VS 2008 Checkbox In Gridview Header

I wnat to have checkbox in Gridview Header. On the checkbox changed I wnat to check and uncheck all the checkboxes of the Gridview Rows. Currently I am doing it on the button click. But I want to have checkbox in Gridview header, How to have checkbox in Gridview Header,can somebody tell me??

protected void btnCheckAll_Click(object sender, EventArgs e)
{
Check_Uncheck_Checkboxes(true);
}
protected void btnunCheckAll_Click(object sender, EventArgs e)
{
Check_Uncheck_Checkboxes(false);
}
private void Check_Uncheck_Checkboxes(bool bState)
{
CheckBox chk;
foreach (GridViewRow rowItem in GrdCompanies.Rows)
{
chk = (CheckBox)(rowItem.Cells[1].FindControl("chkSelect"));
chk.Checked = bState;
}
}