Check box within grid view is not selectable

My code is as below and my problem is that when I used this code in Visual Studio
2008 it will not work properly. (I used this in my office, but not working)
Actually problem is that when I try to select the check box it cannot be selected.
Auto post back is true.

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
            BackColor="White" BorderColor="#CC9966" BorderWidth="1px" 
            CellPadding="4" DataKeyNames="Eid,Fname" 
            onrowediting="GridView1_RowEditing" onrowupdating="GridView1_RowUpdating" 
            onselectedindexchanged="GridView1_SelectedIndexChanged" ShowFooter="True" 
            
            style="z-index: 1; left: 126px; top: 262px; position: absolute; height: 140px; width: 448px" 
            onrowcommand="GridView1_RowCommand" AllowSorting="True" 
            onpageindexchanging="GridView1_PageIndexChanging" onsorting="GridView1_Sorting" 
            PageSize="2" AllowPaging="True" BorderStyle="None">
            <pagersettings firstpagetext="First" lastpagetext="Last" nextpagetext="Next">
                PreviousPageText="Prev" />
            <footerstyle backcolor="#FFFFCC" forecolor="#330099" />
            <columns>
                <asp:TemplateField HeaderText="Eid" SortExpression="Eid">
                    <edititemtemplate>
                        <asp:Label ID="Label1" runat="server" Text='<%# Eval("EId") %>'>
                    </edititemtemplate>
                    <footertemplate>
                        <asp:TextBox ID="TextBox4" runat="server" 
                           >
                    </footertemplate>
                    <itemtemplate>
                        <asp:Label ID="Label1" runat="server" Text='<%# Bind("EId") %>'>
                    </itemtemplate>
                


                


                <asp:TemplateField HeaderText="Salary" SortExpression="Salary">
                    <edititemtemplate>
                        <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("Salary") %>'>
                    </edititemtemplate>
                    <footertemplate>
                        <asp:TextBox ID="TextBox5" runat="server" 
                           >
                    </footertemplate>
                    <itemtemplate>
                        <asp:Label ID="Label2" runat="server" Text='<%# Bind("Salary") %>'>
                    </itemtemplate>
                
                <asp:TemplateField HeaderText="Name" SortExpression="Name">
                    <edititemtemplate>
                        <asp:DropDownList ID="DropDownList1" runat="server" 
                            SelectedValue='<%# Eval("FName") %>' 
                            DataSourceID="SqlDataSource1" DataTextField="Fname" DataValueField="Fname">
                        
                        <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
                            ConnectionString="<%$ ConnectionStrings:masterConnectionString %>" 
                            SelectCommand="SELECT [Fname] FROM [Employee]">
                    </edititemtemplate>
                    <footertemplate>
                        <asp:TextBox ID="TextBox6" runat="server" 
                            >
                    </footertemplate>
                    <itemtemplate>
                        <asp:Label ID="Label3" runat="server" Text='<%# Bind("FName") %>'>
                    </itemtemplate>
                
                <%--<asp:CheckBoxField DataField="IsTemp" />--%>
                <asp:TemplateField HeaderText="Edit" ShowHeader="False">
                    <edititemtemplate>
                        <asp:LinkButton ID="LinkButton1" runat="server" CausesValidation="True" 
                            CommandName="Update" Text="Update">
                         <asp:LinkButton ID="LinkButton2" runat="server" CausesValidation="False" 
                            CommandName="Cancel" Text="Cancel">
                    </edititemtemplate>
                    <footertemplate>
                        <asp:LinkButton ID="LinkButton3" runat="server" CommandName="Insert">Insert
                    </footertemplate>
                    <itemtemplate>
                        <asp:LinkButton ID="LinkButton1" runat="server" CausesValidation="False" 
                            CommandName="Edit" Text="Edit">
                    </itemtemplate>
                
                <asp:CommandField ShowDeleteButton="True" />
                <asp:CommandField ShowSelectButton="True" />
                <asp:TemplateField>
                    <HeaderTemplate>
                        <asp:CheckBox ID="CheckBox2" runat="server" AutoPostBack="True" 
                            oncheckedchanged="CheckBox2_CheckedChanged"/>
                    </HeaderTemplate>
                    <itemtemplate>
                        <asp:CheckBox ID="CheckBox1" runat="server" AutoPostBack="True"  OnCheckedChanged="showtxtbox" Checked="False" Enabled="True" EnableViewState="True" ViewStateMode="Inherit" /><asp:TextBox ID="TextBox7" runat="server" Visible="false">
                    </itemtemplate>
                
                <asp:HyperLinkField DataNavigateUrlFields="Salary" 
                    DataNavigateUrlFormatString="Default2.aspx?sal={0}" DataTextField="FName" />
                <asp:ButtonField ButtonType="Button" DataTextField="Salary" Text="Button" />
                <asp:TemplateField HeaderText="txtvalue" SortExpression="txtvalue">
                    <edititemtemplate>
                        <asp:Label ID="Label12" runat="server" Text='<%# Eval("txtvalue") %>'>
                    </edititemtemplate>
                    
                    <itemtemplate>
                        <asp:Label ID="Label12" runat="server" Text='<%# Bind("txtvalue") %>'>
                    </itemtemplate>
                
            </columns>
            
            <pagerstyle backcolor="#FFFFCC" forecolor="#330099">
                HorizontalAlign="Center" />
            <rowstyle backcolor="White" forecolor="#330099" />
            <SelectedRowStyle BackColor="#FFCC66" ForeColor="#663399" Font-Bold="True" />
            <HeaderStyle BackColor="#990000" Font-Bold="True" ForeColor="#FFFFCC" />
            <sortedascendingcellstyle backcolor="#FEFCEB" />
            <sortedascendingheaderstyle backcolor="#AF0101" />
            <sorteddescendingcellstyle backcolor="#F6F0C0" />
            <sorteddescendingheaderstyle backcolor="#7E0000" />

My cs file code is:

protected void showtxtbox(object sender, EventArgs e)
    {
        
        CheckBox cb = (CheckBox)sender;
        GridViewRow gvr=(GridViewRow) cb.NamingContainer;
        int rrowindex = gvr.RowIndex;
        CheckBox cbb = (CheckBox)GridView1.Rows[rrowindex].Cells[6].Controls[1];
        TextBox txt=(TextBox)GridView1.Rows[rrowindex].Cells[6].Controls[2];
       
        
            cbb.Checked=cb.Checked;
           
            if (cbb.Checked)
            {
                txt.Visible = true;
               
                
            }
            if (cbb.Checked == false)
            {
                txt.Visible = false;
            }
        
    }

Tags: ASP.NET