I have a GridView (Winforms v. 2017.3.1017.40, same problem with 2017.2.502.40) configured as on the attached screenshot (made some groups, made a search, and used the "excel-like" filter on a column to uncheck some values). Then I try to click on some column headers to sort the grid. After a few clicks, I get this error : InvalidOperationException: La collection a été modifiée ; l'opération d'énumération peut ne pas s'exécuter. I got different stacktrace with the same error : InvalidOperationException: La collection a été modifiée ; l'opération d'énumération peut ne pas s'exécuter. at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource) at System.Collections.Generic.List`1.Enumerator.MoveNextRare() at System.Collections.Generic.List`1.Enumerator.MoveNext() at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext() at Telerik.WinControls.UI.GridViewGroupRowInfo.get_ChildRows() at Telerik.WinControls.UI.GridViewRowInfo.HasChildRows() at Telerik.WinControls.UI.PrintGridTraverser.CanStepInHierarchy() at Telerik.WinControls.UI.GridTraverser.StepInHierarchy() at Telerik.WinControls.UI.GridTraverser.MoveNextCore() at Telerik.WinControls.UI.GridTraverser.MoveNext() at Telerik.WinControls.UI.GridViewSearchRowInfo.worker_DoWork(Object sender, DoWorkEventArgs e) at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e) at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument) ----- InvalidOperationException: Failed to compare two elements in the array. ---> NullReferenceException: Object reference not set to an instance of an object. at Telerik.WinControls.UI.GridViewRowInfoComparer.CompareRows(GridViewRowInfo x, GridViewRowInfo y, SortDescriptorCollection context) at System.Collections.Generic.ArraySortHelper`1.InternalBinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer) at System.Collections.Generic.ArraySortHelper`1.BinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer) --- End of inner exception stack trace --- at System.Collections.Generic.ArraySortHelper`1.BinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer) at System.Collections.Generic.List`1.BinarySearch(Int32 index, Int32 count, T item, IComparer`1 comparer) at Telerik.Collections.Generic.HybridIndex`1.PerformWithQuickSort() at Telerik.Collections.Generic.HybridIndex`1.get_Items() at Telerik.Collections.Generic.Index`1.GetEnumerator() at Telerik.WinControls.Data.GroupBuilder`1.Perform(IReadOnlyCollection`1 items, Int32 level, Group`1 parent) at Telerik.WinControls.Data.GroupBuilder`1.get_Groups() at Telerik.WinControls.UI.GridViewInfo.Refresh() at Telerik.WinControls.UI.GridViewInfo.get_ChildRows() at Telerik.WinControls.UI.ViewInfoTraverser.SetCollectionForStage(Boolean initializeCollection) at Telerik.WinControls.UI.ViewInfoTraverser.ChangeCollectionForward() at Telerik.WinControls.UI.ViewInfoTraverser.MoveNextCore() at Telerik.WinControls.UI.ViewInfoTraverser.MoveNext() at Telerik.WinControls.UI.GridTraverser.MoveNextCore() at Telerik.WinControls.UI.GridTraverser.MoveNext() at Telerik.WinControls.UI.BaseGridNavigator.NavigateToRow(GridViewRowInfo row, Boolean select) at Telerik.WinControls.UI.BaseGridNavigator.ProcessViewChangedEvent(GridViewEvent eventData) at Telerik.WinControls.UI.BaseGridNavigator.ProcessEventCore(GridViewEvent eventData) at Telerik.WinControls.UI.GridViewEventProcessEntity.ProcessCollection(GridViewEvent gridEvent, PriorityWeakReferenceList list, GridEventProcessMode processMode) at Telerik.WinControls.UI.GridViewEventProcessEntity.ProcessEvent(GridViewEvent gridEvent) at Telerik.WinControls.UI.GridViewSynchronizationService.NotifyListeners(GridViewEvent gridEvent) at Telerik.WinControls.UI.GridViewSynchronizationService.FlushEvents() at Telerik.WinControls.UI.GridViewSynchronizationService.DispatchEvent(GridViewEvent gridEvent) at Telerik.WinControls.UI.GridViewSynchronizationService.DispatchEvent(GridViewTemplate template, GridViewEvent eventData, Boolean postUI) at Telerik.WinControls.UI.GridViewTemplate.DispatchDataViewChangedEvent(Object sender, DataViewChangedEventArgs args) at Telerik.WinControls.UI.GridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e) at Telerik.WinControls.UI.MasterGridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e) at Telerik.WinControls.Data.RadDataView`1.RebuildData(Boolean notify) at Telerik.WinControls.Data.RadDataView`1.OnNotifyPropertyChanged(PropertyChangedEventArgs e) at Telerik.WinControls.Data.RadCollectionView`1.OnNotifyPropertyChanged(String propertyName) at Telerik.WinControls.UI.GridViewSortDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args) at System.ComponentModel.PropertyChangedEventHandler.Invoke(Object sender, PropertyChangedEventArgs e) at Telerik.WinControls.Data.SortDescriptor.set_Direction(ListSortDirection value) at Telerik.WinControls.UI.GridViewColumn.Sort(RadSortOrder sortOrder, Boolean multiSortMode) at Telerik.WinControls.UI.GridHeaderCellElement.Sort(RadSortOrder sortOrder) at Telerik.WinControls.UI.GridHeaderRowBehavior.OnMouseUp(MouseEventArgs e) at Telerik.WinControls.UI.BaseGridBehavior.OnMouseUp(MouseEventArgs e) at Telerik.WinControls.UI.RadGridView.OnMouseUp(MouseEventArgs e) at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) at System.Windows.Forms.Control.WndProc(Message& m) at Telerik.WinControls.RadControl.WndProc(Message& m) at Telerik.WinControls.UI.RadGridView.WndProc(Message& m) at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) ------ InvalidOperationException: Collection was