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: 
Asuming baseType contains a System.Type object that you want to check against and matchType contains a System.Type object with the type of your current iteration (through a foreach-loop or whatever):

If you want to check wheather matchType is derived from the class represented by baseType I'd use 

```
matchType.IsSubclassOf(baseType)

```

And if you want to check wheather matchType implements the interface represented by baseType I'd use

```
matchType.GetInterface(baseType.ToString(), false) != null

```

Of course I'd store baseType.ToString() as a global variable so I wouldn't need to call it all the time. And since you probably would need this in a context where you have a lot of types, you could also consider using the System.Threading.Tasks.Parallel.ForEach-Loop to iterate through all your types...
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.