Question

I have the following WPF Combobox:

<Window.Resources>
    <CollectionViewSource x:Key="performanceItemsource" Source="{Binding Path=SelectedReport.Performances}"  >
        <CollectionViewSource.SortDescriptions>
            <scm:SortDescription PropertyName="Name"/>
        </CollectionViewSource.SortDescriptions>
    </CollectionViewSource>
</Window.Resources>
 ...
    <ComboBox Name="cbxPlanPerf" Grid.ColumnSpan="2"
      SelectedValuePath="MSDPortfolioID" DisplayMemberPath="Name" 
      SelectedValue="{Binding Path=PlanPerfID}"
      ItemsSource="{Binding Source={StaticResource performanceItemsource}}"/>

The Source for the CollectionViewSource is:

public List<MSDExportProxy> Performances
{
  get
  {
    if (Portfolio != null)
    {
      return (from a in Portfolio.Accounts where a.MSDPortfolioID != null select new MSDExportProxy(a))
        .Concat<MSDExportProxy>(from g in Portfolio.Groups where g.MSDPortfolioID != null select new MSDExportProxy(g))
        .Concat<MSDExportProxy>(from p in new[] { Portfolio } where p.MSDPortfolioID != null select new MSDExportProxy(p))
        .ToList<MSDExportProxy>();
    }
    return new List<MSDExportProxy>();
  }
}

The bound property PlanPerfID is a string.

I move between records using a ListBox control. The ComboBox works fine if the previous record had no items in its ComboBox.ItemsSource. If there were any items in the previous record’s ComboBox.ItemsSource then the new record won’t find its matching item in the ItemsSource collection. I’ve tried setting the ItemsSource in both XAML and the code-behind, but nothing changes this odd behavior. How can I get this darn thing to work?

Answer 1

I found a quick and dirty solution to my problem. I just happen to have a public NotifyPropertyChanged() method on my Report entity and I discovered that if I called SelectedReport.NotifyPropertyChanged("PlanPerfID") in the Report ListBox’s SelectionChanged event that it was enough of a jolt to get the ComboBox to re-evaluate and find its matching item in the ItemsSource. Yeah, it’s KLUGE…

UPDATE: I also wound up needing to add SelectedReport.NotifyPropertyChanged("Performances") for some situations…

UPDATE 2: Okay, turns out the above wasn’t bullet proof and I ran across a situation that broke it so I had to come up with a better workaround:

1. Altered the SelectedReport property in the Window’s code-behind, adding a private flag (_settingCombos) to keep the Binding from screwing up the bound values until the dust has settled from changin the ItemSource:

   private bool _settingCombos = false;
   private Report _SelectedReport;
   public Report SelectedReport
   {
     get { return _SelectedReport; }
     set 
     {
       _settingCombos = true;
       _SelectedReport = value;
       NotifyPropertyChanged("SelectedReport");
     }
   }
   

2. Created a proxy to bind to in the Window code-behind that will refuse to update the property’s value if the _settingCombos flag is true:

   public string PlanPerfID_Proxy
   {
     get { return SelectedReport.PlanPerfID; }
     set
     {
       if (!_settingCombos)
       {
         SelectedReport.PlanPerfID = value;
         NotifyPropertyChanged("PlanPerfID_Proxy");
       }
     }
   }
   

3. Added an extra Notification in the Report ListBox’s SelectionChanged event along with code to reset the _settingCombos flag back to false:

   private void lbxReports_SelectionChanged(object sender, SelectionChangedEventArgs e)
   {
     //KLUGE: Couldn't get the ComboBoxes associated with these properties to work right
     //this forces them to re-evaluate after the Report has loaded
     if (SelectedReport != null)
     {
       NotifyPropertyChanged("PlanPerfID_Proxy");
       _settingCombos = false;
     }
   }
   

4. Bound the ComboBox to the PlanPerfID_Proxy property (instead of directly to the SelectedReport.PlanPerfID property.

Wow, what a hassle! I think that this is simply a case of .NET’s binding logic getting confused by the dynamic nature of the ComboBox.ItemSource, but this seems to have fixed it. Hope it helps someone else.