Rename ThrowHelper to Preconditions and make it public - we'll want to use it from the generated code soon.

Additionally, change it to return the value passed, and make it generic with a class constraint.
A separate method doesn't have the class constraint, for more unusual scenarios.

Makefile.am

@@ -107,6 +107,7 @@ csharp_EXTRA_DIST=                                                           \
   csharp/src/Google.Protobuf/LimitedInputStream.cs                           \
   csharp/src/Google.Protobuf/MessageExtensions.cs                            \
   csharp/src/Google.Protobuf/MessageParser.cs                                \
+  csharp/src/Google.Protobuf/Preconditions.cs                                \
   csharp/src/Google.Protobuf/Properties/AssemblyInfo.cs                      \
   csharp/src/Google.Protobuf/Reflection/DescriptorBase.cs                    \
   csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs                    \
@@ -133,7 +134,6 @@ csharp_EXTRA_DIST=                                                           \
   csharp/src/Google.Protobuf/Reflection/RepeatedFieldAccessor.cs             \
   csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs                 \
   csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs               \
-  csharp/src/Google.Protobuf/ThrowHelper.cs                                  \
   csharp/src/Google.Protobuf/WellKnownTypes/Any.cs                           \
   csharp/src/Google.Protobuf/WellKnownTypes/Api.cs                           \
   csharp/src/Google.Protobuf/WellKnownTypes/Duration.cs                      \

csharp/src/Google.Protobuf/Collections/MapField.cs

@@ -112,13 +112,13 @@ namespace Google.Protobuf.Collections
 
         public bool ContainsKey(TKey key)
         {
-            ThrowHelper.ThrowIfNull(key, "key");
+            Preconditions.CheckNotNullUnconstrained(key, "key");
             return map.ContainsKey(key);
         }
 
         public bool Remove(TKey key)
         {
-            ThrowHelper.ThrowIfNull(key, "key");
+            Preconditions.CheckNotNullUnconstrained(key, "key");
             LinkedListNode<KeyValuePair<TKey, TValue>> node;
             if (map.TryGetValue(key, out node))
             {
@@ -151,7 +151,7 @@ namespace Google.Protobuf.Collections
         {
             get
             {
-                ThrowHelper.ThrowIfNull(key, "key");
+                Preconditions.CheckNotNullUnconstrained(key, "key");
                 TValue value;
                 if (TryGetValue(key, out value))
                 {
@@ -161,11 +161,11 @@ namespace Google.Protobuf.Collections
             }
             set
             {
-                ThrowHelper.ThrowIfNull(key, "key");
+                Preconditions.CheckNotNullUnconstrained(key, "key");
                 // value == null check here is redundant, but avoids boxing.
                 if (value == null && !allowNullValues)
                 {
-                    ThrowHelper.ThrowIfNull(value, "value");
+                    Preconditions.CheckNotNullUnconstrained(value, "value");
                 }
                 LinkedListNode<KeyValuePair<TKey, TValue>> node;
                 var pair = new KeyValuePair<TKey, TValue>(key, value);
@@ -187,7 +187,7 @@ namespace Google.Protobuf.Collections
 
         public void Add(IDictionary<TKey, TValue> entries)
         {
-            ThrowHelper.ThrowIfNull(entries, "entries");
+            Preconditions.CheckNotNull(entries, "entries");
             foreach (var pair in entries)
             {
                 Add(pair.Key, pair.Value);
@@ -374,7 +374,7 @@ namespace Google.Protobuf.Collections
 
         void IDictionary.Remove(object key)
         {
-            ThrowHelper.ThrowIfNull(key, "key");
+            Preconditions.CheckNotNull(key, "key");
             if (!(key is TKey))
             {
                 return;
@@ -403,7 +403,7 @@ namespace Google.Protobuf.Collections
         {
             get
             {
-                ThrowHelper.ThrowIfNull(key, "key");
+                Preconditions.CheckNotNull(key, "key");
                 if (!(key is TKey))
                 {
                     return null;

csharp/src/Google.Protobuf/Google.Protobuf.csproj

@@ -96,7 +96,7 @@
     <Compile Include="Reflection\RepeatedFieldAccessor.cs" />
     <Compile Include="Reflection\ServiceDescriptor.cs" />
     <Compile Include="Reflection\SingleFieldAccessor.cs" />
-    <Compile Include="ThrowHelper.cs" />
+    <Compile Include="Preconditions.cs" />
     <Compile Include="WellKnownTypes\Any.cs" />
     <Compile Include="WellKnownTypes\Api.cs" />
     <Compile Include="WellKnownTypes\Duration.cs" />

csharp/src/Google.Protobuf/JsonFormatter.cs

@@ -120,7 +120,7 @@ namespace Google.Protobuf
 
         public string Format(IMessage message)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
+            Preconditions.CheckNotNull(message, "message");
             StringBuilder builder = new StringBuilder();
             // TODO(jonskeet): Handle well-known types here.
             // Our reflection support needs improving so that we can get at the descriptor

csharp/src/Google.Protobuf/MessageExtensions.cs

@@ -41,8 +41,8 @@ namespace Google.Protobuf
     {
         public static void MergeFrom(this IMessage message, byte[] data)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
-            ThrowHelper.ThrowIfNull(data, "data");
+            Preconditions.CheckNotNull(message, "message");
+            Preconditions.CheckNotNull(data, "data");
             CodedInputStream input = CodedInputStream.CreateInstance(data);
             message.MergeFrom(input);
             input.CheckLastTagWas(0);
@@ -50,8 +50,8 @@ namespace Google.Protobuf
 
