The name 'HttpContext' does not exist in the current context

put using System.Web; and using System; into the source file...

You need [] instead of ():

string[] strUserInitials = System.Web.HttpContext.Current.Request.ServerVariables["LOGON_USER"].Split(System.Convert.ToChar(@"\"));

You have to reference to System.Web and import the namespace System.Web:

using System.Web;

I would not use Convert at all:

string[] strUserInitials = System.Web.HttpContext.Current.Request.ServerVariables["LOGON_USER"].Split('\\'));