How do I invoke an extension method using reflection?

Question

I appreciate that similar questions have been asked before, but I am struggling to invoke the Linq Where method in the following code. I am looking to use reflection to dynamically call this method and also dynamically build the delegate (or lambda) used in the Where clause. This is a short code sample that, once working, will help to form part of an interpreted DSL that I am building. Cheers.

    public static void CallWhereMethod()
    {
        List<MyObject> myObjects = new List<MyObject>(){new MyObject{Name="Jon Simpson"}};
        System.Delegate NameEquals = BuildEqFuncFor<MyObject>("Name", "Jon Simpson");
        object[] atts = new object[1] ;
        atts[0] = NameEquals;

        var ret = typeof(List<MyObject>).InvokeMember("Where", BindingFlags.InvokeMethod, null, InstanceList,atts);
    }

    public static Func<T, bool> BuildEqFuncFor<T>(string prop, object val)
    {
        return t => t.GetType().InvokeMember(prop,BindingFlags.GetProperty,
                                             null,t,null) == val;
    }

Answer 1

Extention methods are a c# compiler trick, and they don't exist in the type concerned. They (these particular ones) exist in static classes within the System.Linq name spaces. I'd suggest reflecting this in reflector and then invoking reflection on these types.

Answer 2

Extension methods are really just static methods underwater. An extension method call like foo.Frob(arguments) is really just SomeClass.Frob(foo, arguments). In the case of the Where method, you're looking for System.Linq.Enumerable.Where. So get the typeof Enumerable and invoke Where on that.

Answer 3

Your code sample is a little confusing... unless MyObject is an enumerable.

Using reflection you'll have to invoke Where on System.Linq.Enumerable, passing in the enumerable you want to preform Where on.

Answer 4

As others said, extensions methods are compiler magic, you can alway use VS right click, go to definition to find the real type that implements the static method.

From there it gets fairly hairy. Where is overloaded, so you need to find the actual definition that matches the signature you want. GetMethod has some limitations with generic types so you have to find the actual one using a search.

Once you find the method you must make the methodinfo specific using the MakeGenericMethod call.

Here is a full working sample:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Reflection;

namespace ConsoleApplication9 {
    class Program {

        class MyObject {
            public string Name { get; set; }
        } 

        public static void CallWhereMethod() {
            List<MyObject> myObjects = new List<MyObject>() { 
                new MyObject { Name = "Jon Simpson" },
                new MyObject { Name = "Jeff Atwood" }
            };


            Func<MyObject, bool> NameEquals = BuildEqFuncFor<MyObject>("Name", "Jon Simpson");


            // The Where method lives on the Enumerable type in System.Linq
            var whereMethods = typeof(System.Linq.Enumerable)
                .GetMethods(BindingFlags.Static | BindingFlags.Public)
                .Where(mi => mi.Name == "Where"); 

            Console.WriteLine(whereMethods.Count());
            // 2 (There are 2 methods that are called Where)

            MethodInfo whereMethod = null;
            foreach (var methodInfo in whereMethods) {
                var paramType = methodInfo.GetParameters()[1].ParameterType;
                if (paramType.GetGenericArguments().Count() == 2) {
                    // we are looking for  Func<TSource, bool>, the other has 3
                    whereMethod = methodInfo;
                }
            }

            // we need to specialize it 
            whereMethod = whereMethod.MakeGenericMethod(typeof(MyObject));

            var ret = whereMethod.Invoke(myObjects, new object[] { myObjects, NameEquals }) as IEnumerable<MyObject>;

            foreach (var item in ret) {
                Console.WriteLine(item.Name);
            }
            // outputs "Jon Simpson"

        }

        public static Func<T, bool> BuildEqFuncFor<T>(string prop, object val) {
            return t => t.GetType().InvokeMember(prop, BindingFlags.GetProperty,
                                                 null, t, null) == val;
        }

        static void Main(string[] args) {
            CallWhereMethod();
            Console.ReadKey();

        }
    }
}

Answer 5

thank for your code...i tranformed in vb.net...works fine::::