Class Properties Changing in Unexpected Ways

Question

Class Properties Changing in Unexpected Ways

RogerSchlueter-7899 1,671

Consider this Person class:

Public Class Person
	Implements INotifyPropertyChanged
	
	Private _PersonID As Integer
	Public Property PersonID As Integer
		Get
			Return _PersonID
		End Get
		Set(value As Integer)
			_PersonID = value
		End Set
	End Property
	
	Private _JointMemberID As Integer
	Public Property JointMemberID As Integer
		Get
			Return _JointMemberID
		End Get
		Set(value As Integer)
			_JointMemberID = value
			OnPropertyChanged(NameOf(JointMemberID))
		End Set
	End Property

	<< Numerous other properties >>
End Class

The JointMemberID property is used when two people are linked together.

When my project starts all Person instances are populated and added to ocPeople, an ObservableCollection(Of Person).

The window which displays all the person data has a ComboBox (cbxPeople) which the user selects a person whose data is then displayed. Included is another ComboBox (cbxJoint) which displays the joint person if there is one. Here is the code which runs when a person is selected for display:

Private Sub SelectPerson(sender As Object, e As SelectionChangedEventArgs) Handles 
cbxPeople.SelectionChanged
	CurrentPerson = CType(cbxPeople.SelectedItem, Person)
	cbxJoint.ItemsSource = ocPeople.Except({CurrentPerson})
End Sub

If the selected person has JointPersonID > 0 (that is, a joint person does exist) the second statement changes that value. For example if PersonID = 1 has JointPersonID = 2, then that statement changes the JointPersonID value to 1. I don't understand why.

The second problem arises when I first select some person who does not have a joint person (that is, JointPersonID = 0). That is no problem. But then if I select a person who does have a JointPersonID > 0 then the above routine runs four times, changing the JointPersonID each time. For example using the situation as above, a Debug.Write(CurrentPerson.JointMemberID) statement as the last line in the above sub yields 1,2,1,2 (each on its own line). That I also don't understand.

I hope this is sufficiently clear to allow you to diagnose what is wrong.

Viorel 126.8K Reputation points

2026-01-10T17:13:13.4933333+00:00

Are there also handlers for cbxJoint and other places related to CurrentPerson and comboboxes?

By the way, the Debugger, breakpoints and Call Stack window help in investigating the callers of functions.
RogerSchlueter-7899 1,671 Reputation points

2026-01-11T16:13:32.48+00:00

There are no handlers for cbxJoint but, of course, there are several other places where CurrentPerson can change.

If I put a debug statement as the first line of setter of JointMemberID, it is invoked immediately after the second line of the SelectPerson sub. I have assumed this implies that no other managed code is involved. Is that good assumption?

I guess I'm gonna have to start using the Watch and Stack windows in order to figure out this puzzle.

2 answers

Your answer

Viorel 126.8K Reputation points

2026-01-10T17:13:13.4933333+00:00

Are there also handlers for cbxJoint and other places related to CurrentPerson and comboboxes?

By the way, the Debugger, breakpoints and Call Stack window help in investigating the callers of functions.
RogerSchlueter-7899 1,671 Reputation points

2026-01-11T16:13:32.48+00:00

There are no handlers for cbxJoint but, of course, there are several other places where CurrentPerson can change.

If I put a debug statement as the first line of setter of JointMemberID, it is invoked immediately after the second line of the SelectPerson sub. I have assumed this implies that no other managed code is involved. Is that good assumption?

I guess I'm gonna have to start using the Watch and Stack windows in order to figure out this puzzle.

Answer 1

Hi,

Try adding this to the JointMemberID setter to stop extra repeats:


Set(value As Integer)

    If _JointMemberID = value Then Return

    _JointMemberID = value

    OnPropertyChanged(NameOf(JointMemberID))

End Set

It won’t stop the value changing from 2 to 1 if something else is truly setting it, but it can reduce repeated updates and make the problem easier to track.

Also, I tried your Except(...) line in a small test and it didn’t change any values by itself. So the change is likely coming from the UI (MainWindow XAML), especially if cbxJoint has a TwoWay binding like SelectedValue / SelectedItem bound to JointMemberID. When you replace cbxJoint.ItemsSource, the ComboBox may re-pick a selection and write that back into JointMemberID.

Can you share the XAML for cbxPeople and cbxJoint (the ComboBox lines), and any cbxJoint.SelectionChanged code? That will show what’s writing the value.

Danny Nguyen (WICLOUD CORPORATION) 6,535 Reputation points Microsoft External Staff Moderator

2026-02-03T10:53:13.6166667+00:00

Hi, just checking in, do you have any update on this post?

Answer 2

It seems that the unexpected behavior you are experiencing with the JointMemberID property in your Person class may be related to the way property change notifications are being handled and how the ComboBox is interacting with the data.

Issue 1: JointMemberID Changing Unexpectedly

The JointMemberID property is correctly raising the OnPropertyChanged event when it is set. However, if the ComboBox is bound to a collection that is being updated, it may trigger the property setter multiple times. Ensure that the JointMemberID is not being set elsewhere in your code inadvertently. You might want to check if there are any bindings or event handlers that are modifying this property when the ComboBox selection changes.

Issue 2: Multiple Change Notifications

The behavior where the SelectPerson method runs multiple times could be due to the SelectionChanged event being triggered multiple times. This can happen if the ItemsSource of cbxJoint is being set in a way that causes the SelectionChanged event to fire again. To mitigate this, you can add a check to ensure that the CurrentPerson is actually different from the previously selected person before proceeding with the logic that updates cbxJoint.

Here’s a modified version of your SelectPerson method that includes a check:

Private Sub SelectPerson(sender As Object, e As SelectionChangedEventArgs) Handles cbxPeople.SelectionChanged
    Dim selectedPerson As Person = CType(cbxPeople.SelectedItem, Person)
    If selectedPerson IsNot CurrentPerson Then
        CurrentPerson = selectedPerson
        cbxJoint.ItemsSource = ocPeople.Except({CurrentPerson})
    End If
End Sub

This way, the logic to update the cbxJoint will only execute if the selected person has actually changed, which should help prevent the multiple updates you are seeing.

Summary

Ensure that the JointMemberID is not being modified elsewhere in your code.
Add a check to prevent the SelectPerson method from executing its logic multiple times unnecessarily.

By addressing these points, you should be able to resolve the unexpected changes in the JointMemberID and the multiple executions of your selection logic.

References:

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Class Properties Changing in Unexpected Ways

2 answers

Issue 1: JointMemberID Changing Unexpectedly

Issue 2: Multiple Change Notifications

Summary

Your answer