modified; enumeration operation may not execute. at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource) at System.Collections.Generic.List`1.Enumerator.MoveNextRare() at System.Collections.Generic.List`1.Enumerator.MoveNext() at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext() [...] at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source) at Alto.Client.Controls.Dispatch.DispatchResources.DeliveryPointFilterManager.get_DeliveryPointsVisible() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\DeliveryPointFilterManager.cs:line 38 at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.get_Title() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 1520 at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.updatePanelTitle() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 543 at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.onFilterChanged(Object sender, GridViewCollectionChangedEventArgs e) in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 547 at Telerik.WinControls.UI.GridViewCollectionChangedEventHandler.Invoke(Object sender, GridViewCollectionChangedEventArgs e) at Telerik.WinControls.UI.RadGridView.OnFilterChanged(Object sender, GridViewCollectionChangedEventArgs e) at Telerik.WinControls.UI.EventDispatcher.RaiseEvent[T](Object eventKey, Object sender, T args) at Telerik.WinControls.UI.GridViewFilterDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args) at Telerik.Collections.Generic.NotifyCollection`1.InsertItem(Int32 index, T item) at Telerik.WinControls.UI.GridViewFilterDescriptorCollection.InsertItem(Int32 index, FilterDescriptor item) at System.Collections.ObjectModel.Collection`1.Add(T item) at Telerik.WinControls.UI.GridViewDataColumn.SetFilterDescriptor(FilterDescriptor value) at Telerik.WinControls.UI.GridHeaderCellElement.filterPopup_FilterConfirmed(Object sender, EventArgs e) at Telerik.WinControls.UI.BaseFilterPopup.OnFilterConfirmed() at Telerik.WinControls.UI.RadListFilterPopup.OnButtonOkClick(EventArgs e) at Telerik.WinControls.UI.RadListFilterPopup.ButtonOK_Click(Object sender, EventArgs e) at Telerik.WinControls.RadElement.OnClick(EventArgs e) at Telerik.WinControls.UI.RadButtonItem.OnClick(EventArgs e) at Telerik.WinControls.UI.RadButtonElement.OnClick(EventArgs e) at Telerik.WinControls.RadElement.DoClick(EventArgs e) at Telerik.WinControls.RadElement.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args) at Telerik.WinControls.RadItem.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args) at Telerik.WinControls.RadElement.RaiseRoutedEvent(RadElement sender, RoutedEventArgs args) at Telerik.WinControls.RadElement.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args) at Telerik.WinControls.RadItem.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args) at Telerik.WinControls.RadElement.RaiseRoutedEvent(RadElement sender, RoutedEventArgs args) at Telerik.WinControls.RadElement.DoMouseUp(MouseEventArgs e) at Telerik.WinControls.ComponentInputBehavior.OnMouseUp(MouseEventArgs e) at Telerik.WinControls.RadControl.OnMouseUp(MouseEventArgs e) at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.ScrollableControl.WndProc(Message& m) at Telerik.WinControls.RadControl.WndProc(Message& m) at Telerik.WinControls.UI.RadPopupControlBase.WndProc(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) ---- InvalidOperationException: Collection was modified; enumeration operation may not execute. at System.Collections.Generic.List`1.Enumerator.MoveNextRare() at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext() at Telerik.WinControls.Data.GroupBuilder`1.Perform(IReadOnlyCollection`1 items, Int32 level, Group`1 parent) at Telerik.WinControls.Data.DataItemGroup`1.get_Groups() at Telerik.WinControls.UI.DataGroup.get_Groups() at Telerik.WinControls.UI.GridViewTemplate.InvalidateSummaryRowsCore(DataGroup group, Boolean invalidateChildren) at Telerik.WinControls.UI.GridViewTemplate.InvalidateSummaryRowsCore(DataGroup group, Boolean invalidateChildren) at Telerik.WinControls.UI.GridViewTemplate.InvalidateGroupSummaryRows(DataGroup group, Boolean invalidateChildren) at Telerik.WinControls.UI.GridViewTemplate.RefreshSummaryRowsInGroup(NotifyCollectionChangedEventArgs e) at Telerik.WinControls.UI.GridViewTemplate.RefreshAggregates(NotifyCollectionChangedEventArgs e) at Telerik.WinControls.UI.MasterGridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e) at Telerik.WinControls.Data.RadDataView`1.RebuildData(Boolean notify) at Telerik.WinControls.Data.RadDataView`1.OnNotifyPropertyChanged(PropertyChangedEventArgs e) at Telerik.WinControls.Data.RadCollectionView`1.OnNotifyPropertyChanged(String propertyName) at Telerik.WinControls.UI.GridViewSortDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args) at Telerik.WinControls.UI.GridViewSortDescriptorCollection.RemoveItem(Int32 index) at Telerik.WinControls.UI.GridViewColumn.Sort(RadSortOrder sortOrder, Boolean multiSortMode) at Telerik.WinControls.UI.GridHeaderCellElement.Sort(RadSortOrder sortOrder) at Telerik.WinControls.UI.GridHeaderRowBehavior.OnMouseUp(MouseEventArgs e) at Telerik.WinControls.UI.BaseGridBehavior.OnMouseUp(MouseEventArgs e) at Telerik.WinControls.UI.RadGridView.OnMouseUp(MouseEventArgs e) at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) at System.Windows.Forms.Control.WndProc(Message& m) at Telerik.WinControls.RadControl.WndProc(Message& m) at Telerik.WinControls.UI.RadGridView.WndProc(Message& m) at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)