Plugin Architecture with Interfaces; Interface check doesn't work

Question

I am implementing a simple plugin architecture in an application. The plugin requirements are defined using an interface (IPlugin) that is in a *.dll that is referenced by the application and the plugin. The application has a plugin manager (also in the same *.dll) which loads the plugins by looking for all the *.dll's in a plugins folder, loads them, then checks that the plugin implements the interface. I have done this checking two different ways [previously by a simple if (plugin is IPlugin)], but neither one will recognize when a plugin implements the interface. Here's the code:

Assembly pluginAssembly = Assembly.LoadFrom(currFile.FullName);
if (pluginAssembly != null)
{
    foreach (Type currType in pluginAssembly.GetTypes())
    {
        if (currType.GetInterfaces().Contains(typeof(IPlugin)))
        {
            // Code here is never executing
            // even when the currType derives from IPlugin
        }
    }                    
}

I used to test a particular class name ("Plugin"), but then I allowed it to cycle through all the classes in the assembly to no avail. (This follows an example that I found elsewhere.) To make this a little more complicated, there are two interfaces each of which implements the original interface (IPluginA, IPluginB). The plugin actually implements one of the more specific interfaces (IPluginB). However, I have tried it with the plugin just implementing the more general interface (IPlugin), and this still does not work.

[Edit: in response to the two responses that I first received] Yes, I have tried using IsAssignableFrom. See the following:

Assembly pluginAssembly = Assembly.LoadFrom(currFile.FullName);
if (pluginAssembly != null)
{
    foreach (Type currType in pluginAssembly.GetTypes())
    {
        if (typeof(IPlugin).IsAssignableFrom(currType))
        {
            string test = "test";
        }
    }
}

Answer 1

Have you tried:

typeof(IPlugin).IsAssignableFrom(currType)

Also, types implement interfaces, but they do not derive from them. The BaseType property and IsSubclassOf method show derivation, where IsAssignableFrom shows derivation or implementation.

Edit: are your assemblies signed? They could be loading side-by-side versions of your assembly, and since Type objects are compared with ReferenceEquals, the same type in two side-by-side assemblies would be completely independent.

Edit 2: Try this:

public Type[] LoadPluginsInAssembly(Assembly otherAssembly)
{
    List<Type> pluginTypes = new List<Type>();
    foreach (Type type in otherAssembly.GetTypes())
    {
        // This is just a diagnostic. IsAssignableFrom is what you'll use once
        // you find the problem.
        Type otherInterfaceType =
            type.GetInterfaces()
            .Where(interfaceType => interfaceType.Name.Equals(typeof(IPlugin).Name, StringComparison.Ordinal)).FirstOrDefault();

        if (otherInterfaceType != null)
        {
            if (otherInterfaceType == typeof(IPlugin))
            {
                pluginTypes.Add(type);
            }
            else
            {
                Console.WriteLine("Duplicate IPlugin types found:");
                Console.WriteLine("  " + typeof(IPlugin).AssemblyQualifiedName);
                Console.WriteLine("  " + otherInterfaceType.AssemblyQualifiedName);
            }
        }
    }

    if (pluginTypes.Count == 0)
        return Type.EmptyTypes;

    return pluginTypes.ToArray();
}