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

#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/
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//
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#endregion

using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Reflection;

namespace Google.Protobuf.Reflection
{
    /// <summary>
    /// The methods in this class are somewhat evil, and should not be tampered with lightly.
    /// Basically they allow the creation of relatively weakly typed delegates from MethodInfos
    /// which are more strongly typed. They do this by creating an appropriate strongly typed
    /// delegate from the MethodInfo, and then calling that within an anonymous method.
    /// Mind-bending stuff (at least to your humble narrator) but the resulting delegates are
    /// very fast compared with calling Invoke later on.
    /// </summary>
    internal static class ReflectionUtil
    {
        /// <summary>
        /// Empty Type[] used when calling GetProperty to force property instead of indexer fetching.
        /// </summary>
        internal static readonly Type[] EmptyTypes = new Type[0];

        /// <summary>
        /// Creates a delegate which will cast the argument to the appropriate method target type,
        /// call the method on it, then convert the result to object.
        /// </summary>
        internal static Func<IMessage, object> CreateFuncIMessageObject(MethodInfo method)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(IMessage), "p");
            Expression downcast = Expression.Convert(parameter, method.DeclaringType);
            Expression call = Expression.Call(downcast, method);
            Expression upcast = Expression.Convert(call, typeof(object));
            return Expression.Lambda<Func<IMessage, object>>(upcast, parameter).Compile();
        }

        /// <summary>
        /// Creates a delegate which will cast the argument to the appropriate method target type,
        /// call the method on it, then convert the result to the specified type.
        /// </summary>
        internal static Func<IMessage, T> CreateFuncIMessageT<T>(MethodInfo method)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(IMessage), "p");
            Expression downcast = Expression.Convert(parameter, method.DeclaringType);
            Expression call = Expression.Call(downcast, method);
            Expression upcast = Expression.Convert(call, typeof(T));
            return Expression.Lambda<Func<IMessage, T>>(upcast, parameter).Compile();
        }

        /// <summary>
        /// Creates a delegate which will execute the given method after casting the first argument to
        /// the target type of the method, and the second argument to the first parameter type of the method.
        /// </summary>
        internal static Action<IMessage, object> CreateActionIMessageObject(MethodInfo method)
        {
            ParameterExpression targetParameter = Expression.Parameter(typeof(IMessage), "target");
            ParameterExpression argParameter = Expression.Parameter(typeof(object), "arg");
            Expression castTarget = Expression.Convert(targetParameter, method.DeclaringType);
            Expression castArgument = Expression.Convert(argParameter, method.GetParameters()[0].ParameterType);
            Expression call = Expression.Call(castTarget, method, castArgument);
            return Expression.Lambda<Action<IMessage, object>>(call, targetParameter, argParameter).Compile();
        }

        /// <summary>
        /// Creates a delegate which will execute the given method after casting the first argument to
        /// the target type of the method.
        /// </summary>
        internal static Action<IMessage> CreateActionIMessage(MethodInfo method)
        {
            ParameterExpression targetParameter = Expression.Parameter(typeof(IMessage), "target");
            Expression castTarget = Expression.Convert(targetParameter, method.DeclaringType);
            Expression call = Expression.Call(castTarget, method);
            return Expression.Lambda<Action<IMessage>>(call, targetParameter).Compile();
        }        
    }
}