dotnet/wpf · error · ArgumentException

SR.InvalidAbsoluteUriInFontFamilyName

Error message

SR.InvalidAbsoluteUriInFontFamilyName

What it means

Fonts.GetFontFamilies throws this ArgumentException when the 'location' string parses as an absolute Uri but its scheme is not supported for absolute font family references (Util.IsSupportedSchemeForAbsoluteFontFamilyUri fails). The library only allows specific schemes (pack, file, http/https) in absolute font locations.

Solutions

  1. Use a supported scheme (pack, file, or http/https) in the location.
  2. For a network path, copy fonts locally or serve over a supported scheme.
  3. Pass the location as a relative string plus an absolute baseUri instead.

Example fix

// before
Fonts.GetFontFamilies(null, "ftp://server/fonts");
// after
Fonts.GetFontFamilies(null, "file:///server/fonts");
Defensive patterns

Strategy: validation

Validate before calling

if (Uri.TryCreate(location, UriKind.Absolute, out var abs)
    && !(abs.Scheme == Uri.UriSchemeFile || abs.Scheme == Uri.UriSchemeHttp || abs.Scheme == Uri.UriSchemeHttps || abs.Scheme == "pack"))
    throw new ArgumentException("unsupported scheme", nameof(location));

Try / catch

try { Fonts.GetFontFamilies(null, location); }
catch (ArgumentException) { /* unsupported absolute scheme; reformat location */ }

Prevention

When it happens

Trigger: Calling GetFontFamilies(baseUri, "ftp://server/fonts") or any absolute Uri with an unsupported scheme (ftp, ldap, custom app schemes).

Common situations: Pointing font enumeration at a network share via a non-HTTP UNC-style or custom protocol string that happens to parse as an absolute Uri.

Understand the failure class

Background: "Invalid URL" / "URL cannot be empty": fix the malformed or missing URL behind request-construction failures — this error's family across 50 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ee3d749fe0cdec4d. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Fonts.cs:92

        /// <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"));

                // the location part must include a path component, otherwise we'll look in windows fonts and ignore the base URI
                if (string.IsNullOrEmpty(location))
                    location = "./";
                else if (Util.IsReferenceToWindowsFonts(location))
                    location = "./" + location;

                fontLocation = new Uri(baseUri, location);

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