Unable to edit DDL of the row

Sorry, where is the 'status' I should adjust?

Sorry, please disregard my last reply.

if (Session["status"] != "Edit")
        ddl.Enabled = false;
else
        ddl.Enabled = true;
Your code checks for the "Edit" value in the session variable 'status'. If it is "Edit", it enables it.

So, you are somewhere setting the value of 'status' variable. Debug and see if the Session["status"] gives the value "Edit"

Sorry, once I've selected one value to that DDL, I get this

Server Error in '/Start_dev' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 


[NullReferenceException: Object reference not set to an instance of an object.]
   Start._Default.ddl_IndexChanged(Object sender, EventArgs e) +143
   System.Web.UI.WebControls.ListControl.OnSelectedIndexChanged(EventArgs e) +129
   System.Web.UI.Page.RaiseChangedEvents() +332
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3571

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.34237

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while this is the event

        protected void ddl_IndexChanged(object sender, EventArgs e)
        {
            DropDownList ddl = (sender as DropDownList);
            GridViewRow row = ddl.NamingContainer as GridViewRow;
            TextBox tb2 = row.FindControl("tb_ite_code") as TextBox;
            tb2.Text = ddl.SelectedItem.Text;
        }
...

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why?

Your code is not finding the DropDownList. What event is this code attached to?

Sorry, I click the DDL of 1st field of Gridview and the current error comes out. Why?

did you write the code for ddl_IndexChanged? if not write the code for it and check

One last thing, here is Gridview

    <asp:GridView ID="GridView1" runat="server"  Width = "550px"
    AutoGenerateColumns = "false" Font-Names = "Arial"
    Font-Size = "11pt" AlternatingRowStyle-BackColor = "#C2D69B" 
    HeaderStyle-BackColor = "green" AllowPaging ="true" Data  
    ShowFooter = "true" OnPageIndexChanging="GridView1_PageIndexChanging" 
    OnRowCancelingEdit="GridView1_RowCancelingEdit" OnRowEditing="GridView1_RowEditing" 
    OnRowUpdating="GridView1_RowUpdating" PageSize = "10" OnRowDeleting="GridView1_RowDeleting" OnRowDataBound="GridView1_RowDataBound">
    <Columns>
    <asp:TemplateField HeaderText="Currency">
        <ItemTemplate>
            <asp:DropDownList ID="ddl" OnSelectedIndexChanged="ddl_IndexChanged" AutoPostBack="true" runat="server">
            </asp:DropDownList>
        </ItemTemplate>
        <EditItemTemplate>
            <asp:DropDownList ID="ddl2" OnSelectedIndexChanged="ddl2_IndexChanged" AutoPostBack="true" runat="server">
            </asp:DropDownList>
        </EditItemTemplate>
        <FooterTemplate>
            <asp:DropDownList ID="ddl3" OnSelectedIndexChanged="ddl3_IndexChanged" AutoPostBack="true" runat="server">
            </asp:DropDownList>
        </FooterTemplate>
    </asp:TemplateField>
    <asp:TemplateField Visible="false">
        <ItemTemplate>
            <asp:TextBox id="tb_currency_code" runat="server" Text='<%# Eval("currency_code") %>' Visible="false" />
        </ItemTemplate>
        <EditItemTemplate>
            <asp:TextBox id="tb_curr_cd" runat="server" Text='<%# Eval("currency_code") %>' Visible="false" />
        </EditItemTemplate>
        <FooterTemplate>
            <asp:TextBox id="tb_curr_cd2" runat="server" Text='<%# Eval("currency_code") %>' Visible="false" />
        </FooterTemplate>
    </asp:TemplateField>
    <asp:TemplateField ItemStyle-Width = "150px"  HeaderText = "Saving Rate">
        <ItemTemplate>
            <asp:Label ID="curr_rate" runat="server"
                Text='<%# Eval("Currency_rate")%>'>
            </asp:Label>
        </ItemTemplate>
        <EditItemTemplate>
            <asp:Label ID="curr_rate" runat="server" 
                Text='<%# Eval("Currency_rate")%>' >
            </asp:Label>
        </EditItemTemplate>
        <FooterTemplate>
            <asp:Label ID="curr_rate2" runat="server" 
                Text='<%# Eval("Currency_rate")%>' >
            </asp:Label>
        </FooterTemplate>
    </asp:TemplateField>
    <asp:TemplateField HeaderText="Delete?">
        <ItemTemplate>
            <span onclick="return confirm('Are you sure to delete the record?')">
                <asp:LinkButton CssClass="forbutton" ID="lnkdelete" runat="server" Text="Delete" ForeColor="Red" CommandName="Delete"/>
            </span>
        </ItemTemplate>
        <FooterTemplate>
            <asp:Button CssClass="forbutton" ID="btnadd" runat="server" Text="Add" OnClick="AddNewRec" />
        </FooterTemplate>
    </asp:TemplateField>
    <asp:CommandField ShowEditButton="true" />
    </Columns>
    <AlternatingRowStyle BackColor="White"/>
    <EditRowStyle BackColor="#efefef" />
    <FooterStyle BackColor="#507CD1" Font-Bold="true" ForeColor="White" />
    <HeaderStyle BackColor="#507CD1" Font-Bold="true" ForeColor="White" />
    <PagerStyle BackColor="#2461BF" ForeColor="White" HorizontalAlign="Center" />
    <RowStyle BackColor="#EFF3FB" />
    <SelectedRowStyle BackColor="#D1DDF1" Font-Bold="true" ForeColor="#333333" />
    <SortedAscendingCellStyle BackColor="#F5F7FB" />
    <SortedAscendingHeaderStyle BackColor="#6D95E1" />
    <SortedDescendingCellStyle BackColor="#E9EBEF" />
    <SortedDescendingHeaderStyle BackColor="#4870BE" />
    </asp:GridView>
    ...

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and here is the event to update the record

        protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
        {
            GridViewRow row = GridView1.Rows[e.RowIndex] as GridViewRow;
            TextBox tb_currency_code = row.FindControl("tb_curr_cd") as TextBox;
            Label curr_rate = row.FindControl("curr_rate") as Label;
            using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["Mssqlconn2"].ConnectionString))
            //using (OleDbConnection conn = new OleDbConnection(@"Provider=Microsoft.ACE.OLEDB.12.0;Jet OLEDB:Database Password=213461;Data Source=C:\inetpub\VS2012\DB1.accdb"))
            {
                string sql = "Update [savingrate] set [Currency_rate]= @Currency_rate" + " where [currency_code]=@currency_code";
                using (SqlCommand cmd = new SqlCommand(sql, conn))
                //using (OleDbCommand cmd = new OleDbCommand(sql, conn))
                {
                    cmd.Parameters.AddWithValue(
                    "@currency_code", tb_currency_code.Text.Trim());
                    cmd.Parameters.AddWithValue(
                    "@Currency_rate", Convert.ToDecimal(curr_rate.Text));
                    conn.Open();
                    cmd.ExecuteNonQuery();
                    conn.Close();
                }
            }
            lb_msg.Text = "Record updated successfully!";
            GridView1.EditIndex = -1;
            this.PopulateData();
        }
        ...

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When I press Update to the row, how to ensure that the DDL is showing the current value I've selected, as now it is not?