unoplatform/uno · error · FormatException

Color code must start with #

Error message

Color code must start with #

What it means

ParseColorCode parses XAML hex color literals and requires the value to begin with '#'. Any input lacking the leading '#' throws FormatException. This runs during XAML code generation whenever a color attribute/resource resolves to a hex string.

Source

Thrown at src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/Utils/ColorCodeParser.cs:18

#nullable enable

using System;

namespace Uno.UI.SourceGenerators.XamlGenerator.Utils
{
	internal static class ColorCodeParser
	{
		public static string ParseColorCode(string colorCode)
		{
			if (colorCode == null)
			{
				throw new ArgumentNullException(nameof(colorCode));
			}

			if (!colorCode.StartsWith("#", StringComparison.Ordinal))
			{
				throw new FormatException("Color code must start with #");
			}

			byte a;
			byte r;
			byte g;
			byte b;

			if (colorCode.Length == 4)
			{
				a = 0xFF;
				r = Convert.ToByte(new string(colorCode[1], 2), 16);
				g = Convert.ToByte(new string(colorCode[2], 2), 16);
				b = Convert.ToByte(new string(colorCode[3], 2), 16);
			}
			else if (colorCode.Length == 5)
			{
				a = Convert.ToByte(new string(colorCode[1], 2), 16);
				r = Convert.ToByte(new string(colorCode[2], 2), 16);

View on GitHub (pinned to 0418340488)

Solutions

1. Prefix the color value with '#', e.g. Color="#FFFF0000".
2. Use a XAML-recognized named color or a {ThemeResource}/{StaticResource} brush instead of a raw hex string where appropriate.
3. Validate color literals before they reach the generator.

Example fix

<!-- before -->
<SolidColorBrush Color="FF0000" />
<!-- after -->
<SolidColorBrush Color="#FFFF0000" />

static bool IsValidHexColor(string s) => !string.IsNullOrEmpty(s) && s.StartsWith("#", StringComparison.Ordinal);
// reject color literals before they reach the generator if !IsValidHexColor(value)

When it happens

Trigger: A color value passed to the parser does not start with '#', e.g. a bare hex 'FF0000', a named color like 'Red' routed where hex is expected, or an empty/garbage string.

Common situations: Hand-edited XAML with a Color attribute missing the '#'; a resource/binding that evaluates to a non-hex string at generation time.

Related errors

• Failed to parse color code: #{colorCode}
• The {0} string passed in the colorString argument is not a r
• The {0} string passed in the colorString argument is not a r
• The {0} string passed in the colorString argument is not a r
• Expected end of x:Bind expression or start of argument list.

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/d7afb0c51b8ea045. Report an issue: GitHub.