         public static void MergeFrom(this IMessage message, ByteString data)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
-            ThrowHelper.ThrowIfNull(data, "data");
+            Preconditions.CheckNotNull(message, "message");
+            Preconditions.CheckNotNull(data, "data");
             CodedInputStream input = data.CreateCodedInput();
             message.MergeFrom(input);
             input.CheckLastTagWas(0);
@@ -59,8 +59,8 @@ namespace Google.Protobuf
 
         public static void MergeFrom(this IMessage message, Stream input)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
-            ThrowHelper.ThrowIfNull(input, "input");
+            Preconditions.CheckNotNull(message, "message");
+            Preconditions.CheckNotNull(input, "input");
             CodedInputStream codedInput = CodedInputStream.CreateInstance(input);
             message.MergeFrom(codedInput);
             codedInput.CheckLastTagWas(0);
@@ -68,8 +68,8 @@ namespace Google.Protobuf
 
         public static void MergeDelimitedFrom(this IMessage message, Stream input)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
-            ThrowHelper.ThrowIfNull(input, "input");
+            Preconditions.CheckNotNull(message, "message");
+            Preconditions.CheckNotNull(input, "input");
             int size = (int) CodedInputStream.ReadRawVarint32(input);
             Stream limitedStream = new LimitedInputStream(input, size);
             message.MergeFrom(limitedStream);
@@ -77,7 +77,7 @@ namespace Google.Protobuf
 
         public static byte[] ToByteArray(this IMessage message)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
+            Preconditions.CheckNotNull(message, "message");
             byte[] result = new byte[message.CalculateSize()];
             CodedOutputStream output = CodedOutputStream.CreateInstance(result);
             message.WriteTo(output);
@@ -87,8 +87,8 @@ namespace Google.Protobuf
 
         public static void WriteTo(this IMessage message, Stream output)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
-            ThrowHelper.ThrowIfNull(output, "output");
+            Preconditions.CheckNotNull(message, "message");
+            Preconditions.CheckNotNull(output, "output");
             CodedOutputStream codedOutput = CodedOutputStream.CreateInstance(output);
             message.WriteTo(codedOutput);
             codedOutput.Flush();
@@ -96,8 +96,8 @@ namespace Google.Protobuf
 
         public static void WriteDelimitedTo(this IMessage message, Stream output)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
-            ThrowHelper.ThrowIfNull(output, "output");
+            Preconditions.CheckNotNull(message, "message");
+            Preconditions.CheckNotNull(output, "output");
             CodedOutputStream codedOutput = CodedOutputStream.CreateInstance(output);
             codedOutput.WriteRawVarint32((uint)message.CalculateSize());
             message.WriteTo(codedOutput);
@@ -106,7 +106,7 @@ namespace Google.Protobuf
 
         public static ByteString ToByteString(this IMessage message)
         {
-            ThrowHelper.ThrowIfNull(message, "message");
+            Preconditions.CheckNotNull(message, "message");
             return ByteString.AttachBytes(message.ToByteArray());
         }        
     }

csharp/src/Google.Protobuf/MessageParser.cs

@@ -84,7 +84,7 @@ namespace Google.Protobuf
         /// <returns>The newly parsed message.</returns>
         public T ParseFrom(byte[] data)
         {
-            ThrowHelper.ThrowIfNull(data, "data");
+            Preconditions.CheckNotNull(data, "data");
             T message = factory();
             message.MergeFrom(data);
             return message;
@@ -92,7 +92,7 @@ namespace Google.Protobuf
 
         public T ParseFrom(ByteString data)
         {
-            ThrowHelper.ThrowIfNull(data, "data");
+            Preconditions.CheckNotNull(data, "data");
             T message = factory();
             message.MergeFrom(data);
             return message;

csharp/src/Google.Protobuf/ThrowHelper.cs → csharp/src/Google.Protobuf/Preconditions.cs

@@ -1,53 +1,74 @@
-#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;
-
-namespace Google.Protobuf
-{
-    /// <summary>
-    /// Helper methods for throwing exceptions
-    /// </summary>
-    internal static class ThrowHelper
-    {
-        /// <summary>
-        /// Throws an ArgumentNullException if the given value is null.
-        /// </summary>
-        internal static void ThrowIfNull(object value, string name)
-        {
-            if (value == null)
-            {
-                throw new ArgumentNullException(name);
-            }
-        }
-    }
+#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;
+
+namespace Google.Protobuf
+{
+    /// <summary>
+    /// Helper methods for throwing exceptions
+    /// </summary>
+    public static class Preconditions
+    {
+        /// <summary>
+        /// Throws an ArgumentNullException if the given value is null, otherwise
+        /// return the value to the caller.
+        /// </summary>
+        public static T CheckNotNull<T>(T value, string name) where T : class
+        {
+            if (value == null)
+            {
+                throw new ArgumentNullException(name);
+            }
+            return value;
+        }
+
+        /// <summary>
+        /// Throws an ArgumentNullException if the given value is null, otherwise
+        /// return the value to the caller.
+        /// </summary>
+        /// <remarks>
+        /// This is equivalent to <see cref="CheckNotNull"/> but without the type parameter
+        /// constraint. In most cases, the constraint is useful to prevent you from calling CheckNotNull
+        /// with a value type - but it gets in the way if either you want to use it with a nullable
+        /// value type, or you want to use it with an unconstrained type parameter.
+        /// </remarks>
+        internal static T CheckNotNullUnconstrained<T>(T value, string name)
+        {
+            if (value == null)
+            {
+                throw new ArgumentNullException(name);
+            }
+            return value;
+        }
+    }
 }