try this code.
<asp:button id="btn_submit" runat="server" text="submit" xmlns:asp="#unknown">
<asp:label id="lbl" runat="server" xmlns:asp="#unknown">
protected void Page_Load(object sender, EventArgs e)
{
if (Page.IsPostBack)
{
btn_submit.Click += new EventHandler(btn_submit_Click);
}
}
void btn_submit_Click(object sender, EventArgs e)
{
lbl.Text = "hi";
//throw new NotImplementedException();
}