Reflect op_implicit table

问题:

I'm currently looking to build dynamic type converter,

for example, I can easily do :

public struct Tester
{
    public int Hello;

    public static implicit operator int(Tester d)
    {
        return d.Hello;
    }

    public static implicit operator float(Tester d)
    {
        return d.Hello;
    }
}

then

typeof(Tester).GetMethods()

Will return me implicit cast MethodInfo.

However, if I do:

typeof(int).GetMethods()

It will not return any op_implicit

I've seen that you can see the table here , but I was wondering if it's possible to reflect it from the framework itself.

Please note that it's not really a blocking issue, if it's not possible, I'll add converters from the table manually, but I would obviously prefer to have this dynamically built (cleaner and less error prone).

回答1:

The operators for the primitive types aren't defined in the framework - they're part of the CLI itself; they have their own special instructions, basically. There's no IL involved, no methods, so nothing for a MethodInfo to refer to.

If you look at System.Decimal, however, you'll find the operators as that's implemented "just" in the framework itself.

(In a slightly similar way, string doesn't declare a + operator; uses of + within C# are converted to calls to string.Concat.)

回答2:

Jon of course is right. However, it might be useful for you to take a look at System.Linq.Expressions.Expression class (and especially Convert method). For instance, one can quickly build something like this:

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

namespace Tests
{
    static class ConvertTest
    {
        // conceptual point
        static Func<TInput, TOutput> CreateConvertFunc<TInput, TOutput>()
        {
            var source = Expression.Parameter(typeof(TInput), "source");
            // the next will throw if no conversion exists
            var convert = Expression.Convert(source, typeof(TOutput));
            var method = convert.Method;
            if (method != null)
            {
                // here is your method info
            }
            else
            {
                // here is the case of primitive types
                // unfortunately it would not help you, because it's resolved when you call Complile.
                // but you can take a look at reference implementation how they handle it 
            }
            return Expression.Lambda<Func<TInput, TOutput>>(convert, source).Compile();
        }
        // cache
        struct ConverterFunc<TInput, TOutput>
        {
            public static readonly Func<TInput, TOutput> Instance = CreateConvertFunc<TInput, TOutput>();
        }
        // fluent accessor
        struct ConvertSource<T>
        {
            public T source;
            public U To<U>()
            {
                try { return ConverterFunc<T, U>.Instance(source); }
                catch (TypeInitializationException e) { throw e.InnerException; }
            }
        }
        static ConvertSource<T> Convert<T>(this T source) { return new ConvertSource<T> { source = source }; }
        // test
        struct Wrapper<T>
        {
            public T Value;
            public static implicit operator Wrapper<T>(T source) { return new Wrapper<T> { Value = source }; }
            public static implicit operator T(Wrapper<T> source) { return source.Value; }
        }
        class A { }
        class B : A { }
        static void Main(string[] args)
        {
            var v0 = 1;
            var v1 = v0.Convert().To<byte>();
            var v2 = v1.Convert().To<double>();
            var v3 = v2.Convert().To<decimal>();
            var v4 = v3.Convert().To<Wrapper<decimal>>();
            var v5 = v4.Convert().To<decimal?>();
            var v6 = v5.Convert().To<int>();
            var v7 = Enumerable.Empty<B>().Convert().To<IEnumerable<A>>();
            var v8 = v7.Convert().To<int>(); // exception
        }
    }
}