fixed64Opt = 7700194,
+        SFixed32Opt = 7698645,
+        SFixed64Opt = 7685475,
+        FloatOpt = 7675390,
+        DoubleOpt = 7673293,
+        StringOpt = 7673285,
+        BytesOpt = 7673238,
+        EnumOpt = 7673233,
+        MessageTypeOpt = 7665967,
+
+        // Miscellaneous
+        ComplexOpt4 = 7633546,
+        ComplexOpt1 = 7646756,
+        ComplexOpt2 = 7636949,
+        ComplexOpt3 = 7636463,
+
+        // Aggregates
+        AggregateFileOpt = 15478479,
+        AggregateMsgOpt = 15480088,
+        AggregateFieldOpt = 15481374,
+        AggregateEnumOpt = 15483218,
+        AggregateEnumValueOpt = 15486921,
+        AggregateServiceOpt = 15497145,
+        AggregateMethodOpt = 15512713,
+    }
+
+    /// <summary>
+    /// The majority of the testing here is done via parsed descriptors. That's simpler to
+    /// achieve (and more important) than constructing a CodedInputStream manually.
+    /// </summary>
+    public class CustomOptionsTest
+    {
+        delegate bool OptionFetcher<T>(int field, out T value);
+
+        [Test]
+        public void EmptyOptionsIsShared()
+        {
+            var structOptions = Struct.Descriptor.CustomOptions;
+            var timestampOptions = Struct.Descriptor.CustomOptions;
+            Assert.AreSame(structOptions, timestampOptions);
+        }
+
+        [Test]
+        public void SimpleIntegerTest()
+        {
+            var stream = new MemoryStream();
+            var output = new CodedOutputStream(stream);
+            output.WriteTag(MakeTag(1, WireType.Varint));
+            output.WriteInt32(1234567);
+            output.Flush();
+            stream.Position = 0;
+            var input = new CodedInputStream(stream);
+            input.ReadTag();
+
+            var options = CustomOptions.Empty;
+            options = options.ReadOrSkipUnknownField(input);
+
+            int intValue;
+            Assert.True(options.TryGetInt32(1, out intValue));
+            Assert.AreEqual(1234567, intValue);
+
+            string stringValue;
+            // No ByteString stored values
+            Assert.False(options.TryGetString(1, out stringValue));
+            // Nothing stored for field 2
+            Assert.False(options.TryGetInt32(2, out intValue));
+        }
+
+        [Test]
+        public void SimpleStringTest()
+        {
+            var stream = new MemoryStream();
+            var output = new CodedOutputStream(stream);
+            output.WriteTag(MakeTag(1, WireType.LengthDelimited));
+            output.WriteString("value");
+            output.Flush();
+            stream.Position = 0;
+            var input = new CodedInputStream(stream);
+            input.ReadTag();
+
+            var options = CustomOptions.Empty;
+            options = options.ReadOrSkipUnknownField(input);
+
+            string stringValue;
+            Assert.True(options.TryGetString(1, out stringValue));
+            Assert.AreEqual("value", stringValue);
+
+            int intValue;
+            // No numeric stored values
+            Assert.False(options.TryGetInt32(1, out intValue));
+            // Nothing stored for field 2
+            Assert.False(options.TryGetString(2, out stringValue));
+        }
+
+        [Test]
+        public void ScalarOptions()
+        {
+            var options = CustomOptionOtherValues.Descriptor.CustomOptions;
+            AssertOption(-100, options.TryGetInt32, Int32Opt);
+            AssertOption(12.3456789f, options.TryGetFloat, FloatOpt);
+            AssertOption(1.234567890123456789d, options.TryGetDouble, DoubleOpt);
+            AssertOption("Hello, \"World\"", options.TryGetString, StringOpt);
+            AssertOption(ByteString.CopyFromUtf8("Hello\0World"), options.TryGetBytes, BytesOpt);
+            AssertOption((int) TestEnumType.TestOptionEnumType2, options.TryGetInt32, EnumOpt);
+        }
+
+        [Test]
+        public void MessageOptions()
+        {
+            var options = VariousComplexOptions.Descriptor.CustomOptions;
+            AssertOption(new ComplexOptionType1 { Foo = 42, Foo4 = { 99, 88 } }, options.TryGetMessage, ComplexOpt1);
+            AssertOption(new ComplexOptionType2
+                {
+                    Baz = 987, Bar = new ComplexOptionType1 { Foo = 743 },
+                    Fred = new ComplexOptionType4 { Waldo = 321 },
+                    Barney = { new ComplexOptionType4 { Waldo = 101 }, new ComplexOptionType4 { Waldo = 212 } }
