This is because DNN was created in Visual Basic codebase. So the short answer will be, you can't. However, DotNetNuke Corp.
announced[
^] that they will be converting their codebase from VB.Net to C#.
It seems the project is not yet available although they mentioned that they will be releasing it Q2 of 2011.
Update: The code is already available
here[
^] :doh: