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;
    }

Solution 1

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();

        }
    }
}

Solution 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.

Solution 3

I'm a bit off and late but this could help you if you need to call Linq extensions of a IEnumerable wich type is unkown.

IEnumerable<dynamic> test = obj as IEnumerable<dynamic>;

then maybe test obj if not null and

int count = test.Count()

for me that worked very well.

Solution 4

Here's an answer for a general case where the method name is unique (so not in the same case the original posted asked, because Enumerable.Where is overloaded).

Say you have a target object targetObject of the type which is extended, where the extension method is defined in a class TargetClassExtensions and whose extension method's name is ExtensionMethod which takes in an integer parameter and is generic for which you want to call for the class TargetGenericClass.

Then, to call this extension method through reflection, do the following:

int inputInteger = 9; // Example input for the generic method.

object? result = typeof(TargetClassExtensions)
    .GetMethod(nameof(TargetClassExtensions.ExtensionMethod))
    .MakeGenericMethod(typeof(TargetGenericClass))
    .Invoke(null, new object[] { targetObject, inputInteger });

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Comments

[Steven Sudit]

They're a .NET trick, not a C# one. VB.NET can make and use them, too, though the syntax is different, and involves attributes.

[Faisal Ansari]

I get a MissingMethodException using var ret = typeof(System.Linq.Enumerable).InvokeMember("Where", BindingFlags.InvokeMethod, null, InstanceList, atts); Any ideas? Thanks.

[Joren]

The problem is that the method is generic. I found a pretty good blog post on invoking generic methods (blogs.microsoft.co.il/blogs/gilf/archive/2008/10/10/…), but there is one additional problem: There are two overloads of Enumerable.Where, differing only in the type on one of the parameters (Func<T, bool> versus Func<T, int, bool>) and I haven't found out yet how to neatly select the one you want.

[Faisal Ansari]

Thanks for putting the effort into this. Much appreciated.

[Joren]

I suppose then that there is no direct way to have the framework select the appropriate method? That was what I was looking for, but wasn't able to find anything. +1 For working it out.

[ICR]

They're still a compiler trick, it's just the VB.NET compiler also uses the same trick.

[Preet Sangha]

I love the irony of using the where method to find the where method :-)

[Faisal Ansari]

Thanks for this, dynamic was a game changer and I have since changed my entire approach!