Comboxbox auto select first item when data is available

I am looking for way to select the first item when data became available. But if no data in the source , then do not select. How to do it ? I am very new to WPF.

<ComboBox Grid.Row="5" Grid.Column="1"
          IsEditable="False"
          ItemsSource="{Binding Source={x:Static l:DirectXResolution.Resolutions}}"
          ToolTip="Resolutions">
    <ComboBox.Resources>
        <l:Res开发者_JAVA技巧olutionConverter x:Key="resolutionConverter"/>
    </ComboBox.Resources>
    <ComboBox.Text>
        <MultiBinding Converter="{StaticResource resolutionConverter}">
            <Binding Path="GameWidth" Mode="OneWayToSource"/>
            <Binding Path="GameHeight" Mode="OneWayToSource"/>
        </MultiBinding>
    </ComboBox.Text>
</ComboBox>

The easiest way is to use SelectedIndex. Please, check the code below.

<ComboBox Grid.Row="5" Grid.Column="1"
          IsEditable="False"
          ItemsSource="{Binding Source={x:Static l:DirectXResolution.Resolutions}}"
          ToolTip="Resolutions"
          SelectedIndex="0">
....

DirectXResolution.Resolutions must be ObservableCollection<T> otherwise your ComboBox will not be updated when the data becomes available. You can use CollectionChanged event of ObservableCollection<T> to select the first item.

If DirectXResolution.Resolutions is not ObservableCollection, create a wrapper for this collection and inherit INotifyCollectionChanged

Here is how to do it in code:

Items.CollectionChanged += (sender, e) =>
{
    if (!Items.Contains(Selected))
    {
        Selected = Items.FirstOrDefault();
    }
};

Items is an ObservableCollection that may be updated. Selected is a two-way property of the selected item. This code should be placed in the constructor of your view-model.