单个应用程序实例的WPF互斥锁无法正常工作

I'm trying to use the mutex method for only allowing one instance of my app to run. That is - I only want a max of one instance for all users on a machine. I've read through the various other threads on this issue and the solution seems straightforward enough but in testing I can't get my second instance to not run. Here is my code...I'm trying to use the mutex method for only all