populate loggedin username in master page

I am trying to display logged username in all the pages. So trying to do that in master page.
 I can do that in a single page using codebehind .
single page aspx:
<td>          Logged as <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>          </td>
cs:
protected void Page_Load(object sender, EventArgs e)     {         NameValueCollection coll = HttpContext.Current.Request.Headers;         TextBox2.Text = coll["SMUSER"];       }
I tried like below but nothing populated. Any help much appreciated.
Masterpage:
<body>     <form runat="server">     <div class="page" ...