Ok I replaced my code with yours and it seems to work, thanks.

I still suggest that you provide an API that will do this for me. I have a feeling that I must assume too much of the TelerikGrid's internals.

If it helps, I've refactored some one the extension methods and added new resuable one:

        public static string? GetFilterMember(this IFilterDescriptor filter)
        {
            var filterDescriptor = filter;

            while (filterDescriptor is CompositeFilterDescriptor)
            {
                filterDescriptor = ((CompositeFilterDescriptor)filterDescriptor).FilterDescriptors?.ElementAtOrDefault(0);
            }

            return (filterDescriptor as FilterDescriptor)?.Member; //added save casting in case someting else than filter parameter is something else than FilterDescriptor or CompositeFilterDescriptor
        }

        public static async Task SetFilterWithReplaceInCollection<TItem>(this TelerikGrid<TItem> grid, FilterDescriptor filterDescriptor)
        {

            var state = grid.GetState();

            foreach (var existingFilter in state.FilterDescriptors.GetFiltersForMember(filterDescriptor.Member).OfType<FilterDescriptorBase>().ToArray())
            {
                state.FilterDescriptors.Remove(existingFilter);
            }

            CompositeFilterDescriptor theFilterDescriptor = new CompositeFilterDescriptor()
            {
                FilterDescriptors = new FilterDescriptorCollection()
                {
                    filterDescriptor,
                    new FilterDescriptor()
                    {
                        Member = filterDescriptor.Member,
                        Operator = FilterOperator.IsEqualTo,
                        Value = null,
                    }
                },
                LogicalOperator = FilterCompositionLogicalOperator.Or
            };

            state.FilterDescriptors.Add(theFilterDescriptor);

            await grid.SetState(state);
        }

Hi Martin,

although I managed to set the filter menu from code, I still don't get the filter icon highlighted:

Perfect, thanks!