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

#region Copyright notice and license
// Protocol Buffers - Google's data interchange format
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#endregion

using System;
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) =>
            GetReflectionHelper(method.DeclaringType, method.ReturnType).CreateFuncIMessageObject(method);

        /// <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, int> CreateFuncIMessageInt32(MethodInfo method) =>
            GetReflectionHelper(method.DeclaringType, typeof(object)).CreateFuncIMessageInt32(method);

        /// <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) =>
            GetReflectionHelper(method.DeclaringType, method.GetParameters()[0].ParameterType).CreateActionIMessageObject(method);

        /// <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) =>
            GetReflectionHelper(method.DeclaringType, typeof(object)).CreateActionIMessage(method);        

        /// <summary>
        /// Creates a reflection helper for the given type arguments. Currently these are created on demand
        /// rather than cached; this will be "busy" when initially loading a message's descriptor, but after that
        /// they can be garbage collected. We could cache them by type if that proves to be important, but creating
        /// an object is pretty cheap.
        /// </summary>
        private static IReflectionHelper GetReflectionHelper(Type t1, Type t2) =>
            (IReflectionHelper) Activator.CreateInstance(typeof(ReflectionHelper<,>).MakeGenericType(t1, t2));

        // Non-generic interface allowing us to use an instance of ReflectionHelper<T1, T2> without statically
        // knowing the types involved.
        private interface IReflectionHelper
        {
            Func<IMessage, int> CreateFuncIMessageInt32(MethodInfo method);
            Action<IMessage> CreateActionIMessage(MethodInfo method);
            Func<IMessage, object> CreateFuncIMessageObject(MethodInfo method);
            Action<IMessage, object> CreateActionIMessageObject(MethodInfo method);
        }

        private class ReflectionHelper<T1, T2> : IReflectionHelper
        {
            public Func<IMessage, int> CreateFuncIMessageInt32(MethodInfo method)
            {
                var del = (Func<T1, int>) method.CreateDelegate(typeof(Func<T1, int>));
                return message => del((T1) message);
            }

            public Action<IMessage> CreateActionIMessage(MethodInfo method)
            {
                var del = (Action<T1>) method.CreateDelegate(typeof(Action<T1>));
                return message => del((T1) message);
            }

            public Func<IMessage, object> CreateFuncIMessageObject(MethodInfo method)
            {
                var del = (Func<T1, T2>) method.CreateDelegate(typeof(Func<T1, T2>));
                return message => del((T1) message);
            }

            public Action<IMessage, object> CreateActionIMessageObject(MethodInfo method)
            {
                var del = (Action<T1, T2>) method.CreateDelegate(typeof(Action<T1, T2>));
                return (message, arg) => del((T1) message, (T2) arg);
            }
        }
    }
}