Combobox SelectedItem doesn't update when source changes

Question

I have a viewmodel which implement INotifyPropertyChanged. On this viewModel is a property called SubGroupingView. This property is bound to the selected item of a combo box. When i change the combo box, the source property is being updated fine, but when I change the source property or when the control is initialized, the combobox.selectedItem is NOT reflecting what exists in the property.
Here is some code to get you started:

<ComboBox Grid.Column="3" Grid.Row="1" 
          Margin="0,1,4,1" 
          SelectedItem="{Binding Path=SubGroupingView, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay, diag:PresentationTraceSources.TraceLevel=High}" 
          ItemsSource="{Binding Columns}" 
          DisplayMemberPath="DisplayName">

The property raises the PropertyChanged event and the TraceSource output shows me that the binding detected it and transferred the value, its just that the combobox isn't reflecting it. Any ideas would be most welcome!

EDIT:
output from the trace source is this:

System.Windows.Data Warning: 91 : BindingExpression (hash=23631369): Got PropertyChanged event from ReportViewModel (hash=52844413)  
System.Windows.Data Warning: 97 : BindingExpression (hash=23631369): GetValue at level 0 from ReportViewModel (hash=52844413) using RuntimePropertyInfo(SubGroupingView):         DataColumnViewModel (hash=58231222)  
System.Windows.Data Warning: 76 : BindingExpression (hash=23631369): TransferValue - got raw value DataColumnViewModel (hash=58231222)  
System.Windows.Data Warning: 80 : BindingExpression (hash=23631369): TransferValue - implicit converter produced DataColumnViewModel (hash=58231222)  
System.Windows.Data Warning: 85 : BindingExpression (hash=23631369): TransferValue - using final value DataColumnViewModel (hash=58231222)

Here is the code for the source property:

public class ReportViewModel : ViewModelBase, IReportTemplate
{
    public DataColumnViewModel SubGroupingView
    {
        get
        {
            return GetViewModel(_report.SubGrouping);
        }
        set
        {
            if (_report.SubGrouping == value.ColumnName)
                return;
            _report.SubGrouping = value.ColumnName;
            RefreshDataSeries();
            base.OnPropertyChanged("SubGroupingView");
            base.OnPropertyChanged("IsReady");
        }

    }
}

Note: ViewModelBase implements INotifyPropertyChange.

ANSWER
I overloaded the ==, != operators, GetHashCode(), and Equals(object) and now it is working nicely. Thanks for all of your help!

Out of interest, try setting the ItemsSource before the SelectedItem in your XAML declaration there. — Matt Hamilton, Aug 17 '10 at 04:26
Interesting... could you possibly post a snippet of your ViewModel (specifically the definition of SubGroupingView)? — Mark Carpenter, Aug 17 '10 at 04:33
How do you change source property? Is it really the object from the list, not any copy of it? — Maxim Alexeyev, Aug 18 '10 at 03:55
Since its a ViewModel approach, the object is created. I will try using the Columns property of the list — TerrorAustralis, Aug 18 '10 at 13:13

score 19 · Accepted Answer · edited May 23 '17 at 10:29

19

The object returned from your SubGroupingView must be "equal" to one of the objects in the ComboBox.Items (which means it must be in your Columns collection). So if you perform an "a.Equals(b)", it would need to return true.

If they are functionally the same, but not returning true when compared then that's your problem. You would need to either return the same object, or override the Equals method (and potentially the == and != operators).

If this is your issue, it's the same problem as in this question.

edited May 23 '17 at 10:29

Community

1
1

answered Aug 20 '10 at 00:47

CodeNaked

40,753
6
122
148

WELL worth the bounty! i'd vote you up even further if i could!! What implications does this have for MVVM? should all MVVM VMs implement a comparer on the base class? (cant award bounty for another three hours tho) – TerrorAustralis Aug 20 '10 at 01:39
It's hard to say definitively, since every situation is different. If you're ViewModel isn't used in ItemsControls, then it may not matter if you override Equals. But you would run into the same sitution if you did a simple List.Contains(something). Since the Contains is performing an equality test also. – CodeNaked Aug 20 '10 at 11:49

score 4 · Answer 2 · answered Aug 17 '10 at 05:00

4

Is IsSynchronizedWithCurrentItem on your ComboBox perhaps set to false? You could try explicitly setting IsSynchronizedWithCurrentItem="True" and see if that helps.

answered Aug 17 '10 at 05:00

Zach Johnson

23,678
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Thanks mate, but no luck, If i set IsSynchronizedWithCurrentItem, it doesnt fire of the setter on the source – TerrorAustralis Aug 17 '10 at 05:23
3

This helped me when updating the list of items and the selected index stayed the same. – Dave Mar 07 '13 at 22:17
If I set it, I get "Failed to assign to property 'Windows.UI.Xaml.Controls.Primitives.Selector.IsSynchronizedWithCurrentItem'." during initialize (It's an UWP application, the property was suggested by Intellisense, so it exists) – II ARROWS Feb 17 '18 at 20:19

Alex34758 · Answer 3 · 2016-10-03T21:58:10.607

0

The @CodeNaked 's answer is right. But in my case just overriding Object.Equals throws StackOverflowException. I think the whole answer is to implement full IEquatable that means implementing its Equals method and overriding Object.Equals(Object) and Object.GetHashCode methods as in this example (see the end of "Remarks" section - "Notes to Implementers" - and "Examples" section).

edited Oct 03 '16 at 21:58

answered Jul 26 '16 at 12:59

Alex34758

396
4
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score 0 · Answer 4 · answered Aug 31 '22 at 00:26

As my experience, You need set the SelectedItem by searching from ItemsSource.

For MVCC example, If you created a combobox with

<Combobox ItemsSource={Binding Path = Items} 
SelectedItem={Binding Path=OneItem} 
DisplayMember="item_name"/>

Note that: "item_name" is a property of item element in Items collection. Then you set

Items= new ObservableCollection<Item>(<some items list>);
RaisePropertyChanged("Items");

It's okay for now, But if you want to change the SelectedItem, you cannot set

OneItem= new Item{item_name="abc"};
RaisePropertyChanged("OneItem");

This will not change the SelectedItem on UI.

Instead, you must set the OneItem by searching for item from Items collection

OneItem = Items.Where(x=> x.item_name=="abc").LastOrDefault();
RaisePropertyChanged("OneItem");

Hope this help someone.

Combobox SelectedItem doesn't update when source changes

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