The way Mono and Xcode work together is less than perfect. MonoDevelop will create copies of the project that are used by Xcode for the user interface, etc., And the debugger also uses the Xcode system.
I had this problem and fixed it by closing MonoDevelop and dragging the .obj and .bin directories to the trash and freeing the trash before performing a complete rebuild of the project.
It also seems to fix other errors, for example, when Xcode refuses to edit the XIB when MonoDevelop is called.
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