Update DataTable row If exists else Insert using C# and VB.Net in ASP.Net

I am inserting row from textboxes and DropDownList into GridView.

Now i want that when insert record into GridView then DropDownList value qty sum get display into label at the time of insert, if same description item exit in GridView then it get sum qty then display into label.

html

<asp:TextBox ID="TextBox1" AutoPostBack="true" runat="server" Placeholder="Input Bale No"
    OnTextChanged="TextBox1_TextChanged" Height="16px" Width="100px" Style="margin-left: 0px"></asp:TextBox>
<asp:DropDownList ID="DropDownList1" runat="server" Enabled="false" Height="20px"
    Width="130px">
</asp:DropDownList>
<asp:TextBox ID="txtorderno" Placeholder="Order No" ReadOnly="true" runat="server"
    Height="16px" Width="38px"></asp:TextBox>
<asp:TextBox ID="txtqty" Placeholder="QTY" ReadOnly="true" runat="server" Height="16px"
    Width="21px"></asp:TextBox>
&nbsp;<asp:TextBox ID="txtpid" ReadOnly="true" runat="server" Height="19px" Width="55px"></asp:TextBox>
&nbsp;<asp:Button ID="Button1" runat="server" Text="Add" Visible="false" OnClick="Button1_Click"
    Height="26px" Width="35px" />
<asp:GridView ID="GridView1" runat="server" HeaderStyle-BackColor="#66ccff" HeaderStyle-ForeColor="White"
    CssClass="mygrdContent" PagerStyle-CssClass="pager" HeaderStyle-CssClass="header"
    RowStyle-CssClass="rows" AutoGenerateColumns="false" OnRowDeleting="GridView1_RowDeleting"
    OnRowEditing="GridView1_RowEditing" OnRowUpdating="GridView1_RowUpdating" AllowSorting="True"
    OnRowDataBound="GridView1_RowDataBound" ShowFooter="True">
    <Columns>
        <asp:TemplateField HeaderText="Bale_No">
            <ItemTemplate>
                <asp:Label ID="Prdno" runat="server" Text='<%#Bind("Prdno")%>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Description">
            <ItemTemplate>
                <asp:Label ID="Descriptionitem" runat="server" Text='<%#Bind("Descriptionitem")%>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Orderno">
            <ItemTemplate>
                <asp:Label ID="Orderno" runat="server" Text='<%#Bind("Orderno")%>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="QTY">
            <ItemTemplate>
                <asp:Label ID="QTY" runat="server" Text='<%#Bind("QTY")%>'></asp:Label>
            </ItemTemplate>
            <FooterTemplate>
                <asp:Label ID="lblTotal" runat="server"></asp:Label></FooterTemplate>
        </asp:TemplateField>
        <asp:CommandField ShowDeleteButton="True" ButtonType="Button" />
    </Columns>
    <HeaderStyle BackColor="#66CCFF" ForeColor="White"></HeaderStyle>
    <SelectedRowStyle BackColor="Yellow" />
</asp:GridView>

C#

private void insertbalgv()
{
    if (txtpid.Text != "" && txtpid.Text != "1")
    {
        Response.Write("<script language='javascript'>alert('Bale Already Packed');</script>");
    }
    else if (txtOrder.Text == "")
    {
        Response.Write("<script language='javascript'>alert('Please Input Orderno');</script>");
        Label1.Text = "";
        TextBox1.Text = "";
        txtqty.Text = "";
        txtorderno.Text = "";
        DropDownList1.SelectedValue = "";
    }
    else
    {
        if
                (txtorderno.Text == "")
        {
            Response.Write("<script language='javascript'>alert('Please Input Bale No');</script>");
        }
        else if (txtqty.Text == "")
        {
            Response.Write("<script language='javascript'>alert('Please Input QTY');</script>");
        }
        else if (txtqty.Text != "1")
        {
            Response.Write("<script language='javascript'>alert('QTY is more then 1 ');</script>");
        }
        else if ((int.Parse(txtOrder.Text) != (int.Parse(txtorderno.Text))))
        {
            lbmsg.Text = "Bale is Not in this order";
            txtorderno.ForeColor = Color.Red;
            lbpk.Text = "";
            Label1.Text = "";
        }

        else if (txtorderno.Text == "")
        {
            Response.Write("<script language='javascript'>alert('Please Input Orderno');</script>");
        }

        else if (DropDownList1.SelectedValue == "")
        {
            Response.Write("<script language='javascript'>alert('Please Input Item');</script>");
        }
        else
        {
            {
                if (DropDownList1.SelectedItem != null)
                {
                    // get value and text which item you have selected
                    int codeitem = Convert.ToInt32(DropDownList1.SelectedItem.Value.ToString());
                    string Descriptionitem = DropDownList1.SelectedItem.Text.ToString();


                    //add the selected item to gridview

                    DataTable dtgridview = ViewState["dt"] as DataTable;
                    DataRow dr2 = dtgridview.NewRow();
                    bool ifExist = false;
                    foreach (DataRow dr in dtgridview.Rows)
                    {
                        if (dr["Prdno"].ToString() == TextBox1.Text.Trim())
                        {
                            ifExist = true;
                            break;
                        }
                    }
                    if (!ifExist)
                    {
                        dr2["Prdno"] = TextBox1.Text.Trim();
                        dr2["Codeitem"] = codeitem;
                        dr2["Descriptionitem"] = Descriptionitem;
                        dr2["Orderno"] = txtorderno.Text;

                        dr2["QTY"] = txtqty.Text;
                        if (dtgridview.Rows.Count < Convert.ToInt16(DDLPktype.SelectedItem.Value))
                        {
                            dtgridview.Rows.Add(dr2);
                        }
                        else
                        {
                            ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('You can not insert more than " + dtgridview.Rows.Count + " rows')", true);
                        }
                        // dtgridview.Rows.Add(dr2);
                        ViewState["dt"] = dtgridview;
                        GridView1.DataSource = dtgridview;
                        GridView1.DataBind();
                    }
                    else
                    {
                        // this.lbgvck.Visible = true;
                        lbgv.Text = "Bale Already Added...";
                    }

                    clear();
                }
            }
        }
    }
}