Parameter Ordering

Although the two ComboBox samples below look very similar they actually function a little differently.
The first sample sets the selected item based on the SelectedItem binding and then almost instantaneously sets the SelectedItem based on the first record in the ItemsSource binding.

<ComboBox
DisplayMemberPath="Description"
SelectedItem="{Binding Path=EditedCourse.Centre, Mode=TwoWay}"
SSForm:FormItem.LabelContent="Centre"
IsEditable="{Binding Path=EditedCourse.IsCentreEditable}">
<ComboBox.ItemsSource>
<Binding
Mode="OneWay">
<Binding.Source>
<CollectionViewSource
x:Name="PART_CentresView"
Source="{Binding Source={StaticResource ViewModel}, Path=Centres, Mode=OneWay}" />
</Binding.Source>
</Binding>
</ComboBox.ItemsSource>
</ComboBox>

The second sample sets the selected item based on the SelectedItem binding only and therefore does what I would have expected to happen from the first sample.

<ComboBox
DisplayMemberPath="Description"
SSForm:FormItem.LabelContent="Centre"
IsEditable="{Binding Path=EditedCourse.IsCentreEditable}">
<ComboBox.ItemsSource>
<Binding
Mode="OneWay">
<Binding.Source>
<CollectionViewSource
x:Name="PART_CentresView"
Source="{Binding Source={StaticResource ViewModel}, Path=Centres, Mode=OneWay}" />
</Binding.Source>
</Binding>
</ComboBox.ItemsSource>
<ComboBox.SelectedItem>
<Binding
Path="EditedCourse.Centre"
Mode="TwoWay" />
</ComboBox.SelectedItem>
</ComboBox>

The implication is that the selected item is not necessarily what you would expect it to be.

In the first sample, the final selected item is the first (default) entry in the item source collection, and in the second sample the final selected item is the item bound to in the SelectedItem property.