csharp/src/ProtocolBuffers/MessageUtil.cs

#region Copyright notice and license

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// http://github.com/jskeet/dotnet-protobufs/
// Original C++/Java/Python code:
// http://code.google.com/p/protobuf/
//
// 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;

namespace Google.ProtocolBuffers
{
    /// <summary>
    /// Utilities for arbitrary messages of an unknown type. This class does not use
    /// generics precisely because it is designed for dynamically discovered types.
    /// </summary>
    public static class MessageUtil
    {
        /// <summary>
        /// Returns the default message for the given type. If an exception is thrown
        /// (directly from this code), the message will be suitable to be displayed to a user.
        /// </summary>
        /// <param name="type"></param>
        /// <exception cref="ArgumentNullException">The type parameter is null.</exception>
        /// <exception cref="ArgumentException">The type doesn't implement IMessage, or doesn't
        /// have a static DefaultMessage property of the same type, or is generic or abstract.</exception>
        /// <returns></returns>
        public static IMessage GetDefaultMessage(Type type)
        {
            if (type == null)
            {
                throw new ArgumentNullException("type", "No type specified.");
            }
            if (type.IsAbstract || type.IsGenericTypeDefinition)
            {
                throw new ArgumentException("Unable to get a default message for an abstract or generic type (" +
                                            type.FullName + ")");
            }
            if (!typeof(IMessage).IsAssignableFrom(type))
            {
                throw new ArgumentException("Unable to get a default message for non-message type (" + type.FullName +
                                            ")");
            }
            PropertyInfo property = type.GetProperty("DefaultInstance",
                                                     BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic);
            if (property == null)
            {
                throw new ArgumentException(type.FullName + " doesn't have a static DefaultInstance property");
            }
            if (property.PropertyType != type)
            {
                throw new ArgumentException(type.FullName + ".DefaultInstance property is of the wrong type");
            }
            return (IMessage) property.GetValue(null, null);
        }

        /// <summary>
        /// Returns the default message for the type with the given name. This is just
        /// a convenience wrapper around calling Type.GetType and then GetDefaultMessage.
        /// If an exception is thrown, the message will be suitable to be displayed to a user.
        /// </summary>
        /// <param name="typeName"></param>
        /// <exception cref="ArgumentNullException">The typeName parameter is null.</exception>
        /// <exception cref="ArgumentException">The type doesn't implement IMessage, or doesn't
        /// have a static DefaultMessage property of the same type, or can't be found.</exception>
        public static IMessage GetDefaultMessage(string typeName)
        {
            if (typeName == null)
            {
                throw new ArgumentNullException("typeName", "No type name specified.");
            }
            Type type = Type.GetType(typeName);
            if (type == null)
            {
                throw new ArgumentException("Unable to load type " + typeName);
            }
            return GetDefaultMessage(type);
        }
    }
}