dotnet/wpf · warning · FormatException

SR.Parsers_IllegalToken

Error message

SR.Parsers_IllegalToken

What it means

Parsers.ParseHexChar converts a single character to its hex value and throws FormatException (SR.Parsers_IllegalToken) for any character outside 0-9, a-f, A-F. It is used by ParseHexColor when parsing #-prefixed color strings like #AARRGGBB in WPF XAML.

Solutions

1. Fix the color string so it contains only hex characters after '#' (formats: #RGB, #ARGB, #RRGGBB, #AARRGGBB)
2. Validate with a try-parse before use, e.g. ColorConverter.ConvertFromString in a try/catch or Regex ^#([0-9a-fA-F]{3,8})$
3. Correct the source XAML/config file where the malformed token originates

Example fix

// before
Color c = (Color)ColorConverter.ConvertFromString("#GG20FF"); // FormatException
// after
var m = System.Text.RegularExpressions.Regex.Match(input, "^#([0-9a-fA-F]{3}|[0-9a-fA-F]{4}|[0-9a-fA-F]{6}|[0-9a-fA-F]{8})$");
if (m.Success)
    Color c = (Color)ColorConverter.ConvertFromString(input);

static readonly Regex HexColorRegex = new Regex("^#([0-9a-fA-F]{3}|[0-9a-fA-F]{4}|[0-9a-fA-F]{6}|[0-9a-fA-F]{8})$");
bool IsValidHexColor(string s) => s != null && HexColorRegex.IsMatch(s);

try { color = (Color)ColorConverter.ConvertFromString(input); } catch (FormatException ex) { color = Colors.Transparent; LogParseFailure(input, ex); }

When it happens

Trigger: Parsing a color string containing an invalid hex digit, e.g. #GGHHII, a too-short/odd-length hex string whose character is non-hex, or locale/typo mistakes in XAML color attributes.

Common situations: Typo in XAML Color values (e.g. Background="#Ff00G0"), hand-built color strings from user input or config, generated XAML with corrupted values.

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.

Related errors

• SR.CompositeFontInvalidUnicodeRange
• SR.Format(SR.InvalidStringCornerRadius, s)
• SR.Format(SR.LengthFormatError, span.ToString())
• errMsg (dynamic: message of caught…
• FileFormatException(new Uri(fileName…

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/0a7c3f62fc6a8b21. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Parsers.cs:35

        private static int ParseHexChar(char c )
        {
            int intChar = (int) c;

            if ((intChar >= s_zeroChar) && (intChar <= (s_zeroChar+9)))
            {
                return (intChar-s_zeroChar);
            }

            if ((intChar >= s_aLower) && (intChar <= (s_aLower+5)))
            {
                return (intChar-s_aLower + 10);
            }

            if ((intChar >= s_aUpper) && (intChar <= (s_aUpper+5)))
            {
                return (intChar-s_aUpper + 10);
            }
            throw new FormatException(SR.Parsers_IllegalToken);
        }

        private static Color ParseHexColor(string trimmedColor)
        {
            int a,r,g,b;
            a = 255;

            if ( trimmedColor.Length > 7 )
            {
                a = ParseHexChar(trimmedColor[1]) * 16 + ParseHexChar(trimmedColor[2]);
                r = ParseHexChar(trimmedColor[3]) * 16 + ParseHexChar(trimmedColor[4]);
                g = ParseHexChar(trimmedColor[5]) * 16 + ParseHexChar(trimmedColor[6]);
                b = ParseHexChar(trimmedColor[7]) * 16 + ParseHexChar(trimmedColor[8]);
            }
            else if ( trimmedColor.Length > 5)
            {
                r = ParseHexChar(trimmedColor[1]) * 16 + ParseHexChar(trimmedColor[2]);
                g = ParseHexChar(trimmedColor[3]) * 16 + ParseHexChar(trimmedColor[4]);

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