Dynamic dropdownlist is always returning the first value, not the selected value

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ASP.NET

C#5.0

ASP.NET5

Hello Everyone,

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:

What I have tried:

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---");
                    }

                }

Binding Method:

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"));
    }

My DropdownList Source code:

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

Reading Value:

if (this.ddlAppClass.SelectedIndex != 0)
                {
                    SchoolAge(this.ddlAppClass.SelectedItem.Text);                    
                }

SchoolAge Method:

private void SchoolAge(string _classType)
        {
            try
            {
                var select = "SELECT [ClassType],[age] FROM [Corona Schools_ Trust Council$AddAge] WHERE [ClassType] = @ClassType";
                SqlDataReader rder = null;
                using (var connection = new SqlConnection(ConfigurationManager.ConnectionStrings["DatabaseDemo Database NAV (8-0)1"].ConnectionString))
                {
                    using (var command = new SqlCommand(select, connection))
                    {
                        command.CommandType = CommandType.Text;
                        command.Parameters.Add("@ClassType", SqlDbType.VarChar, 50).Value = _classType;
                        connection.Open();
                        rder = command.ExecuteReader();
                        while (rder.Read())
                        {

                            ValidateAge.Text = (rder["age"].ToString());

                        }

                    }
                }
            }
            catch (SqlException ex)
            {
                //lblMsg.Text = ex.Message;
                //lblMsg.ForeColor = System.Drawing.Color.Red;
            }

        }

I appreciate your assistance!