How to properly cast objects created through reflection

Question

I'm trying to wrap my head around reflection, so I decided to add plugin capability to a program that I'm writing. The only way to understand a concept is to get your fingers dirty and write the code, so I went the route of creating a simple interface library consisting of the IPlugin and IHost interfaces, a plugin implementation library of classes that implement IPlugin, and a simple console project that instantiates the IHost implementation class that does simple work with the plugin objects.

Using reflection, I wanted to iterate through the types contained inside my plugin implementation dll and create instances of types. I was able to sucessfully instantiate classes with this code, but I could not cast the created object to the interface.

I tried this code but I couldn't cast object o as I expected. I stepped through the process with the debugger and the proper constructor was called. Quickwatching object o showed me that it had the fields and properties that I expected to see in the implementation class.

loop through assemblies
  loop through types in assembly
    // Filter out unwanted types
    if (!type.IsClass || type.IsNotPublic || type.IsAbstract )
      continue;
    // This successfully created the right object
    object o = Activator.CreateInstance(type);
    // This threw an Invalid Cast Exception or returned null for an "as" cast
    // even though the object implemented IPlugin      
    IPlugin i = (IPlugin) o;

I made the code work with this.

using System.Runtime.Remoting;
ObjectHandle oh = Activator.CreateInstance(assembly.FullName, type.FullName);
// This worked as I intended
IPlugin i = (IPlugin) oh.Unwrap();
i.DoStuff();

Here are my questions:

1. Activator.CreateInstance(Type t) returns an object, but I couldn't cast the object to an interface that the object implemented. Why?
2. Should I have been using a different overload of CreateInstance()?
3. What are the reflection related tips and tricks?
4. Is there some crucial part of reflection that I'm just not getting?

Answer 1

I'm just guessing here because from your code it's not obvious where do you have definition of IPlugin interface but if you can't cast in your host application then you are probably having IPlugin interface in your host assembly and then at the same time in your plugin assembly. This won't work.

The easiest thing is to make this work is to have IPlugin interface marked as public in your host assembly and then have your Plugin assembly reference host application assembly, so both assemblies have access to the very same interface.