private string GetUserName()
    {
        string retVal = string.Empty;
        //Pull the username out of the domain\user string.  
        retVal = Page.User.Identity.Name.Split('\\')[1]; 
        
        DirectoryEntry userEntry = new DirectoryEntry("WinNT://" + userName +",User");
        retVal = (string)userEntry.Properties["fullname"].Value;
        return retVal; 
    }

Typically won't work if the ASP.Net service identity (in the machine.config ProcessModel section) is a local account.

ASP.Net Code to Get an Active Directory User Name from the Integrated Authentication Principal Name

security, aspnet

C#