+                },
+                options.TryGetMessage, ComplexOpt2);
+            AssertOption(new ComplexOptionType3 { Qux = 9 }, options.TryGetMessage, ComplexOpt3);
+        }
+
+        [Test]
+        public void OptionLocations()
+        {
+            var fileOptions = UnittestCustomOptionsProto3Reflection.Descriptor.CustomOptions;
+            AssertOption(9876543210UL, fileOptions.TryGetUInt64, FileOpt1);
+
+            var messageOptions = TestMessageWithCustomOptions.Descriptor.CustomOptions;
+            AssertOption(-56, messageOptions.TryGetInt32, MessageOpt1);
+
+            var fieldOptions = TestMessageWithCustomOptions.Descriptor.Fields["field1"] .CustomOptions;
+            AssertOption(8765432109UL, fieldOptions.TryGetFixed64, FieldOpt1);
+
+            var enumOptions = TestMessageWithCustomOptions.Descriptor.EnumTypes[0].CustomOptions;
+            AssertOption(-789, enumOptions.TryGetSFixed32, EnumOpt1);
+
+            var enumValueOptions = TestMessageWithCustomOptions.Descriptor.EnumTypes[0].FindValueByNumber(2).CustomOptions;
+            AssertOption(123, enumValueOptions.TryGetInt32, EnumValueOpt1);
+
+            var service = UnittestCustomOptionsProto3Reflection.Descriptor.Services
+                .Single(s => s.Name == "TestServiceWithCustomOptions");
+            var serviceOptions = service.CustomOptions;
+            AssertOption(-9876543210, serviceOptions.TryGetSInt64, ServiceOpt1);
+
+            var methodOptions = service.Methods[0].CustomOptions;
+            AssertOption((int) UnitTest.Issues.TestProtos.MethodOpt1.Val2, methodOptions.TryGetInt32, CustomOptionNumber.MethodOpt1);
+        }
+
+        [Test]
+        public void MinValues()
+        {
+            var options = CustomOptionMinIntegerValues.Descriptor.CustomOptions;
+            AssertOption(false, options.TryGetBool, BoolOpt);
+            AssertOption(int.MinValue, options.TryGetInt32, Int32Opt);
+            AssertOption(long.MinValue, options.TryGetInt64, Int64Opt);
+            AssertOption(uint.MinValue, options.TryGetUInt32, UInt32Opt);
+            AssertOption(ulong.MinValue, options.TryGetUInt64, UInt64Opt);
+            AssertOption(int.MinValue, options.TryGetSInt32, SInt32Opt);
+            AssertOption(long.MinValue, options.TryGetSInt64, SInt64Opt);
+            AssertOption(uint.MinValue, options.TryGetUInt32, Fixed32Opt);
+            AssertOption(ulong.MinValue, options.TryGetUInt64, Fixed64Opt);
+            AssertOption(int.MinValue, options.TryGetInt32, SFixed32Opt);
+            AssertOption(long.MinValue, options.TryGetInt64, SFixed64Opt);
+        }
+
+        [Test]
+        public void MaxValues()
+        {
+            var options = CustomOptionMaxIntegerValues.Descriptor.CustomOptions;
+            AssertOption(true, options.TryGetBool, BoolOpt);
+            AssertOption(int.MaxValue, options.TryGetInt32, Int32Opt);
+            AssertOption(long.MaxValue, options.TryGetInt64, Int64Opt);
+            AssertOption(uint.MaxValue, options.TryGetUInt32, UInt32Opt);
+            AssertOption(ulong.MaxValue, options.TryGetUInt64, UInt64Opt);
+            AssertOption(int.MaxValue, options.TryGetSInt32, SInt32Opt);
+            AssertOption(long.MaxValue, options.TryGetSInt64, SInt64Opt);
+            AssertOption(uint.MaxValue, options.TryGetFixed32, Fixed32Opt);
+            AssertOption(ulong.MaxValue, options.TryGetFixed64, Fixed64Opt);
+            AssertOption(int.MaxValue, options.TryGetSFixed32, SFixed32Opt);
+            AssertOption(long.MaxValue, options.TryGetSFixed64, SFixed64Opt);
+        }
+
+        [Test]
+        public void AggregateOptions()
+        {
+            // Just two examples
+            var messageOptions = AggregateMessage.Descriptor.CustomOptions;
+            AssertOption(new Aggregate { I = 101, S = "MessageAnnotation" }, messageOptions.TryGetMessage, AggregateMsgOpt);
+
+            var fieldOptions = AggregateMessage.Descriptor.Fields["fieldname"].CustomOptions;
+            AssertOption(new Aggregate { S = "FieldAnnotation" }, fieldOptions.TryGetMessage, AggregateFieldOpt);
+        }
+
+        private void AssertOption<T>(T expected, OptionFetcher<T> fetcher, CustomOptionNumber field)
+        {
+            T actual;
+            Assert.IsTrue(fetcher((int) field, out actual));
+            Assert.AreEqual(expected, actual);
+        }
+    }
+}