ComboBox itemsSource and default selected value doesn't work - wpf

Question

I have this code:
<ComboBox Name="cbxWorkers" HorizontalContentAlignment="Right"
ItemsSource="{Binding Workers}">
<ComboBoxItem IsSelected="True" Content="Select" />
<ComboBox.ItemTemplate>
<DataTemplate>
<ComboBoxItem Content="{Binding LastName}" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
Everything works fine except the second line.
It gives me a runtime exception:
Items collection must be empty before using ItemsSource.
How can I deal with that so I will get also all the Workers, and also the item - "Select" as the first item of the combobox?
Thanks a lot :)

Answer 1

You can't have two sources. You'd have to specify in code from the ItemsSource which item you want selected.
<ComboBox Name="cbxWorkers" HorizontalContentAlignment="Right" ItemsSource="{Binding Workers}">
<ComboBox.ItemTemplate>
<DataTemplate>
<ComboBoxItem Content="{Binding LastName}" IsSelected="{Binding isSelected}" />
</DataTemplate>
</ComboBox.ItemTemplate>
You can do this, or you can just make an extra first Item in C#/VB and make sure it's selected.

Answer 2

You can do this with a CompositeCollection:
<ComboBox x:Name="cbxWorkers" SelectedIndex="0">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=LastName}" />
</DataTemplate>
</ComboBox.ItemTemplate>
<ComboBox.ItemsSource>
<CompositeCollection>
<ComboBoxItem Content="Select" />
<CollectionContainer Collection="{Binding Workers}" />
</CompositeCollection>
</ComboBox.ItemsSource>
</ComboBox>
Note: you need to set the SelectedIndex="0" on the ComboBox, because when the ComboBoxItem is in the ItemsSource, its IsSelected property won't set the selection on the ComboBox.
Edit concerning CollectionContainer
As #H.B. pointed out, the Binding on CollectionContainer won't work this way. You have a couple of options. They are spelled out for you by this CodeProject article, so I won't repeat them here. The one method that doesn't mention is the newish (as of .NET 4) x:Reference option. It would be used like this:
<CollectionContainer Collection="{Binding DataContext.Workers, Source={x:Reference cbxWorkers}}" />