dotnet/wpf · warning · InvalidOperationException

SR.DoesNotSupportMultipleSelection

Error message

SR.DoesNotSupportMultipleSelection

What it means

WindowsCalendar.CalendarDay's ISelectionItemProvider.AddToSelection() throws InvalidOperationException(SR.DoesNotSupportMultipleSelection) when WindowsCalendar.IsMultiSelect(_hwnd) is false. A single-selection MonthCalendar cannot add a second date to the selection, so AddToSelection is invalid.

Solutions

1. Check whether the calendar supports multi-select before calling AddToSelection (e.g. query the control style or SelectionPattern.CanSelectMultiple).
2. Use Select() instead of AddToSelection() on single-selection calendars.
3. Enable multi-select on the underlying control (MSMultiSelect style / MaxSelectionCount) if multiple dates are required.
4. Catch InvalidOperationException and fall back to Select() to replace the current selection.

Example fix

// before
selectionItemPattern.AddToSelection();
// after
if (selectionPattern.Current.CanSelectMultiple)
{
    selectionItemPattern.AddToSelection();
}
else
{
    selectionItemPattern.Select();
}

if (selectionPattern.Current.CanSelectMultiple) { selectionItemPattern.AddToSelection(); } else { selectionItemPattern.Select(); }

try { selectionItemPattern.AddToSelection(); } catch (InvalidOperationException) { selectionItemPattern.Select(); /* single-select fallback */ }

When it happens

Trigger: Calling AddToSelection() on a day element when the native calendar is single-select (no MCS_MULTISELECT style / IsMultiSelect false) and another date is or would be selected.

Common situations: UIA clients using generic multi-selection logic on the standard (single-select) MonthCalendar; scripts assuming SelectionItem.AddToSelection is always supported.

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

• SR.UIA_OperationCannotBePerformed
• Can only call SelectAll when SelectionMode is Multiple or…
• ElementNotEnabledException
• Microsoft.Windows.Controls.SR.UIA_OperationCannotBePerformed
• Operation cannot be performed.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/01ec297bc45d7876. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClientSideProviders/MS/Internal/UnsupportedAutomationProxies/WindowsCalendar.cs:3679

                    // Make sure that the control is enabled
                    if (!SafeNativeMethods.IsWindowEnabled(_hwnd))
                    {
                        throw new ElementNotEnabledException();
                    }

                    ISelectionItemProvider selProvider = (ISelectionItemProvider) this;

                    // check if item already selected
                    if (selProvider.IsSelected)
                    {
                        return;
                    }

                    // if does not support multiple selection
                    // and other element(not us) is already selected return false
                    if (!WindowsCalendar.IsMultiSelect(_hwnd))
                    {
                        throw new InvalidOperationException(SR.DoesNotSupportMultipleSelection);
                    }

                    DateTime [] dateSelection =
                        WindowsCalendar.CreateDateTimeFromSystemTime (
                            WindowsCalendar.GetRawValue (_hwnd, WindowsCalendar.CalendarData.Selection));
                    DateTime itemDateTime = GetDateTime();

                    // Valid grid item but no date int the cell, before beginning or end of the week
                    if (itemDateTime == DateTime.MinValue)
                    {
                        throw new InvalidOperationException(SR.OperationCannotBePerformed);
                    }

                    // The day can only be added if it 
                    TimeSpan ts = dateSelection[0] - itemDateTime;

                    if (ts.Days == 1)
                    {

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