i neeed to assign string to repeater label on button click but it is showing error object reference not set
//Label date = (Label)Repeater.FindControl("ltpl_datefor");
Label date = Repeater.Items[0].FindControl("ltpl_datefor") as Label;
date.Text = dateforloop.ToString().Substring(0);
<asp:Repeater ID="Repeater" runat="server">
<HeaderTemplate>
</HeaderTemplate>
<ItemTemplate>
<table width="95%" align="center" cellpadding="1" cellspacing="0" border="1">
<tr>
<td align="center" width="5%">
<%#(((RepeaterItem)Container).ItemIndex+1).ToString()%>
</td>
<td width="15%">
<asp:Label ID="ltpl_datefor" runat="server"></asp:Label>
</td>
<td width="80%" align="Center" colspan="2">
<asp:Repeater ID="Repeater1" runat="server">
<HeaderTemplate>
</HeaderTemplate>
<ItemTemplate>
<table border="1" width="98%">