This inherited property is not used by the DeclarativeCatalogPart control. The property is overridden only for the purpose of preventing it from appearing in Microsoft Visual Studio 2005 designer tools.
Assembly: System.Web (in System.Web.dll)
The DeclarativeCatalogPart control, is not used anywhere within the Web Parts control set for rendering the control. Therefore, you should not use this property; setting it has no effect on rendering.property, as a member of the
To prevent developers from using this property, code attributes are set on it to hide it in Visual Studio. When you work with a DeclarativeCatalogPart control in Visual Studio, the property is not visible in the Properties pane or IntelliSense.
If you want to use this property, you need to create a custom class that inherits from the CatalogPart class. The custom class can either use the inherited DefaultButton property as it is, or override it and change its behavior.
Available since 2.0