.DataContext = _mainViewModel;
.dataFilteQuery.FilterDescriptors.LogicalOperator = Telerik.Windows.Data.FilterCompositionLogicalOperator.Or;
countryFilter.FieldFilter.LogicalOperator = Telerik.Windows.Data.FilterCompositionLogicalOperator.Or;
I suggest you to check this help article, where you can find detailed information about programmatic filtering.
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Please excuse me for misleading you. In my previous post I had shown you how to set the logical operator for RadGridView.
Indeed, the logical operator of RadDataFilter has to be set as you tried. For your convenience I have prepared a sample project. Please find it attached for a reference.
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Indeed, it is working as expected. You can check the attached project. As you can see, I set DataFilter's source in the XAML.