Essentially everything you need to implement plug-ins in your application is some way to dynamically instantiate classes at runtime. As long as they all implement the same interface, you just use it to access its properties and methods.

In COM world this was achieved by calling the CreateObject function. In the managed world you should use AppDomain.CreateInstanceAndUnwrap as demonstrated by the following example:

IBind pluginInstance = (IBind)AppDomain.CurrentDomain.CreateInstanceAndUnwrap("MCPP", "DamirsCorner.Samples.LateBinding.MCPP");

Everything you need to know at runtime is the full class name (including the namespace) and the containing assembly. To get those you can either require their previous registration within the application or you can dynamically discover them by loading assemblies from a predefined location (Assembly.Load or Assembly.LoadFrom) and enumerating their classes (Assembly.GetTypes) whichever suits you situation best.