dotnet/wpf · error · ArgumentException

SR.AccessKeyManager_NotAUnicodeCharacter

Error message

SR.AccessKeyManager_NotAUnicodeCharacter

What it means

AccessKeyManager.NormalizeKey throws ArgumentException when the given access key string is not a single Unicode character (a text element). WPF access keys are exactly one character, so multi-character strings are rejected before normalization via ToUpperInvariant. The SR resource AccessKeyManager_NotAUnicodeCharacter supplies the message naming the offending 'key' argument.

Solutions

1. Pass exactly one character: key.Substring(0,1) or a char.ToString() value.
2. Validate the string with StringInfo.GetNextTextElement(key) == key before calling the API.
3. Strip modifier prefixes (e.g. "_") and take only the access character, which is what UIHelper/Label does for _Mnemonics.

Example fix

// before
AccessKeyManager.Register("Ctrl+K", window);
// after
AccessKeyManager.Register("K", window);

static bool IsSingleUnicodeChar(string key) =>
    !string.IsNullOrEmpty(key) && StringInfo.GetNextTextElement(key) == key;

static string ToAccessKey(object value)
{
    var s = value as string;
    return (s != null && s.Length == 1) ? s.ToUpperInvariant() : null;
}

try { AccessKeyManager.Register(key, target); }
catch (ArgumentException ex) when (ex.Message.Contains("AccessKeyManager"))
{ /* log: key must be a single Unicode character */ }

When it happens

Trigger: Calling AccessKeyManager.Register, Unregister, IsKeyRegistered, or ProcessKey with a key string whose full text differs from its first text element — i.e. any string longer than one grapheme, such as "AB", surrogate-pair misuse, or an empty string (empty yields firstCharacter="" and ""!="" is false only when key==""... actually GetNextTextElement("") returns "" so empty passes null check but throws here only when key.Length>1).

Common situations: Binding an access key to a label substring or a localized string without slicing out a single character; passing a user-typed string or a config value like "F1" or "Esc" instead of a single character; concatenating modifiers with the key ("Ctrl+K").

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

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AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/9c9ab322fc0c8fdb. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/AccessKeyManager.cs:125

            AccessKeyManager akm = AccessKeyManager.Current;
            return (akm.ProcessKeyForScope(scope, key, isMultiple,false) == ProcessKeyResult.MoreMatches);
        }

        /// <summary>
        /// Returns StringInfo.GetNextTextElement(key).ToUpperInvariant() throwing exceptions for null
        /// and multi-char strings.
        /// </summary>
        /// <param name="key"></param>
        /// <returns></returns>
        private static string NormalizeKey(string key)
        {
            ArgumentNullException.ThrowIfNull(key);

            string firstCharacter = StringInfo.GetNextTextElement(key);

            if (key != firstCharacter)
            {
                throw new ArgumentException(SR.Format(SR.AccessKeyManager_NotAUnicodeCharacter, "key"));
            }

            return firstCharacter.ToUpperInvariant();
        }

        /// <summary>
        /// This event is used by elements that want to define a scope for accesskeys, such as Menu and Popup.
        /// This event will never be raised, it is used to identify classes that define new scopes.
        /// </summary>
        public static readonly RoutedEvent AccessKeyPressedEvent = EventManager.RegisterRoutedEvent(
            "AccessKeyPressed", RoutingStrategy.Bubble, typeof(AccessKeyPressedEventHandler), typeof(AccessKeyManager));

        /// <summary>
        ///     Adds a handler for the AccessKeyPressed attached event
        /// </summary>
        /// <param name="element">UIElement or ContentElement that listens to this event</param>
        /// <param name="handler">Event Handler to be added</param>
        public static void AddAccessKeyPressedHandler(DependencyObject element, AccessKeyPressedEventHandler handler)

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