LoadPlugin.cs

public static IPlugin LoadPlugin(string dllPath)
{
    var dllBytes = File.ReadAllBytes(dllPath);
    var an = AssemblyName.GetAssemblyName(dllPath);
    var assembly = Assembly.Load(dllBytes);
    var pluginType = typeof(IPlugin);
    foreach(Type type in assembly.GetTypes())
    {
        if(!type.IsInterface && !type.IsAbstract)
        {
            if(type.GetInterface(pluginType.FullName) != null)
            {
                return assembly.CreateInstance(type.FullName) as IPlugin;
            }
        }
    }
    return null;
}