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: 
If you're just interested in browsing, then .NET Reflector has the ability to do this. However, it isn't something that's really feasible. Would you want all types that are in the currently loaded assemblies? Assemblies referenced by the executing assembly? There are many different ways to obtain a list of Types, and writing something that would account for (and provide options for) would be a pretty big cost with relatively low benefit.

What are you trying to do? There's likely a better (or at least more efficient) way to accomplish it.
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.