HttpContext.Current.User.Identity.Name in C# may just end up null at IIS7 hosting.

But, it doesn't have to be so. I think I've found a fix here and I offer the following eight steps, copied from the bottom of the page, which worked for me.

1. Set the DisableStrictNameChecking registry entry to 1. For more information about how to do this, click the following article number to view the article in the Microsoft Knowledge Base: 281308 (http://support.microsoft.com/kb/281308/ ) Connecting to SMB share on a Windows 2000-based computer or a Windows Server 2003-based computer may not work with an alias name
2. Click Start, click Run, type regedit, and then click OK.
3. In Registry Editor, locate and then click the following registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0
4. Right-click MSV1_0, point to New, and then click Multi-String Value.
5. Type BackConnectionHostNames, and then press ENTER.
6. Right-click BackConnectionHostNames, and then click Modify.
7. In the Value data box, type the host name or the host names for the sites that are on the local computer, and then click OK.
8. Quit Registry Editor, and then restart the IISAdmin service.