What causes "Invalid color string: '{s.ToString()}'." in AvaloniaUI/Avalonia?

Use Color.TryParse(ReadOnlySpan , out Color) to avoid the throw on bad input.

AvaloniaUI/Avalonia · error · FormatException

Invalid color string: '{s.ToString()}'.

Error message

Invalid color string: '{s.ToString()}'.

What it means

The ReadOnlySpan<char> overload of Color.Parse throws the same FormatException as the string overload, but for span inputs. It exists for performance-sensitive parsing paths (e.g. XAML/JSON parsers) that avoid allocating a string. The error message interpolates s.ToString() so the offending input is visible.

Source

Thrown at src/Avalonia.Base/Media/Color.cs:135

                return color;
            }

            throw new FormatException($"Invalid color string: '{s}'.");
        }

        /// <summary>
        /// Parses a color string.
        /// </summary>
        /// <param name="s">The color string.</param>
        /// <returns>The <see cref="Color"/>.</returns>
        public static Color Parse(ReadOnlySpan<char> s)
        {
            if (TryParse(s, out Color color))
            {
                return color;
            }

            throw new FormatException($"Invalid color string: '{s.ToString()}'.");
        }

        /// <summary>
        /// Parses a color string.
        /// </summary>
        /// <param name="s">The color string.</param>
        /// <param name="color">The parsed color</param>
        /// <returns>The status of the operation.</returns>
        public static bool TryParse(string? s, out Color color)
        {
            color = default;

            if (string.IsNullOrEmpty(s))
            {
                return false;
            }

            if (s[0] == '#' &&

View on GitHub (pinned to 11c5427268)

Solutions

Use Color.TryParse(ReadOnlySpan<char>, out Color) to avoid the throw on bad input.
Validate the span content matches a known color/hex format before calling Parse.
Ensure upstream tokenization produces a clean color token.

Example fix

// before
var color = Color.Parse(tokenSpan);

// after
if (!Color.TryParse(tokenSpan, out var color))
    color = Colors.Black;

Defensive patterns

Strategy: validation

Validate before calling

if (!Color.TryParse(spanInput, out var color))
    color = Colors.Black;

Try / catch

Color color;
try { color = Color.Parse(spanInput); }
catch (FormatException) { color = Colors.Black; }

Prevention

Use the span-based TryParse overload to avoid both allocations and exceptions.
Ensure upstream tokenizers produce clean color tokens.
Handle parse failures gracefully in high-throughput parsing paths.

When it happens

Trigger: Calling Color.Parse(ReadOnlySpan<char> s) where the span content is not a valid color. Often reached from internal XAML/markup parsers or when slicing a larger buffer.

Common situations: Internal parser feeding a malformed token. A XAML color attribute with a typo. Slicing a buffer incorrectly leaving non-color characters.

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/b498ebb292eb5f30. Report an issue: GitHub.