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Process.Start() issues with Mono on Mac OS X: My Story

Sat 20 Feb 2010

Over the last month or so I have been spending my time writing .NET code for the new Selenium 2 Bindings. I have spent most of my time working on the Google Chrome Driver. But below is the story of 1 Selenium bug and 1 Mono bug and my exploration of how processes work in .NET on a Mac.

I was quite confident about the .NET bindings since it was extremely easy to get them working on my laptop. I admit I got nervous about how it would work on Non-Windows platforms since it was my first time writing .NET code on a platform that isn't windows. I shouldn't have been nervous. It was fairly easy to install Mono and MonoDevelop on my Ubuntu VM and ran the NUnit tests that we have. Google Chrome started and we had tests passing. Brillant because there was little code changes, I just updated the code where to find the Google Chrome application.

I was confident until I started working Mac OS X. I have very little experience on a Mac. Other than using it to run Selenium Grid or using it with a Browser I know very little about how the OS works.

On the Mac I installed Mono and MonoDevelop and fired them up. I went in and updated the code so that if the .NET code was running on a Mac it knew where to find the Google Chrome application and would then start up the process using the standard Process.Start() method from System.Diagnostics library. Feeling extremely confident I fired up NUnit and clicked Run. They all failed. Why? Well the browser wasn't loading. Selenium without a browser, excluding when used with HTMLUnit, isn't going to work.

I started debugging my code to see why the browser wasn't loading. The first issue that I found was the Selenium 2 .NET code wasn't recognising the Mac OS platform. We have logic that checks which platform you are on and we hadn't put a conditional for Mac OS. Updated my code and ran the tests again and they went red again. Why? No browser again.

Debugging again I came across a rather large and very annoying bug in Mono. Mono when running on Mac OS platform reports that it is running on Unix/Linux. This isn't too ridiculous since Mac OS is based on BSD however it means now that when ever the code needs to launch the browser it needs to up to 3 hits to the disk to see if it is where I think its. It also means the fix I added in Selenium earlier doesn't really matter. Anyway the code now has the ability to launch the browser...or does it? It doesn't as my tests are still failing.

I started debugging again. The error I was getting was that I was trying to open the application and open wouldn't accept the arguments I was passing in because they are not valid for Open. Since I have never developed on a Mac I had to do some research and found out that the defaults for Process.Start() want to Open the file rather than execute it. This is perfect if you want to open text files but not applications. So I tried to see about creating a bash script to start up Google Chrome. One obvious downside if it did start up was that I wouldn't have an easy way to kill it. I was starting to think that I was going to be doing Yak Shaving to get this to work.

I next had a look at using Monobjc. It is a library that allows you to write Cocoa code in .NET and there it uses a bridge to get your code working. It is something that one of the Watin guys had done to get Watin working on a Mac. After a lot of pulling my hair out for a while I gave up and asked a question on StackOverflow.

The 5th answer I was given, and the one I accepted, was that I had forgot to set a property called UseShellExecute. My code for starting up went from

chromeProcess = Process.Start(GetChromeFile(), String.Concat(Arguments, serverUrl));


chromeProcess = Process.Start(new ProcessStartInfo(GetChromeFile(), String.Concat(Arguments, serverUrl)) { UseShellExecute = false });

The subtle change of explicitly creating a new instance of ProcessStartInfo and updating the property { UseShellExecute = false } meant that my code went from opening an application to executing a application. The difference tells Process not to open a shell according to MSDN but on a Mac means it does what I want. I hope this is helpful for anyone else in the future.

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