When you write markup extensions, or any other control that may work differently at runtime then design time, you may want to check if you’re in design time to pick the correct logic. In WPF, you can call the DesignerProperties.GetIsInDesignMode attached property. In Silverlight, you may use the HtmlPage.IsEnabled property. This will work from both Blend and Cider designers. Example: if ( DesignerProperties .GetIsInDesignMode(textBox) { return "In Design Time Mode" ;...