Details about ApplicationUnderTest.Launch() method's fileName and alternateFileName arguments - coded-ui-tests

Question

I'm having a hard time trying to figure out some details about the arguments of the ApplicationUnderTest.Launch(String, String) method. From the MSDN page, they are described like this:
fileName
Type: System.String
The file name of the application to start.
alternateFileName
Type: System.String
The alternate file name that has environment variables.
My questions are:
What is the purpose of alternateFileName? Is it only used when fileName is not found or not valid? Does the Launch() method always try the fileName first?
Does fileName support environment variables in it? Does alternateFileName always require at least one environment variable in it?

Answer 1

Based on my own experience (and a little help from a decompiler), here's what I've discovered.
What is the purpose of alternateFileName?
alternateFileName allows you to use a path with environment variables that are expanded at run time. There is also some special handling to convert environment variables only found on a 64-bit environment to those found on a 32-bit environment, if necessary (for example, converting %programfiles(x86)% to %programfiles%).
Is it only used when fileName is not found or not valid? Does the Launch() method always try the fileName first?
If you use one of the Launch() overloads that include the alternateFileName parameter, it is always checked first. If there is a matching file at the expanded path, then that path is used and fileName is never checked. If there is no matching file at the expanded alternateFileName path, only then will it use fileName.
Note: I just discovered a bug present in Visual Studio 2013 Update 4 where alternateFileName is in fact incorrectly handled on a 32-bit system. In that case, alternateFileName was always getting prepended with the value C:\Windows\System32 so it would end up with a value like C:\Windows\System32C:\Program Files\foo\bar.exe. Since this is an invalid value, the system was only relying on fileName.
Does fileName support environment variables in it?
No, any environment variables in fileName are not expanded.
Does alternateFileName always require at least one environment variable in it?
No, environment variables are not required -- although I can't think of how it would be useful to supply a value for alternateFileName without them.

Answer 2

Here's what I do:
private static ApplicationUnderTest _application;
public static ApplicationUnderTest LaunchApplicationUnderTest(string applicationPath,
bool closeOnPlaybackCleanup)
{
Process[] processes = Process.GetProcessesByName("MyApp");
if (processes.Length > 0)
{
_application = ApplicationUnderTest.FromProcess(processes[0]);
}
else
{
_application = ApplicationUnderTest.Launch(applicationPath);
_application.CloseOnPlaybackCleanup = closeOnPlaybackCleanup;
}
return _application;
}