I recently installed Xamarin Studio, and the next time I went to run brew doctor, I got this warning:
Warning: /Library/Frameworks/Mono.framework detected
This can be picked up by CMake's build system and likely cause the build to fail. You may need to move this file out of the way to compile CMake.
So I moved Mono.framework out of the way to my desktop and that fixed the problem, but when I went to open Xamarin Studio, it said:
Could not launch Xamarin Studio
This application requires the Mono framework.
Please download and install the latest version of Mono.
So then I installed mono with Homebrew, but Xamarin Studio still has the same message when opening.
How can I get Xamarin Studio to use Homebrew's version of Mono?
You can and should leave Mono.framework where you found it in /Library/Frameworks.
If you later discover that you need to build CMake yourself (which is unlikely; Homebrew now typically installs CMake as a pre-compiled "bottle") and you actually encounter an error while trying to rebuild it, then you should move Mono.framework out of the way temporarily, until the CMake build has finished.
I solved this by installing mono-mdk instead of mono from homebrew.
brew cask install mono-mdk
You can also modify your PATH to use mono-mdk's binary:
export PATH=/Library/Frameworks/Mono.framework/Versions/Current/Commands:$PATH
来源：https://stackoverflow.com/questions/23553233/how-to-use-homebrews-version-of-mono-with-xamarin