Q:

How to get the Debug.WriteLine?

is there a possibility to get all things that are write in Debug.WriteLine in a TextBox or something like this?

A:

It's conceivably possible to write a custom trace listener that could do this. 
Alternatively, why not explore something like log4net -- this gives some very flexible approaches to logging that are much more sophisticated than the default logging in the framework. It can even send its output over UDP to an external listener...

