Is there a System.Reflection.Binder (.NET) that binds to generic methods?

Question

The following F# code fails because Type.DefaultBinder does not want to bind to the generic Id method. Is there an alternative Binder that would do this?

open System
open System.Reflection

type Foo() =
    member this.Id<'T>(x: 'T) : 'T = x //'

typeof<Foo>.InvokeMember (
    "F", 
    BindingFlags.InvokeMethod,
    Type.DefaultBinder,
    (new Foo()),
    [| box "test" |]
)

Here is equivalent C#:

using System;
using System.Reflection;

public class Foo {

    T Id<T>(T x) { 
        return x;
    }

    static void Main() {
        typeof(Foo).InvokeMember
        (
         "F",
         BindingFlags.InvokeMethod,
         Type.DefaultBinder,
         (new Foo()),
         new object[] {"test"}
        );
    }
}

Answer 1

The note under "Remarks" on the InvokeMember page indicates that InvokeMember cannot be used to invoke a generic method. Presumably this related to the fact that you can't use typeof<Foo>.GetMethod("Id").Invoke(...) either, since you need to specify a generic parameter somehow.

On the other hand, it looks like you can probably hack something together that has a shot at working:

type MyBinder() =
  inherit System.Reflection.Binder() with
  let bnd = System.Type.DefaultBinder
  override x.SelectProperty(a,b,c,d,e) = bnd.SelectProperty(a,b,c,d,e)
  override x.ChangeType(a,b,c) = bnd.ChangeType(a,b,c)
  override x.BindToField(a,b,c,d) = bnd.BindToField(a,b,c,d)
  override x.ReorderArgumentArray(a,b) = bnd.ReorderArgumentArray(&a,b)
  override x.SelectMethod(a,b,c,d) = bnd.SelectMethod(a,b,c,d)
  override x.BindToMethod(a,meths,args,b,c,d,e) = 
    try 
      bnd.BindToMethod(a,meths,&args,b,c,d,&e)
    with _ ->
      let [| meth |],[| arg |] = meths,args
      upcast (meth :?> System.Reflection.MethodInfo).MakeGenericMethod([| arg.GetType() |])

This handles only non-overloaded generic methods with a single argument, but you could attempt to make it more robust. I wouldn't be surprised if this implementation of BindToMethod breaks all kinds of expected invariants, though, since it returns a method which was not passed in as a candidate.

Answer 2

This doesn't directly answer your question, but if your ultimate goal is just to call the method, you can do e.g.

open System
open System.Reflection

type Foo() =
    member this.Id<'T>(x: 'T) : 'T = x    // '

let ms = typeof<Foo>.GetMethods() 
      |> Array.filter (fun m -> m.Name="Id" && m.GetGenericArguments().Length=1)
assert( ms.Length = 1 )
let m = ms.[0]
let r = m.MakeGenericMethod([|typeof<string>|]).Invoke(new Foo(),[|box "test"|])
printfn "%A" r    

Answer 3

Here is a specific C# solution for finding a generic extension method and it could be modified to implement a binder. FYI: my needs where simple and NOT performance bound, I just needed a working solution so, on that note, I am aware that this needs major tuning and may have gaps. Any feedback is welcomed though.

I hope this helps with your issue

    private MethodInfo FindExtensionMethod(Type instancetype, string methodName, Expression[] args)
    {
        Type[] parametertypes = Enumerable.Repeat(instancetype, 1).Concat(args.Cast<ConstantExpression>().Select(a => a.Value.GetType())).ToArray();
        var methods = AppDomain.CurrentDomain.GetAssemblies()
            .SelectMany(a => a.GetTypes().Where(t => t.IsSealed && !t.IsGenericType && !t.IsNested))
            .SelectMany(t => t.GetMethods(BindingFlags.Static | BindingFlags.Public)
                .Where(m => m.IsDefined(typeof(ExtensionAttribute), false)
                    && m.Name == methodName
                    && CanBeInvokedWith(m, parametertypes))
                .Select(m => EnsureInvokableMethodFor(m, parametertypes)))
            .ToList();

        return methods.FirstOrDefault();
    }

    private MethodInfo EnsureInvokableMethodFor(MethodInfo method, Type[] parameterTypes)
    {
        if (method.ContainsGenericParameters)
        {
            var genericparams = GetGenericParametersFor(method, parameterTypes).ToArray();
            MethodInfo nongenric = method.MakeGenericMethod(genericparams);
            return nongenric;
        }
        else
            return method;
    }

    private IEnumerable<Type> GetGenericParametersFor(MethodInfo method, Type[] parameterTypes)
    {
        IDictionary<int, Type> args = new Dictionary<int, Type>();
        List<Type> genargs = new List<Type>(method.GetGenericArguments());
        int i = 0;
        foreach (var parameter in method.GetParameters())
        {
            if (parameter.ParameterType.IsGenericParameter)
            {
                AddGenArgs(args,
                    genargs.IndexOf(parameter.ParameterType),
                    parameterTypes[i]);
            }
            else
            {
                if (parameter.ParameterType.IsGenericType)
                {
                    int j = 0;
                    foreach (Type genarg in parameter.ParameterType.GetGenericArguments())
                    {
                        if (genarg.IsGenericParameter)
                        {
                            AddGenArgs(args,
                                genargs.IndexOf(genarg),
                                parameterTypes[i].GetGenericArguments()[j]);
                        }
                        j++;
                    }
                }
            }
            i++;
        }

        return args.Values;
    }

    private static void AddGenArgs(IDictionary<int, Type> args, int argindex, Type arg)
    {
        if (args.ContainsKey(argindex))
        {
            if (args[argindex] != arg)
                throw new ArgumentOutOfRangeException();
        }
        else
            args[argindex] = arg;
    }

    private bool CanBeInvokedWith(MethodInfo method, Type[] parametertypes)
    {
        var parameters = method.GetParameters();
        if (parameters.Length != parametertypes.Length)
            return false;
        int i = 0;
        return parameters.All(p => CanBeAssignedFrom(p.ParameterType, parametertypes[i++]));
    }

    private bool CanBeAssignedFrom(Type paramType, Type argType)
    {
        if (paramType.IsGenericType)
        {
            if (argType.IsGenericType)
            {
                if (paramType.GetGenericTypeDefinition() == argType.GetGenericTypeDefinition())
                {
                    return GenericArgsAreCompatible(
                        paramType.GetGenericArguments(),
                        argType.GetGenericArguments());

                }
                else
                    return false;
            }
            else
                return false;
        }
        else
        {
            if (paramType.IsGenericParameter)
                return true;
            else
                return paramType.IsAssignableFrom(argType);
        }
    }

    private bool GenericArgsAreCompatible(Type[] paramArgs, Type[] argArgs)
    {
        if (paramArgs.Length != argArgs.Length)
            return false;

        int i = 0;
        return paramArgs.All(p => TypesAreCompatible(p, argArgs[i++]));
    }

    private bool TypesAreCompatible(Type paramArg, Type argArg)
    {
        if (paramArg.IsGenericParameter)
            return true;
        else
            return paramArg == argArg;
    }