I tried to open Gnome-RDP in Monodevelop, and this is the error that I get:
Could not obtain a C# compiler. C# compiler not found for Mono / .NET 3.5.
The fix is to simply change the compiler target on the project’s properties. Right click on the project and choose “Options”.
Then, change the runtime version.
Changing it to 4.0 fixes the project.
You may need to accept a message like this: