Thank you for writing.
This approach would not work since the GanttViewDataItem has no such property. Moreover, RadObject is mostly used for UI components and the DependencyProperties are used for the theming mechanism. I would highly recommend you to use simple CLR properties along with the INotifyPropertyChanged interface for your types. You should also create a derivative of GanttViewDataItem which will have the needed CLR properties.
Let me know if you have further questions.
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