Introduce a compatiblity shim to support .NET 3.5 delegate creation

csharp/src/Google.Protobuf/Compatibility/MethodInfoExtensions.cs

+#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
+
+#if NET35
+using System;
+using System.Reflection;
+
+namespace Google.Protobuf.Compatibility
+{
+    // .NET Core (at least netstandard1.0) doesn't have Delegate.CreateDelegate, and .NET 3.5 doesn't have
+    // MethodInfo.CreateDelegate. Proxy from one to the other on .NET 3.5...
+    internal static class MethodInfoExtensions
+    {
+        internal static Delegate CreateDelegate(this MethodInfo method, Type type) =>
+            Delegate.CreateDelegate(type, method);
+    }
+}
+#endif

csharp/src/Google.Protobuf/Reflection/ReflectionUtil.cs

@@ -33,6 +33,10 @@
 using System;
 using System.Reflection;
 
+#if NET35
+using Google.Protobuf.Compatibility;
+#endif
+
 namespace Google.Protobuf.Reflection
 {
     /// <summary>