src/ProtocolBuffers/FieldAccess/ReflectionUtil.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/
//
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// modification, are permitted provided that the following conditions are
// met:
//
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using System;
using System.Reflection;

namespace Google.ProtocolBuffers.FieldAccess
{
    /// <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>
        /// Creates a delegate which will execute the given method and then return
        /// the result as an object.
        /// </summary>
        public static Func<T, object> CreateUpcastDelegate<T>(MethodInfo method)
        {
            // The tricky bit is invoking CreateCreateUpcastDelegateImpl with the right type parameters
            MethodInfo openImpl = typeof (ReflectionUtil).GetMethod("CreateUpcastDelegateImpl");
            MethodInfo closedImpl = openImpl.MakeGenericMethod(typeof (T), method.ReturnType);
            return (Func<T, object>) closedImpl.Invoke(null, new object[] {method});
        }

        /// <summary>
        /// Method used solely for implementing CreateUpcastDelegate. Public to avoid trust issues
        /// in low-trust scenarios, e.g. Silverlight.
        /// TODO(jonskeet): Check any of this actually works in Silverlight...
        /// </summary>
        public static Func<TSource, object> CreateUpcastDelegateImpl<TSource, TResult>(MethodInfo method)
        {
            // Convert the reflection call into an open delegate, i.e. instead of calling x.Method()
            // we'll call getter(x).
            Func<TSource, TResult> getter =
                (Func<TSource, TResult>) Delegate.CreateDelegate(typeof (Func<TSource, TResult>), null, method);

            // Implicit upcast to object (within the delegate)
            return delegate(TSource source) { return getter(source); };
        }

        /// <summary>
        /// Creates a delegate which will execute the given method after casting the parameter
        /// down from object to the required parameter type.
        /// </summary>
        public static Action<T, object> CreateDowncastDelegate<T>(MethodInfo method)
        {
            MethodInfo openImpl = typeof (ReflectionUtil).GetMethod("CreateDowncastDelegateImpl");
            MethodInfo closedImpl = openImpl.MakeGenericMethod(typeof (T), method.GetParameters()[0].ParameterType);
            return (Action<T, object>) closedImpl.Invoke(null, new object[] {method});
        }

        public static Action<TSource, object> CreateDowncastDelegateImpl<TSource, TParam>(MethodInfo method)
        {
            // Convert the reflection call into an open delegate, i.e. instead of calling x.Method(y) we'll
            // call Method(x, y)
            Action<TSource, TParam> call =
                (Action<TSource, TParam>) Delegate.CreateDelegate(typeof (Action<TSource, TParam>), null, method);

            return delegate(TSource source, object parameter) { call(source, (TParam) parameter); };
        }

        /// <summary>
        /// Creates a delegate which will execute the given method after casting the parameter
        /// down from object to the required parameter type.
        /// </summary>
        public static Action<T, object> CreateDowncastDelegateIgnoringReturn<T>(MethodInfo method)
        {
            MethodInfo openImpl = typeof (ReflectionUtil).GetMethod("CreateDowncastDelegateIgnoringReturnImpl");
            MethodInfo closedImpl = openImpl.MakeGenericMethod(typeof (T), method.GetParameters()[0].ParameterType,
                                                               method.ReturnType);
            return (Action<T, object>) closedImpl.Invoke(null, new object[] {method});
        }

        public static Action<TSource, object> CreateDowncastDelegateIgnoringReturnImpl<TSource, TParam, TReturn>(
            MethodInfo method)
        {
            // Convert the reflection call into an open delegate, i.e. instead of calling x.Method(y) we'll
            // call Method(x, y)
            Func<TSource, TParam, TReturn> call = (Func<TSource, TParam, TReturn>)
                                                  Delegate.CreateDelegate(typeof (Func<TSource, TParam, TReturn>), null,
                                                                          method);

            return delegate(TSource source, object parameter) { call(source, (TParam) parameter); };
        }

        /// <summary>
        /// Creates a delegate which will execute the given static method and cast the result up to IBuilder.
        /// </summary>
        public static Func<IBuilder> CreateStaticUpcastDelegate(MethodInfo method)
        {
            MethodInfo openImpl = typeof (ReflectionUtil).GetMethod("CreateStaticUpcastDelegateImpl");
            MethodInfo closedImpl = openImpl.MakeGenericMethod(method.ReturnType);
            return (Func<IBuilder>) closedImpl.Invoke(null, new object[] {method});
        }

        public static Func<IBuilder> CreateStaticUpcastDelegateImpl<T>(MethodInfo method)
        {
            Func<T> call = (Func<T>) Delegate.CreateDelegate(typeof (Func<T>), null, method);
            return delegate { return (IBuilder) call(); };
        }
    }
}