Why asp:DropDownList does not respond to code-behind

Question

I have the following ASP drop-down:

<asp:DropDownList ClientIDMode="Static" ID="ddlMain" name="searchPhys" style="width: 365px;" class="default" runat="server" AppendDataBoundItems="true">
        <asp:ListItem Text="BY PHYSICIAN" Value="0" Selected="True" />
        <asp:ListItem Text="BY LOCATION" Value="1" />
        <asp:ListItem Text="BY SPECIALTY" Value="2" />
</asp:DropDownList>
<br /><br />
<asp:DropDownList ClientIDMode="Static" ID="ddlDrillDown" name="searchPhys" style="width: 365px;" class="default" runat="server" AppendDataBoundItems="true">
</asp:DropDownList>

I am trying to make it interactive where if the first select option is changed the second will change as well based on the selection from the first select option.

My C# code looks like this:

public partial class test : System.Web.UI.Page
{
    String cString;
    SqlConnection Conn;
    protected void Page_Load(object sender, EventArgs e) {

    PopulatePhysician();
    //PopulateSpecialty();
    //PopulateLocation();

    }


    public void PopulatePhysician() {
        SqlCommand cmd = new SqlCommand("getPhysicians", new SqlConnection(ConfigurationManager.AppSettings["ConnString"]));
        //cmd.CommandType = Data.CommandType.StoredProcedure
        cmd.Connection.Open();

        SqlDataReader ddlValues = default(SqlDataReader);
        ddlValues = cmd.ExecuteReader();

        //if (!IsPostBack) {
            ddlDrillDown.DataSource = ddlValues;
            ddlDrillDown.DataValueField = "content_id";
            ddlDrillDown.DataTextField = "content_title";
            ddlDrillDown.DataBind();
            //set the default value for the drop down
            ListItem Item = new ListItem();
            Item.Text = "Select a Physician's Name";
            Item.Value = "0";
            //Item.Selected = True
            ddlDrillDown.Items.Insert(0, Item);
        //}
    cmd.Connection.Close();
    cmd.Connection.Dispose();
    }

    public void PopulateSpecialty() {
        SqlCommand cmd = new SqlCommand("getSpecialties", new SqlConnection(ConfigurationManager.AppSettings["ConnString"]));
        cmd.Connection.Open();

        SqlDataReader ddlValues = default(SqlDataReader);
        ddlValues = cmd.ExecuteReader();

        //if (!IsPostBack) {
            ddlDrillDown.DataSource = ddlValues;
            ddlDrillDown.DataValueField = "content_id";
            ddlDrillDown.DataTextField = "content_title";
            ddlDrillDown.DataBind();
            //set the default value for the drop down
            ListItem Item = new ListItem();
            Item.Text = "Select a Specialty";
            Item.Value = "0";
            ddlDrillDown.Items.Insert(0, Item);
        //}
        cmd.Connection.Close();
        cmd.Connection.Dispose();
    }

    public void PopulateLocation() {
        SqlCommand cmd = new SqlCommand("getLocations", new SqlConnection(ConfigurationManager.AppSettings["ConnString"]));
        cmd.Connection.Open();

        SqlDataReader ddlValues = default(SqlDataReader);
        ddlValues = cmd.ExecuteReader();

        //if (!IsPostBack) {
            ddlDrillDown.DataSource = ddlValues;
            ddlDrillDown.DataValueField = "content_id";
            ddlDrillDown.DataTextField = "content_title";
            ddlDrillDown.DataBind();

            //set the default value for the drop down
            ListItem Item = new ListItem();
            Item.Text = "Select a Location";
            Item.Value = "0";
            ddlDrillDown.Items.Insert(0, Item);
        cmd.Connection.Close();
        cmd.Connection.Dispose();
        //}
    }

    public void ddlMain_SelectedIndexChanged(object sender, System.EventArgs e) {
        switch(ddlMain.SelectedItem.Value) {
            case "0":
                PopulatePhysician();
                break;
            case "1":
                PopulateLocation();
                break;
            case "2":
                PopulateSpecialty();
                break;
        }
    }
}

When the page first loads, PopulatePhysician(); works great by populating the select option. But when I call the SelectedIndexChanged() function, nothing happens.

How can I resolve it? Is the case statement correct?

Answer 1

Remove the If (!IsPostBack) clause from your functions. Check for that in page load, but not in each individual function.

Answer 2

You haven't added any delegates to the SelectedIndexChanged event.

Add OnSelectedIndexChanged to the parameters in the asp:DropDownList tag, like so:

<asp:DropDownList ClientIDMode="Static" OnSelectedIndexChanged="ddlMain_SelectedIndexChanged" ID="ddlMain" name="searchPhys" style="width: 365px;" class="default" runat="server" AppendDataBoundItems="true"  >

EDIT

I just noticed you're actually missing the AutoPostBack event as well - it should be set to true, as updating the selection without submitting the form will not fire a postback event, unless AutoPostBack is set to true.

Answer 3

It needs AutoPostback=true on the Dropdown list

<asp:DropDownList AutoPostback="true" ClientIDMode="Static" ID="ddlMain" name="searchPhys" style="width: 365px;" class="default" runat="server" AppendDataBoundItems="true">