Delete multiple rows in GridView with CheckBox selection and with confirmation using C# and VB.Net in ASP.Net

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
            CellPadding="4" ForeColor="#333333" GridLines="None"  Class="table table-striped table-bordered table-hover" 
            onrowcancelingedit="GridView1_RowCancelingEdit" 
            onrowediting="GridView1_RowEditing" onrowupdating="GridView1_RowUpdating" 
            onrowdatabound="GridView1_RowDataBound">
            <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />
            <Columns>
               
                <asp:TemplateField>
                    <HeaderTemplate>
                      <asp:CheckBox ID="checkAll" runat="server" onclick = "checkAll(this);" />
                    </HeaderTemplate>
                   <ItemTemplate>
                     <asp:CheckBox ID="CheckBox1" runat="server" onclick = "Check_Click(this)" />
                   </ItemTemplate>
                </asp:TemplateField>
               
               <asp:TemplateField HeaderText="Reg No.">
               <ItemTemplate >
                   <asp:Label ID="lbl_AdNo" runat="server" Text='<%# Eval("AdmissionNo") %>' ></asp:Label>
               </ItemTemplate>
               </asp:TemplateField>

               <asp:TemplateField HeaderText="Student Name">
                <ItemTemplate >
                    <asp:Label ID="Label1" runat="server" Text='<%# Eval("SName") %>'></asp:Label>
                </ItemTemplate>               
                </asp:TemplateField>

                 <asp:TemplateField HeaderText="Father Name">
                <ItemTemplate >
                    <asp:Label ID="Label2" runat="server" Text='<%# Eval("FName") %>'></asp:Label>
                </ItemTemplate>               
                </asp:TemplateField>
                 
                <asp:TemplateField HeaderText="Subject Name" >
                <ItemTemplate >
                  <asp:Label ID="Label3" runat="server" Text='<%# Eval("SubjectName") %>' Width="75"></asp:Label>
                </ItemTemplate>
                </asp:TemplateField>

                    <asp:TemplateField HeaderText="Total Marks" >
                <ItemTemplate >
                  <asp:Label ID="Label4" runat="server" Text='<%# Eval("MaxMarks") %>' Width="75"></asp:Label>
                </ItemTemplate>
                </asp:TemplateField>

                <asp:TemplateField HeaderText="Pass Marks" >
                <ItemTemplate>
                    <asp:Label ID="Label5" runat="server" Text='<%# Eval("PassMarks") %>'></asp:Label>
                </ItemTemplate>
                </asp:TemplateField>

               <asp:TemplateField HeaderText="Marks" >
                <ItemTemplate>
                    <asp:Label ID="Label6" runat="server" Text='<%# Eval("Marks") %>'></asp:Label>
                </ItemTemplate>
                <EditItemTemplate >
                    <asp:TextBox ID="txtMarks" runat="server" Text='<%# Eval("Marks") %>' Width="75"></asp:TextBox>
                </EditItemTemplate>
                </asp:TemplateField>

               <asp:TemplateField HeaderText="Edit" ShowHeader="false">
                    <ItemTemplate>
                        <asp:LinkButton ID="btnedit" runat="server" CommandName="Edit" Text="Edit" ></asp:LinkButton>
                    </ItemTemplate>
                    <EditItemTemplate>
                        <asp:LinkButton ID="btnupdate" runat="server" CommandName="Update" Text="Update" ></asp:LinkButton>
                        <asp:LinkButton ID="btncancel" runat="server" CommandName="Cancel" Text="Cancel"></asp:LinkButton>
                    </EditItemTemplate>
                  </asp:TemplateField>
            </Columns>
            <FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
            <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />
            <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />
            <HeaderStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
            <EditRowStyle BackColor="#CCCCCC" />
            <AlternatingRowStyle BackColor="White" ForeColor="#284775" />
        </asp:GridView>

and I want to delete selected record and code is :

protected void btnDelete_Click(object sender, EventArgs e)
    {
        for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
        {
            GridViewRow row = GridView1.Rows[i];
            CheckBox Chbox = (CheckBox)row.FindControl("CheckBox1");
            if (Chbox.Checked == true)
            {
                select++;
            }
        }

        if (select == 0)
        {
            ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Please Select Record to Delete');", true);
            
        }

        for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
        {

            string AdNo = GridView1.Rows[i].Cells[1].Text;
           // Label lbl_AdNo = GridView1.Rows[e.RowIndex].FindControl("lbl_AdNo") as Label;
            GridViewRow row = GridView1.Rows[i];
            CheckBox Chbox = (CheckBox)row.FindControl("CheckBox1");
            if (Chbox.Checked == true)
            {
                con = new SqlDbConnect();
                con.SqlQuery("delete from tblSetMarks where AdmissionNo=@AdNo");
                con.Cmd.Parameters.AddWithValue("@AdNo", AdNo);
                con.NonQueryEx();
               // GridView1.Visible = false;
                
            }
        }
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Student's Marks Deteleted Successfully');", true);
        this.BindSectionGrid(); 
    }

This is not deleting the record what is the error in the code