I'm looking for an explanation for the following - I have an assembly I'm loading using

Assembly assembly = Assembly.LoadFrom(filename);

I then loop on all the types in the assembly, and wish to try and find out if a type implements a particular interface and if so I want an instance of that type, I've tried several things which did not work, but when I fell back to the most basic (and probably inefficient) way, I realised there's something more fundamental I don't understand -

            foreach (Type t in assembly.GetTypes())
            {
                foreach (Type i in t.GetInterfaces())
                {
                    if (i.FullName == pluginInterfaceType.FullName)
                    {
                        object o = assembly.CreateInstance(t.ToString());
                        IInterface plugin = (IInterface)o;

That last line causes an InvalidCastException, despite the fact that the type created definitely implements that interface.

Further more - if I use Activator.CreateInstance instead of Assembly.CreateInstance (which I don't want to do), casting to the interface works just fine.