System.Environment.GetEnvironmentVariable("UserName") not working

I need to retrieve the windows login name in my asp.net application. I put System.Environment.GetEnvironmentVariable("UserName") in and it works only when I am debugging my application. However, when I try to run it from the url, and do a response.write it gives an empty string for the "username",  Does anyone know why this happens or how I can get this to work????

Thanks.

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