Clearing and refilling a bound combo box

Question

I have a WPF application with a number of comboboxes that tie together. When I switch comboboxx #1, combobox #2 switches, etc.

Here is the xaml for the 2 comboboxes:

<ComboBox Grid.Row="1" Height="23" HorizontalAlignment="Right" Margin="0,12,286,0" ItemsSource="{Binding}" Name="CboDivision" VerticalAlignment="Top" Width="120" SelectionChanged="CboDivision_SelectionChanged" />
<ComboBox Height="23" HorizontalAlignment="Right" Margin="0,9,32,0" Name="CboCustomerList" ItemsSource="{Binding}" VerticalAlignment="Top" Width="120" SelectionChanged="CboCustomerList_SelectionChanged" Grid.Row="1" />

CboDivision gets populated at the beginning and doesnt need a reset. HEre is the code that calls the change in division, which should trigger a change in customer:

private void CboDivision_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    division = CboDivision.SelectedValue.ToString();
    CboCustomerList.ItemsSource = null;

    BackgroundWorker customerWorker = new BackgroundWorker();
    customerWorker.DoWork += new DoWorkEventHandler(FillCustomers);
    customerWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(customerWorker_RunWorkerCompleted);
    FillCustomers(null, null);

}

When I do an index change it calls a backgroundworker which calls the following code:

    private void FillCustomers(object sender, DoWorkEventArgs e)
    {
        string connectionString = Settings.Default.ProdConnectionString;
        SqlConnection connection = new SqlConnection(connectionString);
        SqlCommand SqlCmd = new SqlCommand();
        Mouse.OverrideCursor = System.Windows.Input.Cursors.Wait;


            SqlCmd.CommandType = CommandType.StoredProcedure;
            SqlCmd.Parameters.Add("@division", SqlDbType.NVarChar).Value = division;
            SqlCmd.Connection = connection;
            SqlCmd.CommandText = "sp_GetCustomers";
            SqlDataReader reader = null;
            connection.Open();
            reader = SqlCmd.ExecuteReader();
            List<string> result = new List<string>();
            while (reader.Read())
            {
                result.Add(reader["NAME"].ToString());
            }
            e.Result = result;


   }

The problem is that i am unable to switch selections on CboDivision and have CboCustomerList clear and reload new values into it. Is it the way I'm binding the values in the xaml? How can I have a change in CboDivision cause a clearing of CboCustomerList items, then the execution of the filling routine?

i am currently resetting the combobox with:

CboCustomerList.SelectedIndex = -1;

but this just appends the new cbocustomerlist query to the end I also tried

CboCustomerList.Items.Clear()

but that just returns a null reference error after the box is refilled and the user selects an item.

Answer 1

Well, you didn't post all of your code, but one problem is that you are not invoking your worker thread.

Replace FillCustomers(null, null) with customerWorker.RunWorkerAsync().

private void CboDivision_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    division = CboDivision.SelectedValue.ToString();
    CboCustomerList.ItemsSource = null;

    BackgroundWorker customerWorker = new BackgroundWorker();
    customerWorker.DoWork += new DoWorkEventHandler(FillCustomers);
    customerWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(customerWorker_RunWorkerCompleted);
    customerWorker.RunWorkerAsync(); // <-- this
}

Let me know if this helps. If there is still more problem, please post the rest of your code.

Answer 2

There are details of code you didn't show. However, here is an example that works: two combos, what that is populated at the beginning, and a second that is populated each time the selection changes in the first. Notice that the data is fetched in a background worker, just as in your case.

XAML

<Window x:Class="CombosRefresh.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
      <Grid.RowDefinitions>
         <RowDefinition Height="Auto" />
         <RowDefinition Height="Auto" />
      </Grid.RowDefinitions>

      <ComboBox Name="CboDivisions" ItemsSource="{Binding}" Grid.Row="0" Margin="5" />
      <ComboBox Name="CboList" ItemsSource="{Binding}" Grid.Row="1" Margin="5" />
   </Grid>
</Window>

C#

   public partial class MainWindow : Window
   {
      private BackgroundWorker bw = new BackgroundWorker();

      public MainWindow()
      {
         InitializeComponent();

         CboDivisions.DataContext = new List<string>() { "red", "blue", "green" };
         CboDivisions.SelectionChanged += CboDivisions_SelectionChanged;

         bw.DoWork += bw_DoWork;
         bw.RunWorkerCompleted += bw_RunWorkerCompleted;
      }

      void CboDivisions_SelectionChanged(object sender, SelectionChangedEventArgs e)
      {
         var division = CboDivisions.SelectedValue as string;
         bw.RunWorkerAsync(division);
      }

      void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
      {
         CboList.DataContext = e.Result as List<string>;
      }

      void bw_DoWork(object sender, DoWorkEventArgs e)
      {
         var division = e.Argument as string;

         var r = new Random();
         var result = new List<string>();

         for(int i = 0; i < r.Next(0, 10); ++i)
         {
            result.Add(string.Format("{0} #{1}", division, i+1));
         }

         e.Result = result;
      }
   }

Answer 3

replace the wrong FillCustomers(null, null); code with: customerWorker.RunWorkerAsync();