Question

I’m trying to use a precompiled DLL with reflection, to instantiate an interface for my class that is in the DLL. I tried by the book, but it won’t work. It throws InvalidCastException when I try to do something like:

ICompute iCompute = (ICompute)Activator.CreateInstance(type);

Where type of course is my class that implements ICompute interface. I’m stuck and don’t know what to do. The complete code follows:

This is the DLL content:

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

namespace ConsoleApplication18
{
    public class ClassThatImplementsICompute : ICompute
    {
       public int sumInts(int term1, int term2)
       {
           return term1 + term2;
       }

       public int diffInts(int term1, int term2)
       {
           return term1 - term2;
       }
    }
}

The actual program:

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

using System.Reflection;

namespace ConsoleApplication18
{

    public interface ICompute
    {
        int sumInts(int term1, int term2);
        int diffInts(int term1, int term2);
    }



    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Loading dll...");
            Assembly assembly = Assembly.LoadFrom("mylib.dll");

            Console.WriteLine("Getting type...");
            Type type = assembly.GetType("ConsoleApplication18.ClassThatImplementsICompute");
            if (type == null) Console.WriteLine("Could not find class type");

            Console.WriteLine("Instantiating with activator...");
            //my problem!!!
            ICompute iCompute = (ICompute)Activator.CreateInstance(type);

            //code that uses those functions...



        }
    }
}

Can anyone help me? Thanks!

Answer 1

The problem is to do with how you load the assembly with Assembly.LoadFrom().

LoadFrom() load the assembly into different context compared to context of the ICompute interface you are trying to cast to. Try to use Assembly.Load() instead if possible. i.e. put the assembly into the bin / probing path folder and load by the full strong name.

Some references:
http://msdn.microsoft.com/en-us/library/dd153782.aspx
http://blogs.msdn.com/b/suzcook/archive/2003/05/29/57143.aspx (see the disadvantage bit for LoadFrom)