"Uncurrying" an instance method in .NET

Question

EDIT: Highlight difficulties with virtual and/or value-type methods

Can you create a delegate of an instance method without specifying the instance at creation time? In other words, can you create a "static" delegate that takes as it's first parameter the instance the method should be called on?

For example, how can I construct the following delegate using reflection?

Func<int, string> = i=>i.ToString();

I'm aware of the fact that I can use methodInfo.Invoke, but this is slower, and does not check for type-correctness until it is called.

When you have the MethodInfo of a particular static method, it is possible to construct a delegate using Delegate.CreateDelegate(delegateType, methodInfo), and all parameters of the static method remain free.

As Jon Skeet pointed out, you can simply apply the same to make an open delegate of an instance method if the method is non-virtual on a reference type. Deciding which method to call on a virtual method is tricky, so that's no so trivial, and value-types look like they don't work at all.

EDIT: For value types, CreateDelegate exhibits really weird behavior:

var func37 = (Func<CultureInfo,string>)(37.ToString);
var toStringMethod = typeof(int).GetMethod("ToString", BindingFlags.Instance | BindingFlags.Public, null, new Type[] {typeof(CultureInfo) }, null);
var func42 = (Func<CultureInfo,string>)Delegate.CreateDelegate(typeof(Func<CultureInfo,string>), 42, toStringMethod,true);
Console.WriteLine( object.ReferenceEquals(func37.Method,func42.Method)); //true
Console.WriteLine(func37.Target);//37
Console.WriteLine(func42.Target);//42
Console.WriteLine(func37(CultureInfo.InvariantCulture));//37
Console.WriteLine(func42(CultureInfo.InvariantCulture));//-201040128... WTF?

Calling CreateDelegate with null as the target object throws a binding exception if the instance method belonged to a value type (this works for reference types).

Answer 1

I'm not sure, but may be Open delegates can help you.

Upd: Follow this link, if first one don't works.

Answer 2

The goog way maybe can be useing the "dynamic" type in .NET 4.0. However the Delegate need the instance (for non-static methods). The problems is more complex then lokks at first time because of polymorfism etc...

Answer 3

You've actually chosen a particularly tricky example, for two reasons:

• ToString() is a virtual method inherited from object but overridden in Int32.
• int is a value type, and there are weird rules with Delegate.CreateDelegate() when it comes to value types and instance methods - basically the first effective parameter becomes ref int rather than int

However, here's an example for String.ToUpper, which doesn't have either of those problems:

using System;
using System.Reflection;

class Test
{
    static void Main()
    {
        MethodInfo method = typeof(string).GetMethod
            ("ToUpper", BindingFlags.Instance | BindingFlags.Public,
             null, new Type[]{}, null);

        Func<string, string> func = (Func<string, string>)
            Delegate.CreateDelegate(typeof(Func<string, string>),
                                    null,
                                    method);

        string x = func("hello");

        Console.WriteLine(x);
    }
}

If that's good enough for you, great... if you really want int.ToString, I'll have to try a bit harder :)

EDIT: Here's an example for a value type, using a new delegate type which takes its first parameter by reference:

using System;
using System.Reflection;

public struct Foo
{
    readonly string value;

    public Foo(string value)
    {
        this.value = value;
    }

    public string DemoMethod()
    {
        return value;
    }
}

class Test
{
    delegate TResult RefFunc<TArg, TResult>(ref TArg arg);

    static void Main()
    {
        MethodInfo method = typeof(Foo).GetMethod
            ("DemoMethod", BindingFlags.Instance | BindingFlags.Public,
             null, new Type[]{}, null);
        RefFunc<Foo, string> func = (RefFunc<Foo, string>)
            Delegate.CreateDelegate(typeof(RefFunc<Foo, string>),
                                    null,
                                    method);

        Foo y = new Foo("hello");
        string x = func(ref y);

        Console.WriteLine(x);
    }
}

Answer 4

You could use Lambdas to get a "somewhat" compiled static wrapper for your instance method.

The sample below isn't exactly blazingly fast, yet it should be significantly faster than any plain dynamic invoke.

The output

100000 iterations took 4 ms 
1000000 iterations took 18 ms 
10000000 iterations took 184 ms

The code

class Program
{

   public sealed class Test
   {
      public String Data { get; set; }
      public override string ToString()
      {
         return Data;
      }
   }

   static void Main(string[] args)
   {
      TestRun(100000);
      TestRun(1000000);
      TestRun(10000000);
   }

   private static void TestRun(int iterations)
   {
      var toString = typeof(Test).GetMethod("ToString",
                                            BindingFlags.Instance
                                            | BindingFlags.Public,
                                            null,
                                            Type.EmptyTypes,
                                            null);
      var call = GetCall<Test, String>(toString);
      var tests
         = (from i in Enumerable.Range(1, iterations)
            select new Test { Data = "..." + i }).ToList();

      var sw = Stopwatch.StartNew();
      tests.ForEach(i => call(i));
      sw.Stop();
      Console.WriteLine("{0} iterations took {1} ms", iterations, sw.ElapsedMilliseconds);
   }

   private static Func<T, M> GetCall<T, M>(MethodInfo methodInfo)
   {
      var input = Expression.Parameter(typeof(T), "input");
      MethodCallExpression member = Expression.Call(input, methodInfo);
      var lambda = Expression.Lambda<Func<T, M>>(member, input);

      return lambda.Compile();
   }
}