未在SelectedIndexChanged事件上加载级联Dropdownlist

Question

I have two dropdownlists named ddFatherEmployeeNumber and ddEmployeeFatherName. Now on load event i am populating ddFatherEmployeeNumber from database. Here is the code

private DataTable LoadComboBoxFatherEmployeeNumber()
    {
        DataTable dtFatherENo = new DataTable();
        using (SqlConnection con = new SqlConnection(Base.GetConnection))
        {
            using (SqlCommand cmd = new SqlCommand("SELECT Id, EmployeeNo FROM TableFatherMaster", con))
            {
                cmd.CommandType = CommandType.Text;
                con.Open();
                SqlDataReader r = cmd.ExecuteReader();
                dtFatherENo.Load(r);
            }
        }
        return dtFatherENo;
    }

Load Event code is

if (!IsPostBack)
        {
            ddFatherEmployeeNumber.DataSource = LoadComboBoxFatherEmployeeNumber();
            ddFatherEmployeeNumber.DataTextField = "EmployeeNo";
            ddFatherEmployeeNumber.DataValueField = "Id";
            ddFatherEmployeeNumber.DataBind();
            ddFatherEmployeeNumber.Items.Insert(0, new ListItem("Select Father Employee No", "0"));
        }

Here is the .aspx code for ddFatherEmployeeNumber

<div class="col-8">
                                    <asp:DropDownList ID="ddFatherEmployeeNumber" runat="server" class="form-control here" AutoPostBack="True" OnSelectedIndexChanged="ddFatherEmployeeNumber_SelectedIndexChanged"></asp:DropDownList>
                                </div>

Now i want to populate the ddEmployeeFatherName on the bases of ddFatherEmployeeNumber selected value. I mean when a user select employee number from ddFatherEmployeeNumber list then from database, Name of that employee loads and populate the ddEmployeeFatherName. For that purpose i already write a code on ddFatherEmployeeNumber_SelectedIndexChanged Event. Here is the Code.

 protected void ddFatherEmployeeNumber_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ddEmployeeFatherName.SelectedIndex == 0)
        {

        }
        else
        {
            using (SqlConnection con = new SqlConnection(Base.GetConnection))
            {
                using (SqlCommand cmd = new SqlCommand("SELECT Id, Name FROM TableFatherMaster WHERE EmployeeNo=@EmployeeNo ", con))
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.Parameters.AddWithValue("@EmployeeNo", ddFatherEmployeeNumber.SelectedValue);
                    con.Open();
                    SqlDataReader r = cmd.ExecuteReader();
                    ddEmployeeFatherName.DataSource = r;
                    ddEmployeeFatherName.DataBind();
                }
            }
        }

    }

Now the problem is, it doesn't loads data into ddEmployeeFatherName. Here is the picture for clear understanding

Answer 1

Try setting DataTextField and DataValueField properties for ddEmployeeFatherName first, because you're not setting them before using DataBind() :

using (SqlConnection con = new SqlConnection(Base.GetConnection))
{
    using (SqlCommand cmd = new SqlCommand("SELECT Id, Name FROM TableFatherMaster WHERE EmployeeNo=@EmployeeNo ", con))
    {
        cmd.CommandType = CommandType.Text;
        cmd.Parameters.AddWithValue("@EmployeeNo", ddFatherEmployeeNumber.SelectedValue);
        con.Open();
        SqlDataReader r = cmd.ExecuteReader();
        ddEmployeeFatherName.DataSource = r;

        // set text and value fields
        ddEmployeeFatherName.DataTextField = "Name";
        ddEmployeeFatherName.DataValueField = "Id";
        ddEmployeeFatherName.DataBind();
    }
}

If the dropdownlist option values are still empty, try loading SqlDataReader contents into DataTable and use it instead:

using (SqlCommand cmd = new SqlCommand("SELECT Id, Name FROM TableFatherMaster WHERE EmployeeNo=@EmployeeNo ", con))
{
    cmd.CommandType = CommandType.Text;
    cmd.Parameters.AddWithValue("@EmployeeNo", ddFatherEmployeeNumber.SelectedValue);
    con.Open();
    SqlDataReader r = cmd.ExecuteReader();

    DataTable dt = new DataTable();
    dt.Load(r);

    ddEmployeeFatherName.DataSource = dt;
    ddEmployeeFatherName.DataTextField = "Name";
    ddEmployeeFatherName.DataValueField = "Id";
    ddEmployeeFatherName.DataBind();
}