CODEWITHSUNDEEP
c#linqreflectiondelegateslambda
Jon Simpson
Jon Simpson

Reputation: 1023

How do I invoke an extension method using reflection?

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;
    }

Upvotes: 43

Views: 32277

Answers (6)

Daniel
Daniel

Reputation: 9464

Here's an answer for a general case where the method name is unique (so not the same case the original question posed, 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 });

Upvotes: 3

Sam Saffron
Sam Saffron

Reputation: 131112

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();

        }
    }
}

Upvotes: 64

Joren
Joren

Reputation: 14746

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.

Upvotes: 10

Jens Marchewka
Jens Marchewka

Reputation: 1443

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.

Upvotes: 3

user1228
user1228

Reputation:

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.

Upvotes: 1

Preet Sangha
Preet Sangha

Reputation: 65476

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.

Upvotes: 0

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