Question

Im stuck in my project here and i really need some help!
I have 2 dropdownlists “DropDownPostal” and “DropDownCity”. When the user/employee logges into the system, he gets the list of Postals in the first dropdown list. Lets say he choose a postal and tabs over to the second dropdown “city”. I want city to be updated with the postal the user choose earlier. I have everything working with the login and retrieving data from the database to the DropDownPostal. But I cannot get the other DropDownCity updated with any data!

Here is my code for the event i made for the postaldropdown, Im trying to make a “onLeave” event.

  using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Web;
   using System.Web.UI;
   using System.Web.UI.WebControls;

 namespace Test
  {
public partial class WebForm1 : System.Web.UI.Page
{

    // Entity
    DanxWebsiteEntities dwe = new DanxWebsiteEntities();

    protected void Page_Load(object sender, EventArgs e)
    {


        //Dropdown postback
        //DropDownListPostal.AutoPostBack = false;

        //Sessions
        if (Session["UserID"] == null && Session["Name"] == null)
        {
            Response.Redirect("Error.aspx");
        }
        else
        {
            msg1.Text = "User id: "+Session["UserID"].ToString();
            msg2.Text = "User Name: " + Session["Name"].ToString();
        }   
        //Update dropdownpostal
        if (!IsPostBack)
        {
            //Guid is structure type 
            Guid id =(Guid) Session["UserID"];
            DropDownListPostal.DataSource = (from custumAdr in dwe.VW_CustumAddress
                                             where custumAdr.UserID ==(Guid)id
                                             select custumAdr).ToList();
            DropDownListPostal.DataTextField = "Postal";
            DropDownListPostal.DataValueField = "UserID";
            DropDownListPostal.DataBind();
        }
    }



    protected void DropDownListPostal_SelectedIndexChanged1(object sender, EventArgs e)
    {
        foreach (var data in dwe.VW_CustumAddress)
            if (data.Postal == DropDownListPostal.SelectedItem.ToString())
            {
                DropDownListBy.Text = data.City;

            }
            else
            {

            }
    }
    }

}

Im not even sure if this code is close to correct.
Detailed help with code will be much appreciated.

Cheers 🙂

Answer 1

If I understood you right, you only need to change the value displayed in dropDownListCity according to the value selected in dropDownListPostal. To do so, you need to bind both dropdown lists in Page_Load and then handle the selected index when SelectedIndexChanged is triggered.

private IEnumerable<KeyValuePair<string, string>> GetData()
{
    using(var dataContext = new DbEntities())
    {
        return dataContext.VW_CustumAddress 
            .ToList() 
            .Select(item => new KeyValuePair<string, string>(item.Postal, item.City))
            .ToArray();
    }
}

protected void Page_Load(object sender, EventArgs e)
{
    if(!IsPostback)
    {
        IEnumerable<KeyValuePair<string,string>> data = GetData();
        //Bind Postal dropdown list
        dropDownListPostal.DataSource = data.Select(kvp => kvp.Key).ToList();
        // ^ Select only 'Postal' column in dropdown list.
        dropDownListPostal.DataBind();
        // Bind City dropdown list
        // We need to bind to a key-value pair to know the correspondence between items
        dropDownListCity.DataValueField = "Key";
        dropDownListCity.DataTextField = "Value";
        dropDownListCity.DataSource = data;
        dropDownListCity.DataBind();
    }
}

Now, after you bound your controls to data you only need to handle SelectedIndexChanged event of dropDownListPostal (don’t forget to set AutoPostback="true" in the markup of dropDownListPostal).
EDIT: Before selecting an item, clear the selection of your dropdown list to make sure only one item is selected.

protected void OnDropDownListPostalSelectedIndexChanged(object sender, EventArgs e)
{
    dropDownListCity.ClearSelection();
    var postal = dropDownListPostal.SelectedValue;
    var listItem = dropDownListCity.Items.FindByValue(postal);
    listItem.Selected = true;
}

That should do it. Hope it helps.