Find generic type over reflection

I have a small snipped of code, which checks, wehter an class exists or not.

At first i load all available types:

List<Type> types = new List<Type>();
foreach (Assembly asm in AppDomain.CurrentDomain.GetAssemblies())
{
    try
    {
        types.AddRange(asm.GetTypes());
    }
    catch (ReflectionTypeLoadException e)
    {
        types.AddRange(e.Types.Where(t => t != null));
    }
}

Than i concat namespace and class name (which should be checked):

string fullName = ns.Trim() + "." + classToProof.Trim();

And in the and, i check wether the class exists:

int found = types.Where(innerItem => innerItem.FullName == fullName).ToList().Count;

But i have the problem, that if i check generic classes, for example System.Collections.Generic.Dictionary, found is always 0 (should be 1).

Does anyone has an idea, why this happens?

Thank you