<script type="text/javascript">
function CheckAndUnCheckCheckBoxes(GridviewId, CndChecked) 
{
    var checkIds = getCheckBoxes(document.getElementById(GridviewId));

    for (i = 0; i <= checkIds.length - 1; i++) 
    {
       checkIds[i].checked = CndChecked; 
    }
}
     
function getCheckBoxes(Id) 
{
    var checkboxesArray = new Array(); 
    var inputGrd = Id.getElementsByTagName("input");

    if (inputGrd.length == 0) null; 

    for (i = 0; i <= inputGrd.length -1; i++)
    {            
        if(inputGrd[i].type == "checkbox")
        {
                checkboxesArray.push(inputGrd[i]); 
        }
    }
    return checkboxesArray;    
}    
</script>

< asp:GridView Id="GridView1" runat="server" AutoGenerateColumns="false" >
   < Columns >
       < asp:BoundField DataField="EmployeeCode" HeaderText="Emp Code" / >
       < asp:BoundField DataField="FirstName" HeaderText="First Name" / >
       < asp:BoundField DataField="LastName" HeaderText="Last Name" / >
       < asp:TemplateField >    
        < HeaderTemplate >
          < input type="checkbox" onclick="CheckAndUnCheckCheckBoxes('< %= GridView1.ClientID % >',this.checked)" id="chk" / >
        < /HeaderTemplate >
    
        < ItemTemplate >
          < asp:CheckBox ID="chkSelect" runat="server" / >
        < /ItemTemplate >
      < /asp:TemplateField >
   < /Columns >
< /asp:GridView >

//Connetion string from web.config

string sCon = ConfigurationSettings.AppSettings["Con"].ToString();

protected void Page_Load(object sender, EventArgs e)
{
   if (!Page.IsPostBack)
   {
      BindGrid();
   }
}

private void BindGrid()
{
    DataSet ds = SqlHelper.ExecuteDataset(sCon, "PS_WFM_UserDetails_Get");

    if (ds.Tables.Count > 0)
    {
       if (ds.Tables[0].Rows.Count > 0)
       {
           GridView1.DataSource = ds.Tables[0];
           GridView1.DataBind();
       }
    }
}