Hi everyone. I have problem. I selected dropdown category and fill gridview. Then before filled second gridview data lost.  How can I do this. My html code shared. 

<asp:DropDownList ID="drpKategori" Style="width: 25%" CssClass="form-control" runat="server"
        OnSelectedIndexChanged="drpKategori_SelectedIndexChanged" AutoPostBack="true">
    </asp:DropDownList>
</div>
<div class="form-group">
    <asp:Button ID="addRow" runat="server" Text="Yeni Satır" />
    <asp:GridView ID="example" CssClass="table table-striped table-bordered table-hover"
        AutoGenerateColumns="false" DataKeyNames="ID" runat="server" ClientIDMode="Static"
        OnPreRender="example_PreRender" OnRowEditing="example_RowEditing" OnRowCancelingEdit="example_RowCancelingEdit"
        OnRowDeleting="example_RowDeleting" OnRowDataBound="example_RowDataBound">
        <Columns>
            <asp:TemplateField HeaderText="Ürün Seç" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:CheckBox ID="chcsec" CssClass="form-control" runat="server" AutoPostBack="true"
                        OnCheckedChanged="chcsec_CheckedChanged" />
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Ürün Ad" ItemStyle-Width="100">
                <ItemTemplate>
                    <asp:Label ID="lblUrun" runat="server" Text='<%# Eval("UrunAd") %>'></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txturunad" runat="server" Text='<%# Eval("UrunAd") %>'></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Stok" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="lblStok" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("Stok") %>'></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtstok" Style="padding: 3px 5px;" runat="server" Text='<%# Eval("Stok") %>'></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Fiyat" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="txtfiyat" Style="padding: 3px 5px;" CssClass="form-control" runat="server">500</asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtfiyat" Style="padding: 3px 5px;" runat="server" CssClass="form-control"></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Adet" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="txtAdet" Style="padding: 3px 5px;" CssClass="form-control" runat="server"></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtAdet" runat="server" Style="padding: 3px 5px;" CssClass="form-control"></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Eleman Seç" ItemStyle-Width="100">
                <ItemTemplate>
                    <asp:DropDownList ID="drpEleman" CssClass="form-control" runat="server">
                    </asp:DropDownList>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Eleman Adet" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="txtElemanAdet" Style="padding: 3px 5px;" CssClass="form-control" runat="server"></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtElemanAdet" runat="server" Style="padding: 3px 5px;" CssClass="form-control"></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Eleman Fiyat" ItemStyle-Width="50">
                <ItemTemplate>
                    <asp:Label ID="txtElemanFiyat" Style="padding: 3px 5px;" CssClass="form-control"
                        runat="server"></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtElemanFiyat" runat="server" Style="padding: 3px 5px;" CssClass="form-control"></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:CommandField ButtonType="Link" ShowEditButton="true" ShowDeleteButton="true"
                ItemStyle-Width="150" />
        </Columns>
    </asp:GridView>

My Checkbox Change. 

 protected void chcsec_CheckedChanged(object sender, EventArgs e)
{
    //List<object> datakeys = ViewState["datakeys"] != null ?
    //(List<object>)ViewState["datakeys"] : new List<object>();

    DataTable dt = new DataTable();
    //dt.Columns.Add("ID", typeof(int));
    dt.Columns.Add("UrunAd", typeof(string));
    dt.Columns.Add("Stok", typeof(string));

    //dt.Columns.Add("ID", typeof(int));
    //dt.Columns.Add("ID", typeof(int));
    //dt.Columns.Add("ID", typeof(int));
    //dt.Columns.Add("ID", typeof(int));
    DataTable dti = null;
    foreach (GridViewRow rw in example.Rows)
    {
        if (rw.RowType == DataControlRowType.DataRow)
        {
            CheckBox chk = (CheckBox)rw.Cells[0].FindControl("chcsec");
            if (chk.Checked)
            {
                Label lb = (Label)rw.Cells[1].FindControl("lblUrun");
                Label lb1 = (Label)rw.Cells[2].FindControl("lblStok");

                //int id = Convert.ToInt32(lb.Text.ToString());
                //string name = lb.ToString();
                //string stok = lb1.ToString();
                dt.Rows.Add(lb.Text, lb1.Text);

                dti = (DataTable)ViewState["datakeys"];

                //datakeys.Add(example.DataKeys[rw.RowIndex].Value);
            }
            //else
            //{
            //    datakeys.Remove(example.DataKeys[rw.RowIndex].Value);
            //}
        }
    }
    gvSiparis.DataSource = dti;
    gvSiparis.DataBind();
}