This quick snippet will give you all the types declared in all the assemblies loaded, filtered by namespace.

List types = new List();
Assembly[] assemblies = AppDomain.CurrentDomain.GetAssemblies();

foreach (Assembly asm in assemblies)
{
        IEnumerable asmTypes = from t in asm.GetTypes()
              where t.IsClass
                && (t.Namespace != null && t.Namespace.StartsWith(targetNamespace))
              select t;
        types.AddRange(asmTypes);
}

I actually got to this point because of the way NUnit wraps an assembly in another AppDomain.

The snippet need Linq so it’s for 3.5 or newer.