动态下拉列表始终返回第一个值，而不是所选值

Question

i have an issue i have been battling with for sometime now, my dynamic DropDownList always return to the first item from database, i have done a few researches on how to resolve this including enabling Viewstate for the page and DropDownList and placing the code on if(!Page.IsPostback) but the issue persists!

Here is my code:

if (!Page.IsPostBack)
{
    string appType2 = Session["applicationType"].ToString();
    if (appType2 == "nur")
    {

        FetchApplicationFeeFromDB("NURSERY");
        this.ddlAppClass.Items.Clear();
        this.ddlApplicationType.Items.Insert(0, "NURSERY SCHOOL");

        string query = string.Format("SELECT [Teaching Room Category] ,[Description] FROM [Corona Schools_ Trust Council$Teaching Room Level] WHERE [Teaching Room Category] = '{0}'", "NURSERY");
        BindDropDownList(this.ddlAppClass, query, "Description", "Teaching Room Category", "---Select---");
    }

}

private void BindDropDownList(DropDownList ddl, string query, string text, string value, string defaultText)
{
    string conString = ConfigurationManager.ConnectionStrings["DatabaseDemo Database NAV (8-0)1"].ConnectionString;
    SqlCommand cmd = new SqlCommand(query);
    using (SqlConnection con = new SqlConnection(conString))
    {
        using (SqlDataAdapter sda = new SqlDataAdapter())
        {
            cmd.Connection = con;
            con.Open();
            ddl.DataSource = cmd.ExecuteReader();
            ddl.DataTextField = text;
            ddl.DataValueField = value;
            ddl.DataBind();
            con.Close();
        }
    }
    ddl.Items.Insert(0, new ListItem(defaultText, "0"));
}

protected void ddlAppClass_SelectedIndexChanged(object sender, EventArgs e){ 
    if (this.ddlAppClass.SelectedIndex != 0) { 
        txtDateOfBirth.Enabled = true; 
    } else { 
        txtDateOfBirth.Enabled = false; 
    } 
} 

My DropdownList Source code:

<asp:DropDownList ID="ddlAppClass" runat="server" AutoPostBack="True" Height="20px" Width="188px" OnSelectedIndexChanged="ddlAppClass_SelectedIndexChanged" EnableViewState="true"></asp:DropDownList>

OnSelectedIndexChanged code:

protected void ddlAppClass_SelectedIndexChanged(object sender, EventArgs e) 
{ 
    if (this.ddlAppClass.SelectedIndex != 0) 
    { 
        txtDateOfBirth.Enabled = true; 
    } 
    else 
    {
        txtDateOfBirth.Enabled = false;
    }
}

Answer 1

You need to bind the items each time the page loads, not just when !PostBack. When you post back, the page is rebuilt from the aspx and selected values are (attempted) set in code but in your case, the dropdownlist will be empty because you are not calling BindDropDownList so its selected value cannot be set and you get the zero.