How do I ensure C#'s Process.Start will expand environment variables?

I'm attempting to create a process like so:

var psi = new ProcessStartInfo
{
    FileName = @"%red_root%\bin\texturepreviewer.exe",
    UseShellExecute = true
};

var proces开发者_如何学编程s = Process.Start(psi);
process.WaitForExit();

Now the environment variable "red_root" definitely exists in the spawned process' environment variables, but the execute doesn't seem to expand the environment variable and so the file isn't found. How can I get the Process.Start to expand the environment variable in the file name?

The Environment.ExpandEnvironmentVariables method should help here.

Replaces the name of each environment variable embedded in the specified string with the string equivalent of the value of the variable, then returns the resulting string.

string unexpandedPath = "%red_root%\\bin\\texturepreviewer.exe";   
psi.FileName = Environment.ExpandEnvironmentVariables(unexpandedPath);

Have you tried System.Environment.GetEnvironmentVariable ("red_root", EnvironmentVariableTarget.Machine) ?