dotnet/wpf · error · InvalidEnumArgumentException

InvalidEnumArgumentException ("button", (int)button…

Error message

InvalidEnumArgumentException ("button", (int)button, typeof(MessageBoxButton))

What it means

MessageBox.ShowCore validates that the button parameter is a defined MessageBoxButton enum value before showing the dialog. An out-of-range or undefined value raises InvalidEnumArgumentException naming parameter "button" and type MessageBoxButton.

Solutions

Pass a defined MessageBoxButton value: OK, OKCancel, YesNo, or YesNoCancel.
Validate external values with Enum.IsDefined(typeof(MessageBoxButton), value) before calling Show.
Check argument order in the Show overload being used to ensure button/icon/defaultResult/options weren't swapped.

Example fix

// before
var b = (MessageBoxButton)cfg.ButtonInt; // may be undefined
MessageBox.Show(msg, title, b);
// after
var b = Enum.IsDefined(typeof(MessageBoxButton), cfg.ButtonInt) ? (MessageBoxButton)cfg.ButtonInt : MessageBoxButton.OK;
MessageBox.Show(msg, title, b);

Defensive patterns

Strategy: validation

Validate before calling

bool valid = Enum.IsDefined(typeof(MessageBoxButton), button);
if (!valid) throw new ArgumentOutOfRangeException(nameof(button));

Type guard

bool IsValidButton(MessageBoxButton b) => b is MessageBoxButton.OK or MessageBoxButton.OKCancel or MessageBoxButton.YesNo or MessageBoxButton.YesNoCancel;

Try / catch

try { MessageBox.Show(msg, title, button); }
catch (InvalidEnumArgumentException ex) when (ex.ParamName == "button") { MessageBox.Show(msg, title, MessageBoxButton.OK); }

Prevention

Validate config-stored button ints with Enum.IsDefined
Never swap the enum parameters in Show overloads
Use named enum members only

When it happens

Trigger: Calling any MessageBox.Show overload with a button argument cast from an invalid int, e.g. (MessageBoxButton)7, or passing a MessageBoxImage/other enum by mistake.

Common situations: Reading the button style from config or a database int column; mapping a Win32 MB_* constant that is out of the MessageBoxButton range; copy-paste swapping parameter order in a Show overload with many enums.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/80e8834767956a03. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/MessageBox.cs:372

                    if (result == MessageBoxResult.Continue) return DEFAULT_BUTTON3;
                    return DEFAULT_BUTTON1;
                default:
                    return DEFAULT_BUTTON1;
            }
        }

        internal static MessageBoxResult ShowCore(
            IntPtr owner,
            string messageBoxText,
            string caption,
            MessageBoxButton button,
            MessageBoxImage icon,
            MessageBoxResult defaultResult,
            MessageBoxOptions options)
        {
            if (!IsValidMessageBoxButton(button))
            {
                throw new InvalidEnumArgumentException ("button", (int)button, typeof(MessageBoxButton));
            }
            if (!IsValidMessageBoxImage(icon))
            {
                throw new InvalidEnumArgumentException ("icon", (int)icon, typeof(MessageBoxImage));
            }
            if (!IsValidMessageBoxResult(defaultResult))
            {
                throw new InvalidEnumArgumentException ("defaultResult", (int)defaultResult, typeof(MessageBoxResult));
            }
            if (!IsValidMessageBoxOptions(options))
            {
                throw new InvalidEnumArgumentException("options", (int)options, typeof(MessageBoxOptions));
            }

            // UserInteractive??
            //
            /*if (!SystemInformation.UserInteractive && (options & (MessageBoxOptions.ServiceNotification | MessageBoxOptions.DefaultDesktopOnly)) == 0) {
                throw new InvalidOperationException("UNDONE: SR.GetString(SR.CantShowModalOnNonInteractive)");

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