Question: Is there a better (more performant or nicer code ;) way to find all derived Types of a Type?
Currently im using something like:

1. get all types in used Assemblies
2. check my type with all those types if it is 'IsAssignable'

I was wondering if theres a better way todo this?

Answer: 
The only optimization you can squeeze out of this is to use `Assembly.GetExportedTypes()` to retrieve only publicly visible types if that's the case. Other than that, there's no way to speed things up. LINQ may help with readability side of things, but not performance-wise.

You can do some short-circuiting to avoid unnecessary calls to `IsAssignableFrom` which is, according to Reflector, quite expensive one, by first testing whether the type in question is of required "class". That is, you're searching for classes only, there's no point in testing enums or arrays for "assignability".
Just create a static dictionary of derived types on start and do lookup with that.
Eg. `public static Dictionay<Type, Type[]> DerivedTypes { get;set; }`
where Type is any type you want to include in the search
and Type[] is a list of derived types.
Fill the dictionary up when app starts and use it during the whole life of the app.