dotnet/wpf · error · ArgumentException

SR.UriNotAbsolute

Error message

SR.UriNotAbsolute

What it means

Fonts.GetFontFamilies(Uri baseUri, string location) throws this ArgumentException when baseUri is non-null but not an absolute Uri. The enumeration needs a resolvable absolute base to locate font resources, matching the FontFamily(Uri, string) contract.

Solutions

  1. Pass an absolute Uri (pack://application:,,,/... or file:///...).
  2. Resolve the relative path against an absolute base before the call.
  3. Pass null for baseUri when location is itself an absolute Uri string.

Example fix

// before
Fonts.GetFontFamilies(new Uri("resources/fonts", UriKind.Relative), null);
// after
Fonts.GetFontFamilies(new Uri("pack://application:,,,/MyApp;component/resources/fonts/"), null);
Defensive patterns

Strategy: validation

Validate before calling

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

Type guard

bool IsValidFontBase(Uri u) => u == null || u.IsAbsoluteUri;

Try / catch

try { var families = Fonts.GetFontFamilies(baseUri, location); }
catch (ArgumentException) { /* relative baseUri; build absolute and retry */ }

Prevention

When it happens

Trigger: Calling GetFontFamilies(relativeUri, "fonts/") with a Uri created as UriKind.Relative, or a scheme-less path Uri.

Common situations: Passing a relative application folder hoping it resolves against the app base automatically instead of building an absolute Pack URI.

Understand the failure class

Background: "Invalid URL" errors: why new URL(), URI.parse, and reqwest::Url reject your string — missing scheme, whitespace, and bad path format — this error's family across 39 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/643eaf8f26a2587e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Fonts.cs:84

        /// Enumerates font families in the font location specified by a base URI and/or a location string.
        /// </summary>
        /// <param name="baseUri">Base URI used to determine the font location if the location parameter is not specified 
        /// or is relative, and the value of the BaseUri property of each FontFamily in the resulting collection. This
        /// parameter can be null if the location parameter specifies an absolute location.</param>
        /// <param name="location">Optional relative or absolute URI reference. The location is used (with baseUri) to 
        /// determine the font folder and is exposed as part of the Source property of each FontFamily in the resulting
        /// collection. If location is null or empty then "./" is implied, meaning same folder as the base URI.</param>
        /// <returns>Collection of FontFamily objects from the specified font location.</returns>
        /// <remarks>
        /// The caller must have FileIOPermission(FileIOPermissionAccess.Read) for the specified font folder.
        /// Each resulting FontFamily object has the specified base Uri as its BaseUri property and includes the
        /// specified location as part of the friendly name specified by the Source property.
        /// </remarks>
        public static ICollection<FontFamily> GetFontFamilies(Uri baseUri, string location)
        {
            // Both Uri parameters are optional but neither can be relative.
            if (baseUri != null && !baseUri.IsAbsoluteUri)
                throw new ArgumentException(SR.UriNotAbsolute, nameof(baseUri));

            // Determine the font location from the base URI and location string.
            Uri fontLocation;
            if (!string.IsNullOrEmpty(location) && Uri.TryCreate(location, UriKind.Absolute, out fontLocation))
            {
                // absolute location; make sure we support absolute font family references for this scheme
                if (!Util.IsSupportedSchemeForAbsoluteFontFamilyUri(fontLocation))
                    throw new ArgumentException(SR.InvalidAbsoluteUriInFontFamilyName, nameof(location));

                // make sure the absolute location is a valid URI reference rather than a Win32 path as
                // we don't support the latter in a font family reference
                location = fontLocation.GetComponents(UriComponents.AbsoluteUri, UriFormat.SafeUnescaped);
            }
            else
            {
                // relative location; we need a base URI
                if (baseUri == null)
                    throw new ArgumentNullException(nameof(baseUri), SR.Format(SR.NullBaseUriParam, "baseUri", "location"));

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