Silverlight：Combobox中的默认值

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);
    }
}