Revert https://codereview.chromium.org/1291903002 (protobuf roll).

third_party/protobuf/csharp/src/Google.Protobuf/Reflection/OneofAccessor.cs

-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2015 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.Reflection;
-using Google.Protobuf.Compatibility;
-
-namespace Google.Protobuf.Reflection
-{
-    /// <summary>
-    /// Reflection access for a oneof, allowing clear and "get case" actions.
-    /// </summary>
-    public sealed class OneofAccessor
-    {
-        private readonly Func<IMessage, int> caseDelegate;
-        private readonly Action<IMessage> clearDelegate;
-        private OneofDescriptor descriptor;
-
-        internal OneofAccessor(PropertyInfo caseProperty, MethodInfo clearMethod, OneofDescriptor descriptor) 
-        {
-            if (!caseProperty.CanRead)
-            {
-                throw new ArgumentException("Cannot read from property");
-            }
-            this.descriptor = descriptor;
-            caseDelegate = ReflectionUtil.CreateFuncIMessageT<int>(caseProperty.GetGetMethod());
-
-            this.descriptor = descriptor;
-            clearDelegate = ReflectionUtil.CreateActionIMessage(clearMethod);
-        }
-
-        /// <summary>
-        /// Gets the descriptor for this oneof.
-        /// </summary>
-        /// <value>
-        /// The descriptor of the oneof.
-        /// </value>
-        public OneofDescriptor Descriptor { get { return descriptor; } }
-
-        /// <summary>
-        /// Clears the oneof in the specified message.
-        /// </summary>
-        public void Clear(IMessage message)
-        {
-            clearDelegate(message);
-        }
-
-        /// <summary>
-        /// Indicates which field in the oneof is set for specified message
-        /// </summary>
-        public FieldDescriptor GetCaseFieldDescriptor(IMessage message)
-        {
-            int fieldNumber = caseDelegate(message);
-            if (fieldNumber > 0)
-            {
-                return descriptor.ContainingType.FindFieldByNumber(fieldNumber);
-            }
-            return null;
-        }
-    }
-}