Even if you could hack the control and add this operator, the LINQ data engine will not support it since it does not know what to do with it.
The entire data engine (which is shared between all of our data controls including RadGridView and RadDataFilter) relies on the FilterDescriptor class which has only a single Value. Even if we add such an operator, the existing infrastructure, i.e. you can only have one value editor per filter, will not allow the user to enter two separate values which are needed by the IsBetween operator.
In other words we would have to dump and re-write the data engine and RadGridView and RadDataFilter's filtering UI from scratch in order to support this custom operator.
Nevertheless, I will log your requirement as a feature request and if this feature generates enough interest among enough people, we will definitely consider its implementation even though that would mean a complete overhaul of the entire data engine logic.
Here you can vote for this feature.
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