MahApps/MahApps.Metro · warning · NotSupportedException

Unknown SelectionMode

Error message

Unknown SelectionMode

What it means

Thrown by the MahApps clear-text command when a MultiSelectionComboBox.SelectionMode value is not Single, Multiple, or Extended. The switch exhausts the standard System.Windows.Controls.SelectionMode enum; the default branch is a defensive guard against an unexpected/extended enum value. In normal usage this is unreachable because SelectionMode only has those three members.

Source

Thrown at src/MahApps.Metro/Controls/Helper/MahAppsCommands.cs:122

                    if (comboBox is MultiSelectionComboBox multiSelectionComboBox)
                    {
                        if (multiSelectionComboBox.HasCustomText)
                        {
                            multiSelectionComboBox.ResetEditableText(true);
                        }
                        else
                        {
                            switch (multiSelectionComboBox.SelectionMode)
                            {
                                case SelectionMode.Single:
                                    multiSelectionComboBox.SetCurrentValue(MultiSelectionComboBox.SelectedItemProperty, null);
                                    break;
                                case SelectionMode.Multiple:
                                case SelectionMode.Extended:
                                    multiSelectionComboBox.SelectedItems?.Clear();
                                    break;
                                default:
                                    throw new NotSupportedException("Unknown SelectionMode");
                            }

                            multiSelectionComboBox.ResetEditableText(true);
                        }

                        comboBox.GetBindingExpression(ComboBox.TextProperty)?.UpdateSource();
                        comboBox.GetBindingExpression(MultiSelectionComboBox.SelectedItemProperty)?.UpdateSource();
                        comboBox.GetBindingExpression(MultiSelectionComboBox.SelectedItemsProperty)?.UpdateSource();
                    }
                    else
                    {
                        if (comboBox.IsEditable)
                        {
                            comboBox.SetCurrentValue(ComboBox.TextProperty, string.Empty);
                            comboBox.GetBindingExpression(ComboBox.TextProperty)?.UpdateSource();
                        }

                        comboBox.SetCurrentValue(Selector.SelectedItemProperty, null);

View on GitHub (pinned to 72099e310b)

Solutions

1. Use only the standard SelectionMode values (Single, Multiple, Extended) on MultiSelectionComboBox.
2. If extending SelectionMode, patch this switch to handle the new case.
3. Report the unexpected value to MahApps if a stock enum legitimately produces it.
4. Avoid casting arbitrary integers into SelectionMode.

// SelectionMode is a closed enum (Single, Multiple, Extended). Validate before relying on it.
if (multiSelectionComboBox.SelectionMode is not (SelectionMode.Single or SelectionMode.Multiple or SelectionMode.Extended))
{
    throw new ArgumentOutOfRangeException(nameof(multiSelectionComboBox.SelectionMode));
}

static bool IsKnownMode(SelectionMode m)
    => m == SelectionMode.Single || m == SelectionMode.Multiple || m == SelectionMode.Extended;

try { /* invoke clear command */ }
catch (NotSupportedException ex) when (ex.Message == "Unknown SelectionMode")
{
    // A future/invalid SelectionMode value; reset to a known mode.
    multiSelectionComboBox.SelectionMode = SelectionMode.Single;
}

When it happens

Trigger: Calling the MahApps 'Clear' textbox command (e.g. clear-text button) on a MultiSelectionComboBox whose SelectionMode was set to a non-standard enum value via reflection or a future framework enum extension; a custom derivative overriding SelectionMode semantics.

Common situations: Effectively unreachable with the current WPF SelectionMode enum; would surface if a fork adds a new SelectionMode member without updating this switch, or if an invalid integer is cast into SelectionMode.

Related errors

• Type must be for an Enum.
• The EnumType must be specified.
• The provided dialog is already visible in the specified wind
• The provided dialog is not visible in the specified window.
• Dialog isn't visible to close

AI-assisted analysis of MahApps/MahApps.Metro@72099e310b (2026-08-13). Data as JSON: /api/errors/a5a85de33bb5638e. Report an issue: GitHub.