Notes to Inheritors
If you are creating a custom container control, make sure that you render the control and all child controls at design time, regardless of whether the Visible property is set to true or false.
The following code example demonstrates how to override the GetDesignTimeHtml method in a custom control designer. If the Text property for the associated control is empty, the GetDesignTimeHtml method calls the GetEmptyDesignTimeHtml method. Otherwise, the GetDesignTimeHtml method creates and renders a Hyperlink control.