unoplatform/uno · error · NotSupportedException

Flag mode is not supported

Error message

Flag mode is not supported

What it means

WeCantSpell.Hunspell's FlagParser switches on the configured FlagParsingMode to wire parse delegates; the default case calls a NoInlining throwNotSupportedFlagMode() that throws NotSupportedException. Flag modes (none/long/numeric/UTF-8) select how .aff dictionary flags are decoded; an unrecognized mode value (or a default(FlagParsingMode) from unconfigured input) lands in the default branch.

Source

Thrown at src/AddIns/Uno.WinUI.SpellChecking/WeCantSpell.Hunspell/FlagParser.cs:129

                    TryParseFlag = TryParseFlagWithUnicodeReDecode;
                    ParseFlagsInOrder = ParseFlagsInOrderWithUnicodeReDecode;
                    ParseFlagSet = ParseFlagSetWithUnicodeReDecode;
                }

                break;
            default:
                throwNotSupportedFlagMode();
                TryParseFlag = null!;
                ParseFlagsInOrder = null!;
                ParseFlagSet = null!;
                break;
        }

#if !NO_EXPOSED_NULLANNOTATIONS
        [System.Diagnostics.CodeAnalysis.DoesNotReturn]
#endif
        [MethodImpl(MethodImplOptions.NoInlining)]
        static void throwNotSupportedFlagMode() => throw new NotSupportedException("Flag mode is not supported");
    }

    public readonly TryParseFlagValueDelegate TryParseFlag;
    public readonly ParseFlagValuesDelegate ParseFlagsInOrder;
    public readonly ParseFlagSetDelegate ParseFlagSet;
    public readonly Encoding Encoding;
    public readonly FlagParsingMode Mode;
    private readonly TextDictionary<FlagSet> _flagSetCache;

    public FlagParser WithEncoding(Encoding encoding) =>
        Encoding.Equals(encoding) ? this : new(Mode, encoding);

    public FlagParser WithMode(FlagParsingMode mode) =>
        Mode == mode ? this : new(mode, Encoding);

    public FlagValue ParseFlagOrDefault(ReadOnlySpan<char> text)
    {
        _ = TryParseFlag(text, out var result);

View on GitHub (pinned to 0418340488)

Solutions

  1. Inspect the .aff file's FLAG: line and correct it to a supported value (long, num, UTF-8) or remove it to use the default single-char mode.
  2. Validate the FlagParsingMode enum value before constructing the FlagParser (is it one of the defined supported modes?).
  3. Upgrade/replace the offending dictionary, or fall back to the default flag parser.

Example fix

// before
var parser = new FlagParser((FlagParsingMode)999, encoding); // throws

// after
var mode = Enum.IsDefined(typeof(FlagParsingMode), raw) ? (FlagParsingMode)raw : FlagParsingMode.Default;
var parser = new FlagParser(mode, encoding);
Defensive patterns

Strategy: validation

Validate before calling

if (!Enum.IsDefined(typeof(FlagParsingMode), raw)) raw = FlagParsingMode.Default;
var parser = new FlagParser((FlagParsingMode)raw, encoding);

Type guard

static bool IsSupportedFlagMode(object raw) => Enum.IsDefined(typeof(FlagParsingMode), raw);

Try / catch

try { new FlagParser(mode, encoding); }
catch (NotSupportedException) { mode = FlagParsingMode.Default; }

Prevention

When it happens

Trigger: Constructing a FlagParser with a FlagParsingMode that is not one of the supported enum values, or an AffReader that read a FLAG: line with an unrecognized token and mapped it to a default/unknown mode.

Common situations: Loading a malformed or exotic .aff dictionary whose FLAG directive contains an unrecognized keyword; a serializer/data path that produced an out-of-range FlagParsingMode enum; a Hunspell dictionary version with a flag mode Uno's bundled Hunspell doesn't support.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/b705fe86adb9ccc6. Report an issue: GitHub.