将此类添加到项目后,可以使用此XAML代码显示默认值：

<local:ExtendedComboBox ItemsSource="{Binding ...Whatever...}" NotSelectedText="Select item..." />

此外,这是带有此控件的test page.我认为第二个组合框就是你所需要的.

这个类的完整代码：

[TemplateVisualState(Name = ExtendedComboBox.StateNormal,GroupName = ExtendedComboBox.GroupItemsSource)]
[TemplateVisualState(Name = ExtendedComboBox.StateNotSelected,GroupName = ExtendedComboBox.GroupItemsSource)]
[TemplateVisualState(Name = ExtendedComboBox.StateEmpty,GroupName = ExtendedComboBox.GroupItemsSource)]
public class ExtendedComboBox : ComboBox
{
    public const string GroupItemsSource = "ItemsSourceStates";
    public const string StateNormal = "Normal";
    public const string StateNotSelected = "NotSelected";
    public const string StateEmpty = "Empty";

    private ContentPresenter selectedContent;

    public ExtendedComboBox()
    {
        this.DefaultStyleKey = typeof(ComboBox);
    }

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        this.selectedContent = this.GetTemplateChild("ContentPresenter") as ContentPresenter;

        // This event can change the NotSelected state
        this.SelectionChanged += (s,e) => this.SetTextIfEmpty();

        // Set a state at start
        this.SetTextIfEmpty();
    }

    // This method can change the Empty state
    protected override void OnItemsChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        base.OnItemsChanged(e);
        this.SetTextIfEmpty();
    }

    /// <summary>
    /// Text if the SelectedItem property is null.
    /// </summary>
    public string NotSelectedText
    {
        get { return (string)GetValue(NotSelectedTextProperty); }
        set { SetValue(NotSelectedTextProperty,value); }
    }

    public static readonly DependencyProperty NotSelectedTextProperty =
        DependencyProperty.Register("NotSelectedText",typeof(string),typeof(ExtendedComboBox),new PropertyMetadata(" "));

    /// <summary>
    /// Text if there are no items in the ComboBox at all.
    /// </summary>
    public string EmptyText
    {
        get { return (string)GetValue(EmptyTextProperty); }
        set { SetValue(EmptyTextProperty,value); }
    }

    public static readonly DependencyProperty EmptyTextProperty =
        DependencyProperty.Register("EmptyText",new PropertyMetadata(null));

    /// <summary>
    /// Changes the state of this control and updates the displayed text.
    /// </summary>
    protected void SetTextIfEmpty()
    {
        if (this.selectedContent == null || !(this.selectedContent.Content is TextBlock))
            return;
        var text = this.selectedContent.Content as TextBlock;

        if (this.SelectedItem == null && this.Items != null && this.Items.Count > 0)
        {
            text.Text = this.NotSelectedText;
            VisualStateManager.GoToState(this,ExtendedComboBox.StateNotSelected,true);
        }
        else if (this.SelectedItem == null)
        {
            text.Text = this.EmptyText ?? this.NotSelectedText;
            VisualStateManager.GoToState(this,ExtendedComboBox.StateEmpty,true);
        }
        else VisualStateManager.GoToState(this,ExtendedComboBox.StateNormal,true);
    }
}