How to call extension method "ElementAt"of List<T> with reflection ?

Question

Hi all,

I have problem that after creating object "oListType01" of type List < MyClass01 > and after assigning it to the another objet "oObjectType " of type "object" I can not access any more function "ElementAt(1)". I tried by using reflection but I am always getting exception(parameter conflict) in "Invoke" method. Does anyone knows why ? Milan

        MyClass01 oMy1 = new MyClass01();
        oMy1._ID = "1";

        MyClass01 oMy2 = new MyClass01();
        oMy2._ID = "3";

        IList<MyClass01> oListType01 = new List<MyClass01>();

        oListType01.Add(oMy1);
        oListType01.Add(oMy2);

        object oObjectType = new object();

        oObjectType = oListType01;

//from here fowrads only object oObjectType //is available (upwards happens in separate //function call in the real case). In VS oObjectType shows two element which I would like //to access per reflection

        MethodInfo mInfo = typeof(System.Linq.Enumerable).GetMethod("ElementAt").MakeGenericMethod(typeof(object));


        object oSingleObject = mInfo.Invoke(oObjectType, new object[] { 1 });

Answer 1

I will assume you have a valid reason to be doing this but it seems a little wrong. That said here is some code that will accomplish what you are trying to do.

MethodInfo mInfo = typeof(System.Linq.Enumerable).GetMethod("ElementAt").MakeGenericMethod(typeof(MyClass01));
object oSingleObject = mInfo.Invoke(oObjectType, new object[] { oObjectType, 1 });

When I run this code I get the second element in the List.

Answer 2

The ElementAt extension method is probably on IEnumerable<T> and so when you treat your list like an object, the extension method won't be available unless you cast it. Either ((List<MyClass01>)oObjectType).ElementAt() or (oObjectType as List<MyClass01>).ElementAt().

I have to ask, though, with all due respect why you'd ever want to do this in the first place? It strikes me that there's something wrong here that could be done a little cleaner using interfaces.

Answer 3

If we can safely assume that:

• oObjectType is a IEnumerable of some T

then here's the code to extract the items from it.

Note that I seriously wonder if this is the right way to go about this, but you haven't given us enough information to help you figure out if there's a better way, so all we're left with is just answering the question as asked.

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

namespace ConsoleApplication2
{
    class MyClass01
    {
        public String _ID;

        public override string ToString()
        {
            return _ID;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            MyClass01 oMy1 = new MyClass01();
            oMy1._ID = "1";

            MyClass01 oMy2 = new MyClass01();
            oMy2._ID = "3";

            IList<MyClass01> oListType01 = new List<MyClass01>();

            oListType01.Add(oMy1);
            oListType01.Add(oMy2);

            object oObjectType = new object();

            oObjectType = oListType01;

            Test(oObjectType);

            Console.In.ReadLine();
        }

        private static void Test(object oObjectType)
        {
            Type tObject = oObjectType.GetType();
            Debug.Assert(tObject.IsGenericType);
            Debug.Assert(tObject.GetGenericArguments().Length == 1);
            Type t = tObject.GetGenericArguments()[0];

            Type tIEnumerable = typeof(IEnumerable<>).MakeGenericType(t);
            Debug.Assert(tIEnumerable.IsAssignableFrom(tObject));

            MethodInfo mElementAt =
                typeof(Enumerable)
                .GetMethod("ElementAt")
                .MakeGenericMethod(t);

            Console.Out.WriteLine("o[0] = " +
                mElementAt.Invoke(null, new Object[] { oObjectType, 0 }));
            Console.Out.WriteLine("o[1] = " +
                mElementAt.Invoke(null, new Object[] { oObjectType, 1 }));
        }
    }
}

Answer 4

Thanks, Lasse V. Karlsen vauuu you are really goood :)

Could you please explain the following line of your code? how is it possible that "object" oObjectType is generic I thought that only methods can be generic ?

Type t = tObject.GetGenericArguments()[0];

Is there any good article about generic ?

Answer 5

This is really similar to your other question, but in this case, the static ElementAt method is actually requiring two parameters. Try this:

object oSingleObject = mInfo.Invoke(null, new object[] { oObjectType, 1 });