Refresh Dropdownlist in webform

<asp:DropDownList ID="UserDropDown" runat="server" AppendDataBoundItems="true" 
 ondatabinding="DropDownList1_DataBinding" DataTextField="key" DataValueField="value"></asp:DropDownList>

 protected void DropDownList1_DataBinding(object sender, EventArgs e)
    {
        var ddl = sender as DropDownList;
        if(ddl!=null)
        {
          //populate list. 
          ddl.Items.Add(new ListItem("test"));
        }
    }

String selectedText = DDL23.SelectedItem.Text;

protected ICollection ddNames() {

        string selectSQL = "mysql stuff";

        string connString = "my string";
        SqlCommand cmd = new SqlCommand(selectSQL, conn);
        SqlDataReader reader;
    // Create a table to store data for the DropDownList control.
     DataTable dt = new DataTable();

     // Define the columns of the table.
     dt.Columns.Add(new DataColumn("NewItemTextField", typeof(String)));
     dt.Columns.Add(new DataColumn("NewItemValueField", typeof(String)));

     dt.Rows.Add(CreateRow("Unassigned", "999999", dt));
        try  {
            conn.Open(); 
            reader = cmd.ExecuteReader();
            while ( reader.Read() )  {
                    // Populate the table with sample values.
                    dt.Rows.Add(CreateRow(DataHelpers.GetUserFirstLastFromID(Convert.ToInt32(reader["someid"])), reader["someid"].ToString(), dt));
            }
            reader.Close();
        }   catch (Exception ex)   {
             throw ex;
        }   finally    {

                if (conn != null)  {
                    conn.Dispose();
                    conn.Close();
                }
         }

    return dt;

    }

  DataRow CreateRow(String Text, String Value, DataTable dt)
  {

     // Create a DataRow using the DataTable defined in the 
     // CreateDataSource method.
     DataRow dr = dt.NewRow();

     // This DataRow contains the NewItemTextField and NewItemValueField 
     // fields, as defined in the CreateDataSource method. Set the 
     // fields with the appropriate value. Remember that column 0 
     // is defined as NewItemTextField, and column 1 is defined as 
     // NewItemValueField.
     dr[0] = Text;
     dr[1] = Value;

     return dr;

  }

 // Specify the data source and field names for the Text 
        // and Value properties of the items (ListItem objects) 
        // in the DropDownList control.
        dd_name.DataSource = ddNames();
        dd_name.DataTextField = "NewItemTextField";
        dd_name.DataValueField = "NewItemValueField";

        // Bind the data to the control.
        dd_name.DataBind();

        // Set the default selected item, if desired.
        dd_name.SelectedIndex = 0;