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

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using Google.Protobuf.Compatibility;
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
    {
        static ReflectionUtil()
        {
            ForceInitialize<string>(); // Handles all reference types
            ForceInitialize<int>();
            ForceInitialize<long>();
            ForceInitialize<uint>();
            ForceInitialize<ulong>();
            ForceInitialize<float>();
            ForceInitialize<double>();
            ForceInitialize<bool>();
            ForceInitialize<int?>();
            ForceInitialize<long?>();
            ForceInitialize<uint?>();
            ForceInitialize<ulong?>();
            ForceInitialize<float?>();
            ForceInitialize<double?>();
            ForceInitialize<bool?>();
            ForceInitialize<SampleEnum>();
            SampleEnumMethod();
        }

        internal static void ForceInitialize<T>() => new ReflectionHelper<IMessage, T>();

        /// <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 type that declares the method,
        /// call the method on it, then convert the result to object.
        /// </summary>
        /// <param name="method">The method to create a delegate for, which must be declared in an IMessage
        /// implementation.</param>
        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 type that declares the method,
        /// call the method on it, then convert the result to the specified type. The method is expected
        /// to actually return an enum (because of where we're calling it - for oneof cases). Sometimes that
        /// means we need some extra work to perform conversions.
        /// </summary>
        /// <param name="method">The method to create a delegate for, which must be declared in an IMessage
        /// implementation.</param>
        internal static Func<IMessage, int> CreateFuncIMessageInt32(MethodInfo method) =>
            GetReflectionHelper(method.DeclaringType, method.ReturnType).CreateFuncIMessageInt32(method);

        /// <summary>
        /// Creates a delegate which will execute the given method after casting the first argument to
        /// the type that declares the method, and the second argument to the first parameter type of the method.
        /// </summary>
        /// <param name="method">The method to create a delegate for, which must be declared in an IMessage
        /// implementation.</param>
        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
        /// type that declares the method.
        /// </summary>
        /// <param name="method">The method to create a delegate for, which must be declared in an IMessage
        /// implementation.</param>
        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)
            {
                // On pleasant runtimes, we can create a Func<int> from a method returning
                // an enum based on an int. That's the fast path.
                if (CanConvertEnumFuncToInt32Func)
                {
                    var del = (Func<T1, int>) method.CreateDelegate(typeof(Func<T1, int>));
                    return message => del((T1) message);
                }
                else
                {
                    // On some runtimes (e.g. old Mono) the return type has to be exactly correct,
                    // so we go via boxing. Reflection is already fairly inefficient, and this is
                    // only used for one-of case checking, fortunately.
                    var del = (Func<T1, T2>) method.CreateDelegate(typeof(Func<T1, T2>));
                    return message => (int) (object) 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);
            }
        }

        // Runtime compatibility checking code - see ReflectionHelper<T1, T2>.CreateFuncIMessageInt32 for
        // details about why we're doing this.

        // Deliberately not inside the generic type. We only want to check this once.
        private static bool CanConvertEnumFuncToInt32Func { get; } = CheckCanConvertEnumFuncToInt32Func();

        private static bool CheckCanConvertEnumFuncToInt32Func()
        {
            try
            {
                // Try to do the conversion using reflection, so we can see whether it's supported.
                MethodInfo method = typeof(ReflectionUtil).GetMethod(nameof(SampleEnumMethod));
                // If this passes, we're in a reasonable runtime.
                method.CreateDelegate(typeof(Func<int>));
                return true;
            }
            catch (ArgumentException)
            {
                return false;
            }
        }

        public enum SampleEnum
        {
            X
        }

        // Public to make the reflection simpler.
        public static SampleEnum SampleEnumMethod() => SampleEnum.X;
    }
}