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".
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.