dotnet/wpf · error · InvalidOperationException

SR.Calendar_OnSelectedDateChanged_InvalidOperation

Error message

SR.Calendar_OnSelectedDateChanged_InvalidOperation

What it means

CheckSelectionMode() throws InvalidOperationException when the Calendar's SelectionMode is None but code attempts to modify or read the selected dates collection in a way that requires selection to be enabled. With SelectionMode.None, the Calendar permits no selection at all, so any selection mutation is an invalid operation.

Solutions

Set Calendar.SelectionMode to SingleDate (or another mode that allows selection) before manipulating SelectedDates.
Guard selection mutations with a check that SelectionMode != CalendarSelectionMode.None.
If the calendar is intentionally display-only, drive highlighting via other means (e.g. styling/BlackoutDates) instead of SelectedDates.

Example fix

// before
calendar.SelectionMode = CalendarSelectionMode.None;
calendar.SelectedDates.Add(DateTime.Today); // throws
// after
calendar.SelectionMode = CalendarSelectionMode.SingleDate;
calendar.SelectedDates.Add(DateTime.Today);

Defensive patterns

Strategy: validation

Validate before calling

if (calendar.SelectionMode == CalendarSelectionMode.None)
{
    // selection is disabled; do not touch SelectedDates
    return;
}
calendar.SelectedDates.Add(date);

Try / catch

try
{
    calendar.SelectedDates.Add(date);
}
catch (InvalidOperationException ex) when (ex.Message.Contains("selection"))
{
    // SelectionMode is None; enable a selection mode or skip
}

Prevention

Verify SelectionMode allows selection before any SelectedDates mutation.
Keep XAML SelectionMode and code-behind selection logic in sync.
Guard shared selection helpers with a SelectionMode != None assertion.

When it happens

Trigger: Calling selectedDates.Add/Clear/etc. (paths that call CheckSelectionMode, e.g. via isCleared logic) while Calendar.SelectionMode == CalendarSelectionMode.None.

Common situations: Setting SelectionMode="None" in XAML for display-only calendars, then programmatically pre-selecting dates in code; a style change switched the mode to None after selection logic was written.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/08deb1f91b7d51b6. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/SelectedDatesCollection.cs:446

            _isAddingRange = true;
        }

        private void EndAddRange()
        {
            Debug.Assert(_isAddingRange);

            _isAddingRange = false;
            RaiseSelectionChanged(this._removedItems, this._addedItems);
            this._removedItems.Clear();
            this._addedItems.Clear();
            this._owner.UpdateCellItems();
        }

        private bool CheckSelectionMode()
        {
            if (this._owner.SelectionMode == CalendarSelectionMode.None)
            {
                throw new InvalidOperationException(SR.Calendar_OnSelectedDateChanged_InvalidOperation);
            }

            if (this._owner.SelectionMode == CalendarSelectionMode.SingleDate && this.Count > 0)
            {
                throw new InvalidOperationException(SR.Calendar_CheckSelectionMode_InvalidOperation);
            }

            // if user tries to add an item into the SelectedDates in SingleRange mode, we throw away the old range and replace it with the new one
            // in order to provide the removed items without an additional event, we are calling ClearInternal
            if (this._owner.SelectionMode == CalendarSelectionMode.SingleRange && !_isAddingRange && this.Count > 0)
            {
                this.ClearInternal();
                return true;
            }
            else
            {
                return false;
            }

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