Helper to get types assignable from specific type, with reflection.

GetAssemblyTypes.cs

public static IEnumerable<Type> GetAssemblyTypes<TType>(params Assembly[] assemblies)
{
    return GetAssemblyTypes(typeof(TType), assemblies);
}

public static IEnumerable<Type> GetAssemblyTypes(Type rootType, params Assembly[] assemblies)
{
    if (assemblies == null || !assemblies.Any())
    {
        assemblies = new[] { rootType.Assembly };
    }

    return assemblies.SelectMany(assembly => assembly.GetTypes().Where(rootType.IsAssignableFrom));
}