Question

I have three ComboBoxes such that C’s items list is dependent on the selected item in B and B’s items list is dependent on the selected item in A. I have ObservableCollections of particular classes for the ItemsSource on each ComboBox, and I want the Name property of the selected item to be pushed to a property in another class. For example:

ObservableCollection<AClass> Items
=> ItemsSource of cbo_A ComboBox

And selectedInstanceOfAClass.Name should be pushed to Data.AClassName property. The problem I have is that when I choose a value in the A ComboBox the first time, the B ComboBox gets the appropriate items based on the selected item in A, as expected. Same for when I select an item in B for the first time–C gets the right items. However, when I choose a different value in A, the items in B get updated but the selected value of B stays the same, and when I try to select a new value in B from its new items, the selection doesn’t change–it stays the same selected value as what I initially selected in B.

Here’s the XAML I have right now:

<ComboBox x:Name="cbo_A"
    ItemsSource="{Binding Path=Lists.AItems, Mode=OneWay}"
    DisplayMemberPath="Name" SelectedValuePath="Name"
    SelectedValue="{Binding Path=Data.AClassName, ValidatesOnDataErrors=True,
        Mode=OneWayToSource}" />

<ComboBox x:Name="cbo_B"
    ItemsSource="{Binding ElementName=cbo_A, Path=SelectedItem.BItems, Mode=OneWay}"
    SelectedValuePath="Name" DisplayMemberPath="Name"
    SelectedValue="{Binding Path=Data.BClassName, ValidatesOnDataErrors=True,
        Mode=OneWayToSource}"/>

<ComboBox x:Name="cbo_C"
    ItemsSource="{Binding ElementName=cbo_B, Path=SelectedItem.CItems, Mode=OneWay}"
    SelectedValuePath="Name" DisplayMemberPath="Name"
    SelectedValue="{Binding Path=Data.CClassName, Mode=OneWayToSource,
        ValidatesOnDataErrors=True}" />

What is causing the weird behavior with the selected value not updating even when I explicitly click the ComboBox and try to change the value?

Edit: when debugging and I had a SelectionChanged handler on cbo_A and another on cbo_B, I entered the cbo_B handler before the cbo_A one, so the selected item from B’s perspective was still the old item, because A had not been updated yet. :/

Edit 2: if I flip the order of my ComboBoxes in XAML, suggested by this question, such that C comes before B comes before A, I enter A’s handler first. When I then enter the handler for cbo_B, the SelectedItem property shows the old value (previously selected, before I chose a new value in cbo_A), even though I clicked a value from the new list of items showing up in the ComboBox.

Edit 3: I’m trying a version of this answer and getting the same problem: I see new values in ComboBox B upon changing the selected value of ComboBox A, but I cannot change the selected value in B after I’ve already set it once. I’m beginning to think I’ve got something else going awry.

Answer 1

Wow. So, don’t go quickly implementing a bunch of IEquatable<T> Equals methods for new classes, because you may end up doing what I did:

if (ReferenceEquals(this, other))
{
    return false;  // Say what??
}

Sigh. I got my ComboBoxes to work by:

• Correcting my BClass.Equals(BClass other) method;
• Using this answer about having another view model class with different properties like SelectedAItem and AItems; and
• Using this answer about putting my XAML declarations in a different order so cbo_C comes before cbo_B and cbo_B comes before cbo_A.

I may try simplifying things and see what parts are absolutely necessary for my ComboBoxes.