Question

I was working with the ComboBox control and couldn’t get the SelectedItem to be set from the property on my viewmodel. Here is the control definition:

<ComboBox x:Name="jobEmployee" Grid.Column="1" Grid.Row="2" 
    Margin="4" HorizontalAlignment="Left" Width="150"
    SelectedItem="{Binding Path=EditingJob.Employee, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}"
    ItemsSource="{Binding Path=Employees, Mode=OneWay}"
    DisplayMemberPath="FullName"/>

I had another Combobox control that worked jut fine. The difference between one that would set the SelectedItem and the one that wouldn’t was the order of the attribute definition. Here is the working control definition:

<ComboBox x:Name="jobEmployee" Grid.Column="1" Grid.Row="2" 
    Margin="4" HorizontalAlignment="Left" Width="150"
    ItemsSource="{Binding Path=Employees, Mode=OneWay}"
    SelectedItem="{Binding Path=EditingJob.Employee, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}"
    DisplayMemberPath="FullName"/>

The difference between the 2 is that the ItemsSource is defined before the SelectedItem on the working one which leads me to believe that in this case at least, the attribute definition order matters. Am I missing something or have others found this to be true? Has it been documented anywhere?

Answer 1

Yes order can matter. Consider that XAML reading involves the creation of objects and the assignment of values to the properties of these objects. Its not possible to assign property values at the same time, clearly one property is going to be assigned followed by another and then another until all properties are assigned.

Since assigning properties in some objects results in side-effects and other code running the order of assigning those properties can impact the result. This of course is a bad thing.