I'd like to know if there is a way to mix C# and Obj-C code in one project. Specifically, I'd like to use Cocos2D for my UI in Obj-C and call some MonoTouch C#-Library that does some computations and get some values back. Is there a way to do this? Or maybe the other way around, i. e. building in MonoTouch and calling Cocos2D-functions? Thanks.
The setup that you describe is possible, but the pipeline is not as smooth as it is when you do your entire project in MonoTouch. This is in fact how we bootstrapped MonoTouch: we took an existing Objective-C sample and we then replaced the bits one by one with managed code.
We dropped those samples as they bitrot.
But you can still get this done, use the mtouch's --xcode command line option to generate a sample program for you, and then copy the bits that you want from the generated template.m into your main.m. Customize the components that you want, and just start the XCode project from there.
During your development cycle, you will continue to use mtouch --xcode