protobuf/src/ProtocolBuffers/FieldAccess/SingleMessageAccessor.cs

// 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.
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// contributors may be used to endorse or promote products derived from
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using System;
using System.Reflection;

namespace Google.ProtocolBuffers.FieldAccess {
  /// <summary>
  /// Accessor for fields representing a non-repeated message value.
  /// </summary>
  internal sealed class SingleMessageAccessor<TMessage, TBuilder> : SinglePrimitiveAccessor<TMessage, TBuilder>
      where TMessage : IMessage<TMessage, TBuilder>
      where TBuilder : IBuilder<TMessage, TBuilder> {

    /// <summary>
    /// The static method to create a builder for the property type. For example,
    /// in a message type "Foo", a field called "bar" might be of type "Baz". This
    /// method is Baz.CreateBuilder.
    /// </summary>
    private readonly Func<IBuilder> createBuilderDelegate;

    internal SingleMessageAccessor(string name) : base(name) {
      MethodInfo createBuilderMethod = ClrType.GetMethod("CreateBuilder", Type.EmptyTypes);
      if (createBuilderMethod == null) {
        throw new ArgumentException("No public static CreateBuilder method declared in " + ClrType.Name);
      }
      createBuilderDelegate = ReflectionUtil.CreateStaticUpcastDelegate(createBuilderMethod);
    }

    /// <summary>
    /// Creates a message of the appropriate CLR type from the given value,
    /// which may already be of the right type or may be a dynamic message.
    /// </summary>
    private object CoerceType(object value) {

      // If it's already of the right type, we're done
      if (ClrType.IsInstanceOfType(value)) {
        return value;
      }
      
      // No... so let's create a builder of the right type, and merge the value in.
      IMessage message = (IMessage) value;
      return CreateBuilder().WeakMergeFrom(message).WeakBuild();
    }

    public override void SetValue(TBuilder builder, object value) {
      base.SetValue(builder, CoerceType(value));
    }

    public override IBuilder CreateBuilder() {
      return createBuilderDelegate();
    }
  }
}