AvaloniaUI/Avalonia · error · ArgumentException

Base uri must be an absolute uri.

Error message

Base uri must be an absolute uri.

What it means

The FontFamily constructor throws ArgumentException when a non-null baseUri is provided but is not absolute. Font asset resolution requires an absolute base URI to correctly locate font resources; a relative base URI would make resolution ambiguous. Pass null (to indicate no base) or a fully-qualified absolute URI.

Source

Thrown at src/Avalonia.Base/Media/FontFamily.cs:41

        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="T:Avalonia.Media.FontFamily" /> class.
        /// </summary>
        /// <param name="baseUri">Specifies the base uri that is used to resolve font family assets.</param>
        /// <param name="name">The name of the <see cref="T:Avalonia.Media.FontFamily" />.</param>
        /// <exception cref="T:System.ArgumentException">Base uri must be an absolute uri.</exception>
        public FontFamily(Uri? baseUri, string name)
        {
            if (string.IsNullOrEmpty(name))
            {
                throw new ArgumentNullException(nameof(name));
            }

            if (baseUri != null && !baseUri.IsAbsoluteUri)
            {
                throw new ArgumentException("Base uri must be an absolute uri.", nameof(baseUri));
            }

            var fontSources = GetFontSourceIdentifier(name);

            FamilyNames = new FamilyNameCollection(fontSources);

            if (fontSources.Count == 1)
            {
                var singleSource = fontSources[0];

                if (singleSource.Source is Uri source)
                {
                    if (source.IsAbsoluteUri)
                    {
                        Key = new FontFamilyKey(source);
                    }
                    else
                    {

View on GitHub (pinned to 11c5427268)

Solutions

1. Pass an absolute URI (e.g. with 'resm:' or 'avares://' scheme) or pass null for baseUri.
2. If you have a relative path, combine it with an absolute base first: new Uri(absoluteBase, relativePath).
3. Use FontFamily.Parse(s, baseUri) only when baseUri is absolute or null.

Example fix

// before
var uri = new Uri("Assets/Fonts", UriKind.Relative);
var ff = new FontFamily(uri, "MyFont");

// after
var ff = new FontFamily(null, "avares://MyApp/Assets/Fonts#MyFont");
// or absolute base:
var baseUri = new Uri("avares://MyApp/", UriKind.Absolute);
var ff = new FontFamily(baseUri, "Assets/Fonts#MyFont");

if (baseUri is not null && !baseUri.IsAbsoluteUri)
    baseUri = null; // or build an absolute URI
var ff = new FontFamily(baseUri, name);

When it happens

Trigger: Calling new FontFamily(baseUri, name) where baseUri is a relative Uri (IsAbsoluteUri == false). Common when constructing a FontFamily from a XAML base URI that was built relatively, or when passing a Uri created without a scheme.

Common situations: Using `new Uri("path/to/fonts", UriKind.Relative)` as a base URI. Passing a XAML baseUri from a context that yielded a relative URI. Manually constructing font asset URIs without a pack/application scheme.

Related errors

• Specified family is not supported.
• Font collection Key should follow the fonts: scheme.
• File path is expected to be an absolute link with "file" or
• Folder path is expected to be an absolute link with "file" o
• Invalid brush string: '{s}'.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/c1baec9d04aa4398. Report an issue: GitHub.