The type or namespace name 'Xrm' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?) when build for release

I'm creating some .Net components that read/write CRM data, but during building it fails.

I've looked at some tutorials and started. All works fine, adding references no problem.

Changed target from '.Net framework 4 client profile' to '.Net fromework 4' and build in debug mode.
0 errors!!!!

Now I changed it to release build and I get 29901 errors. The one that causes the problem in my opinion
is the message 'The type or namespace name 'Xrm' does not exist in the namespace 'Microsoft' (are you missing an assembly reference?)'

Few questions:
- Can I build the custom workflow step in release mode
- If so, what causes my errors?