<asp:GridView ID="gvAll" runat="server"
    AutoGenerateColumns = "false" Font-Names = "Arial"
    Font-Size = "11pt" AlternatingRowStyle-BackColor = "#C2D69B" 
    HeaderStyle-BackColor = "green" AllowPaging ="true"  
    OnPageIndexChanging = "OnPaging" DataKeyNames = "XXXXXXX"
    PageSize = "10" >

   <Columns>

    <asp:TemplateField>
        <HeaderTemplate>

            <asp:CheckBox ID="chkAll" runat="server"
             onclick = "checkAll(this);" />

        </HeaderTemplate>
        <ItemTemplate>

            <asp:CheckBox ID="chk" runat="server"
             onclick = "Check_Click(this)"/>

        </ItemTemplate>
    </asp:TemplateField>

    <asp:BoundField ItemStyle-Width = "150px" DataField = "XYZ"
       HeaderText = "XYZ"/>

    <asp:BoundField ItemStyle-Width = "150px" DataField = "ABC"
       HeaderText = "ABC"/>

    <asp:BoundField ItemStyle-Width = "150px" DataField = "123"
       HeaderText = "123"/>

   </Columns>

   <AlternatingRowStyle BackColor="#C2D69B"  />

</asp:GridView>

<asp:HiddenField ID="hfCount" runat="server" Value = "0" />

<asp:Button ID="btnDelete" runat="server" Text="Delete Checked Records"
   OnClientClick = "return ConfirmDelete();" OnClick="btnDelete_Click" />

private void BindGrid()
{
    string constr = ConfigurationManager
                .ConnectionStrings["conString"].ConnectionString;
    string query = "select * from Table1";
    SqlConnection con = new SqlConnection(constr);
    SqlDataAdapter sda = new SqlDataAdapter(query, con);
    DataTable dt = new DataTable();
    sda.Fill(dt);
    gvAll.DataSource = dt;
    gvAll.DataBind();

}

private void GetData()
{
    ArrayList arr;

    if (ViewState["SelectedRecords"] != null)
        arr = (ArrayList)ViewState["SelectedRecords"];
    else
        arr = new ArrayList();

    CheckBox chkAll = (CheckBox)gvAll.HeaderRow
                        .Cells[0].FindControl("chkAll");

    for (int i = 0; i < gvAll.Rows.Count; i++)
    {
        if (chkAll.Checked)
        {
            if (!arr.Contains(gvAll.DataKeys[i].Value))
            {
                arr.Add(gvAll.DataKeys[i].Value);
            }
        }
        else
        {
            CheckBox chk = (CheckBox)gvAll.Rows[i]
                               .Cells[0].FindControl("chk");
            if (chk.Checked)
            {
                if (!arr.Contains(gvAll.DataKeys[i].Value))
                {
                    arr.Add(gvAll.DataKeys[i].Value);
                }
            }
            else
            {
                if (arr.Contains(gvAll.DataKeys[i].Value))
                {
                    arr.Remove(gvAll.DataKeys[i].Value);
                }
            }
        }
    }
    ViewState["SelectedRecords"] = arr;
}

/*GetData is called in the Page Load event of the ASP.Net web page in the following way*/

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

// setdata function

private void SetData()
{
    int currentCount = 0;
    CheckBox chkAll = (CheckBox)gvAll.HeaderRow
                            .Cells[0].FindControl("chkAll");
    chkAll.Checked = true;
    ArrayList arr = (ArrayList)ViewState["SelectedRecords"];
    for (int i = 0; i < gvAll.Rows.Count; i++)
    {
        CheckBox chk = (CheckBox)gvAll.Rows[i]
                        .Cells[0].FindControl("chk");
        if (chk != null)
        {
            chk.Checked = arr.Contains(gvAll.DataKeys[i].Value);
            if (!chk.Checked)
                chkAll.Checked = false;
            else
                currentCount++;
        }
    }
    hfCount.Value = (arr.Count - currentCount).ToString(); 
}