ArgumentOutOfRangeException in dropdown from c#

Question

I have two dropdown lists one static(Requested) another one is dynamic(Response)."Response" is depends on the "Requested" dropdown list.I am new to dot new framework.I don't no I go a exception in my code.

Ord.aspx

     <%@ Page Title="" Language="C#" AutoEventWireup="true" CodeBehind="o1.aspx.cs" Inherits="C.Portal.ord" %>
    <asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
        <div id="Order">
<table width="100%" border="0">

            <tr>
                        <td><asp:Label ID="Label0" runat="server" Text="Requested:* " /></td>           
                       <td>
                            <asp:RequiredFieldValidator ID="v1" ControlToValidate="ddlreq1" CssClass="error" InitialValue="" Display="Dynamic" SetFocusOnError="true" runat="server">
                                Please select request<br />
                            </asp:RequiredFieldValidator>

                            <asp:DropDownList ID="ddlreq1" Width="100%" 
                                TabIndex="15" AutoPostBack="true" 
                                onselectedindexchanged="ddlRequest_SelectedIndexChanged" runat="server">
                                <asp:ListItem></asp:ListItem>
                                <asp:ListItem>Req1</asp:ListItem>
                                <asp:ListItem>Req2</asp:ListItem>                      
                            </asp:DropDownList>
                        </td>
                    </tr>

                     <tr>
                        <td><asp:Label ID="Label1" runat="server" Text="Response :* " /></td>
                        <td>
                            <asp:RequiredFieldValidator ID="v2" ControlToValidate="ddlresp" CssClass="error" InitialValue="" Display="Dynamic" SetFocusOnError="true" runat="server">
                                Please select a color<br />
                            </asp:RequiredFieldValidator>                    
                            <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                                <ContentTemplate>                
                                    <asp:DropDownList ID="ddlresp" DataTextField="ddlText" Width="100%" TabIndex="16" runat="server" />
                                 </ContentTemplate>
                                <Triggers> 
                                    <asp:AsyncPostBackTrigger ControlID="ddlreq1" EventName="SelectedIndexChanged" />
                                </Triggers>  
                             </asp:UpdatePanel>                    

                        </td>
                    </tr>

        </div>
</table>
    </asp:Content>

Ord.aspx.cs

 using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Web;
    using System.Web.UI;
    using System.Web.UI.WebControls;
    using System.Data;
    using System.Web.Security;
    using System.Configuration;
    using System.Data.SqlClient;
    using System.Net.Mail;

    namespace C.Portal
    {
        public partial class ord : System.Web.UI.Page
        {
             protected void Page_Load(object sender, EventArgs e)
            { 
            // other code 
            if (!IsPostBack)
                {
                DataSet Color = new DataSet();
             Color.ReadXml(MapPath("Project/Color.xml"));
             ddlresp.DataSource = Color;
                  ddlresp.DataBind();
                }

            }
            protected void ddlRequest_SelectedIndexChanged(object sender, EventArgs e)
            {
             if (ddlRequest.SelectedValue != "")
              {
                var requestMatrix = (from f in db.RequestMatrixes
                                       where f.Type == ddlRequest.SelectedValue
                                       select f).SingleOrDefault();
                 if (!requestMatrix.testcolor)
                    {
                        ddlresp.SelectedValue = "None";
                        ddlresp.Enabled = false;
                    }
                    else
                    {
                        ddlresp.SelectedIndex = 0; // here I got a exception 
                        ddlresp.Enabled = true;
                    }
                }
            }

        }
    }

I my code exception details: Response dropdown list not loaded.HOw to load the response dropdown list? some help me.

ddlresp' has a SelectedIndex which is invalid because it does not exist in the list of items. Parameter name: value

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.ArgumentOutOfRangeException: 'ddlresp' has a SelectedIndex which is invalid because it does not exist in the list of items. Parameter name: value

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:

[ArgumentOutOfRangeException: 'ddlresp' has a SelectedIndex which is invalid because it does not exist in the list of items. Parameter name: value]

Answer 1

You need to bind the ddlresp after assigning DataSource using the DataBind() method. If you do not call DataBind the dropdown wont have any items in it and setting SelectedIndex will throw exception

You may also put it in !Page.IsPostBack block if you need it state.

protected void Page_Load(object sender, EventArgs e)
{ 
   if(!Page.IsPostBack)
   {
     // other code 
     DataSet Color = new DataSet();
     Color.ReadXml(MapPath("Project/Color.xml"));
     ddlresp.DataSource = Color;
     ddlresp.DataBind();
   }
}

Edit Your html seems invalid as well, you have tr with table tag.

Put both dropdowns in UpdatePanel, you may put your table in UpdatePanel.