dotnet/wpf · error · InvalidEnumArgumentException

InvalidEnumArgumentException ("defaultResult"…

Error message

InvalidEnumArgumentException ("defaultResult", (int)defaultResult, typeof(MessageBoxResult))

What it means

MessageBox.ShowCore validates that defaultResult is a defined MessageBoxResult value (None, OK, Cancel, Yes, No). This parameter sets the dialog's default button; an undefined value raises InvalidEnumArgumentException naming parameter "defaultResult".

Solutions

Pass a defined MessageBoxResult value: None, OK, Cancel, Yes, or No.
Validate external values with Enum.IsDefined(typeof(MessageBoxResult), value) before calling Show.
Match defaultResult to the button set: e.g. don't use MessageBoxResult.Yes with MessageBoxButton.OKCancel.

Example fix

// before
MessageBox.Show(msg, title, MessageBoxButton.YesNo, MessageBoxImage.Question, (MessageBoxResult)lastInt);
// after
var def = Enum.IsDefined(typeof(MessageBoxResult), lastInt) && lastInt != (int)MessageBoxResult.None ? (MessageBoxResult)lastInt : MessageBoxResult.No;
MessageBox.Show(msg, title, MessageBoxButton.YesNo, MessageBoxImage.Question, def);

Defensive patterns

Strategy: validation

Validate before calling

bool valid = Enum.IsDefined(typeof(MessageBoxResult), defaultResult);
if (!valid) defaultResult = MessageBoxResult.None;

Type guard

bool IsValidResult(MessageBoxResult r) => r is MessageBoxResult.None or MessageBoxResult.OK or MessageBoxResult.Cancel or MessageBoxResult.Yes or MessageBoxResult.No;

Try / catch

try { MessageBox.Show(msg, title, button, icon, defaultResult); }
catch (InvalidEnumArgumentException ex) when (ex.ParamName == "defaultResult") { MessageBox.Show(msg, title, button, icon, MessageBoxResult.None); }

Prevention

Keep MessageBoxResult and MessageBoxButton enums separate
Make defaultResult consistent with the button set (e.g. Yes only with YesNo*)
Validate persisted results before reusing them

When it happens

Trigger: Calling a MessageBox.Show overload that takes a defaultResult with an undefined value, e.g. (MessageBoxResult)99 from a config int, or passing a MessageBoxButton/other enum in the defaultResult slot.

Common situations: Persisting last-used dialog result and feeding it back as default; mixing up MessageBoxResult with MessageBoxButton (both small enums); computing the value from a switch with a bad default.

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/a7ce173026ffde59. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/MessageBox.cs:380

            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)");
            }*/

            if ( (options & (MessageBoxOptions.ServiceNotification | MessageBoxOptions.DefaultDesktopOnly)) != 0)
            {
                if (owner != IntPtr.Zero)
                {
                    throw new ArgumentException(SR.CantShowMBServiceWithOwner);
                }

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