The BasedOn attribute is required for custom styles only when using NoXaml dlls and the Implicit Styles theming mechanism. For more information about this see the Setting a Theme
and Styling Controls
If you are not using NoXaml dlls you can skip setting the BasedOn property. Actually, in this scenario if you set the property, an exception will be thrown because the inherited Style won't be found.
If you are using NoXaml dlls and skip setting the BasedOn property, the custom style will completely override the default style of the control. This includes also its Template. Which means nothing will be displayed.
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