Can only call SelectAll when CanSelectMultipleItems is true.

Error message

Can only call SelectAll when CanSelectMultipleItems is true.

What it means

MultiSelector.SelectAll only works when the selector allows multiple selection; CanSelectMultipleItems is true only when SelectionMode is Extended or Multiple. Calling SelectAll in single-selection mode throws NotSupportedException because selecting all items is meaningless there.

Solutions

Set SelectionMode="Extended" or "Multiple" on the control before calling SelectAll
Guard the call with if (selector.CanSelectMultipleItems)
In single-selection mode, select the desired item with SelectedIndex/SelectedItem instead

Example fix

// before
if (dataGrid.SelectedIndex >= 0) dataGrid.SelectAll();
// after
if (dataGrid.CanSelectMultipleItems)
    dataGrid.SelectAll();
else
    dataGrid.SelectedIndex = 0;

Defensive patterns

Strategy: validation

Validate before calling

if (selector.CanSelectMultipleItems) selector.SelectAll(); else selector.SelectedIndex = 0;

Type guard

static bool CanSelectAll(System.Windows.Controls.Primitives.MultiSelector s) => s.CanSelectMultipleItems;

Try / catch

try { selector.SelectAll(); }
catch (NotSupportedException) { /* SelectionMode is Single - handle single selection instead */ }

Prevention

Check CanSelectMultipleItems before SelectAll/SelectAllCells
Set SelectionMode explicitly (Extended/Multiple) when multi-select is intended
Remember SelectAll is semantically a no-op in single-select mode, hence the throw

When it happens

Trigger: Calling SelectAll()/SelectAllCells() on a DataGrid or MultiSelector whose SelectionMode is Single (or while CanSelectMultipleItems is temporarily false during internal updates).

Common situations: Copy-pasting selection code between a ListBox (Extended default) and a DataGrid configured with SelectionMode="Single", or calling SelectAll before the selection mode is applied.

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/7c93e62f4cb356dd. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/Primitives/MultiSelector.cs:87

            get
            {
                return ((SelectedItemCollection)SelectedItems).IsUpdatingSelectedItems;
            }
        }

        /// <summary>
        ///     Select all the items
        /// Exceptions: InvalidOperationExcpetion if CanSelectMultipleItems is false
        /// </summary>
        public void SelectAll()
        {
            if (CanSelectMultipleItems)
            {
                SelectAllImpl();
            }
            else
            {
                throw new NotSupportedException(SR.MultiSelectorSelectAll);
            }
        }

        /// <summary>
        ///     Clears all of the selected items.
        /// </summary>
        public void UnselectAll()
        {
            UnselectAllImpl();
        }

        #endregion BulkSelection
    }
}

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