SelectedItem in ObservableCollection

Question

I have:

<DataTemplate x:Name="_ComboBoxTemplate" x:Key="_ComboBoxTemplate">
    <StackPanel Orientation="Horizontal">
        <StackPanel>
            <StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
                <TextBlock VerticalAlignment="Center" Text="{Binding}" FontSize="24"/>
            </StackPanel>
        </StackPanel>
    </StackPanel>
</DataTemplate>

<ComboBox x:Name="_criteria" ItemTemplate="{StaticResource _ComboBoxTemplate}" ItemsSource="{Binding}"/>

In the code behind:

this.DataContext = new string[] { "0", "1", "2", "3", "4", "5" };

How do I get it so that the default SelectedItem in ComboBox is 5?

Solution

Create a class containing both your list of items and the selected item:

public class ViewModel
{
    public string[] Items { get; set; }
    public string SelectedItem { get; set; }
}

Initialize it in code behind and set it as DataContext:

DataContext = new ViewModel
{
    Items = new string[] { "0", "1", "2", "3", "4", "5" },
    SelectedItem = "5"
}

Now bind both properties to your ComboBox:

<ComboBox x:Name="_criteria" 
          ItemTemplate="{StaticResource _ComboBoxTemplate}"
          ItemsSource="{Binding Items}"
          SelectedItem="{Binding SelectedItem}"/>

If you set two way binding for SelectedItem the value in DataContext will even update automatically when the user changes it.

Why did you mention ObservableCollection in the title?