In WCF how do I access the user the service is running as?

I have a WCF service hosted in IIS and want to gain access to the user the service runs as - not the caller. Is this possible?

I know about the aspNetCompatibilityEnabled, but that gives me the user calling the service, not the user the servic开发者_开发知识库e is running under.

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How about System.Environment.UserName?