I would like to write to the file system in a directory C: \ directory name, for the moment I have this code:

if (! System.IO.File.Exists (HttpContext.Current.Server.MapPath (filename))) 
{ 
   TXTFile = new System.IO.StreamWriter (HttpContext.Current.Server.MapPath (filename)); 
} 
else 
{ 
   TXTFile = System.IO.File.AppendText (HttpContext.Current.Server.MapPath (filename)); 
}

but in this way writes on the application folder. How to fix it?