How SelectionChanged works in a ComboBox?

Hi,

What I think comboBoxClasses must be populated when I fire comboBoxClasses.SelectionChanged += ComboBoxClasses_SelectionChanged.

The only possibility to populate it is after I select an item in comboBoxDayName, but comboBoxClasses.SelectionChanged += ComboBoxClasses_SelectionChanged destroys the attachment and is empty so the index is -1 and error.

How can I prevent it?

If I put comboBoxClasses.SelectionChanged += ComboBoxClasses_SelectionChanged in Init or OnNavigatedTo it is -1 because no data source is present.

Thanks

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ADRIAN DIBU