select all the checkbox in agridview

Question

Hi frnz
           how can i select all the checkboxes in agridview
<asp:GridView ID="gvApplicable" runat="server" AutoGenerateColumns="False"
AllowSorting="True" AllowPaging="True" PageSize="15" onrowediting="gvApplicable_RowEditing"
onpageindexchanging="gvApplicable_PageIndexChanging" Height="127px"
onselectedindexchanged="gvApplicable_SelectedIndexChanged" Width="213px" >
<Columns>
<asp:TemplateField >
HeaderTemplate>
<asp:CheckBox ID="chkSelectAll" runat="server" AutoPostBack="true"
              OnCheckedChanged="chkSelectAll_CheckedChanged"/>
</HeaderTemplate>

<ItemTemplate>
<asp:CheckBox ID="Chk1" runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField ReadOnly="true" DataField="DeptCode" HeaderText="Code" InsertVisible="false" SortExpression="Code"></asp:BoundField>
<asp:BoundField ReadOnly="true" DataField="DeptName" HeaderText="DeparmentName" InsertVisible="false" SortExpression="Name"></asp:BoundField>
</Columns>
</asp:GridView>