Create object from dynamically load assembly and cast it to interface (.NET 2.0)

Question

I have a problem with dynamically loaded assemblys and casting it to interface. Where is my mistake?

Main app (load plugins):

namespace Console_IFce_Test
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Press any key to find IPlugin library...");
            Console.ReadKey();            

            string[] files = Directory.GetFiles(Directory.GetCurrentDirectory(), "*.dll");
            Console.WriteLine("Loading assembly: {0}", Path.GetFileName(files[0]));

            Assembly asm = Assembly.LoadFrom(files[0]);
            //Trying this, but still have problems
            //Assembly asm = Assembly.Load(File.ReadAllBytes(files[0])); 

            foreach (Type t in asm.GetTypes())
            {
                Console.WriteLine("Searching in type {0}... ", t.FullName);

                foreach (Type iface in t.GetInterfaces())
                {
                    Console.WriteLine("Interface found: {0}", iface.FullName);
                }

                if (t is IPlugin)
                {
                    Console.WriteLine("1 - IPlugin found!");
                    IPlugin plugin = (IPlugin)Activator.CreateInstance(t);
                    return;
                }

                if (typeof(IPlugin).IsAssignableFrom(t))
                {
                    Console.WriteLine("2 - IPlugin found!");
                    IPlugin plugin = (IPlugin)Activator.CreateInstance(t);
                    return;
                }
            }

            Console.WriteLine("All operations done! Press any key to exit...");
            Console.ReadKey();
        }
    }
}

Interface:

namespace Console_IFce_Test
{
    interface IPlugin
    {
        int GetZero();
    }
}

And plugin:

namespace Library
{
    public class Plugin : Console_IFce_Test.IPlugin
    {
        public int GetZero()
        {
            return 0;
        }
    }
}

In directory with .exe - only 1 .dll (plugin). So, it's output:

Press any key to find IPlugin library... 
Loading assembly: Library.dll 
Searching in type Console_IFce_Test.IPlugin... 
Searching in type Library.Plugin... 
Interface found: Console_IFce_Test.IPlugin 
All operations done! Press any key to exit...

You see, that program found IPlugin interface in assembly, but when i trying to compare it with interface (two conditional statements) - they return false. And if i trying to manually cast it - it returns exception "Can't cast".

I found similar problem: Two Types not equal that should be , and author of answer write:

The same class / type loaded by different app domains [.NET] or class loaders [Java] will not compare equal and are not assignable to/from each other directly.

But i can't understand what should i do? And how?

Best way to avoid this trap is create a 3rd assembly with just the interface types. Referenced by both the host and the plugin. Which ensures you won't have the same type in more than one assembly. — Hans Passant
I try. No result. I'm feeling, that i making very fool mistake, but i can't understand - where? — DeniDoman

muratgu muratgu · Accepted Answer · 2011-11-09T00:40:12

You need to somehow make .NET to load the assemblies to the same context to be able to do a cast between the types in those assemblies.

1- Put the assembly in the application path or GAC so that the Load (not the LoadFrom) function will find it.

2- Create an app domain and do all of the work there. You can fully control where the assemblies are being searched for an app domain.

3- Use the add-in model.

Read this article to learn more about your options: Best Practices for Assembly Loading

Create object from dynamically load assembly and cast it to interface (.NET 2.0)

2 Answers