dotnet/wpf · error · ArgumentException

URI must be absolute. Relative URIs are not supported.

Error message

URI must be absolute. Relative URIs are not supported.

What it means

PreloadedPackages keys its package registry by full package Uri; ValidateUriKey rejects null and any relative Uri, throwing ArgumentException (SR.UriMustBeAbsolute) with the 'uri' parameter name. This library throws it because only absolute package URIs can unambiguously identify a package store key.

Solutions

Pass an absolute package URI, e.g. new Uri("pack://application:,,,/", UriKind.Absolute) or PackUriHelper.Create(partUri) results
Resolve relative URIs against a known base: new Uri(baseUri, relativePath)
Check uri.IsAbsoluteUri before calling the API and fail fast with a clearer message

Example fix

// before
packages.AddPackage(new Uri("/app.xps", UriKind.Relative), pkg);
// after
var absUri = new Uri("pack://application:,,,/app.xps", UriKind.Absolute);
packages.AddPackage(System.IO.Packaging.PackUriHelper.GetPackageUri(absUri), pkg);

Defensive patterns

Strategy: validation

Validate before calling

if (uri == null || !uri.IsAbsoluteUri)
    throw new ArgumentException("Package key must be an absolute URI", nameof(uri));
preloadedPackages.AddPackage(uri, package);

Type guard

static bool IsAbsolutePackageUri(Uri u) => u != null && u.IsAbsoluteUri;

Try / catch

try { PreloadedPackages.AddPackage(uri, pkg); } catch (ArgumentException ex) when (ex.ParamName == "uri") { uri = new Uri(packBaseUri, uri); PreloadedPackages.AddPackage(uri, pkg); }

Prevention

Create package URIs with PackUriHelper.Create or absolute pack:// URIs only
Validate uri.IsAbsoluteUri at configuration load time
Resolve relative config paths against a documented base URI before use

When it happens

Trigger: Calling PreloadedPackages.GetPackage, AddPackage, or RemovePackage with a relative Uri such as new Uri("/myPackage.xps", UriKind.Relative) or a default-initialized Uri.

Common situations: Building package URIs from user or config strings that omit the scheme; switching from PackUriHelper-created absolute URIs to hand-built ones; passing pack part URIs that lost their base when resolved.

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/126ec6f7721c43bc. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/IO/Packaging/PreloadedPackages.cs:154

            }
        }

        // Null the instance.  Similar to Dispose, but not quite.
        internal static void Clear()
        {
            lock (_globalLock)
            {
                _packagePairs = null;
            }
        }

        private static void ValidateUriKey(Uri uri)
        {
            ArgumentNullException.ThrowIfNull(uri);

            if (!uri.IsAbsoluteUri)
            {
                throw new ArgumentException(SR.UriMustBeAbsolute, nameof(uri));
            }
        }

        #endregion Internal Methods

        /// <summary>
        /// Package-bool pair where the bool represents the thread-safety status of the package
        /// </summary>
        private class PackageThreadSafePair
        {
            //------------------------------------------------------
            //
            //  Internal Constructors
            //
            //------------------------------------------------------
            internal PackageThreadSafePair(Package package, bool threadSafe)
            {
                Invariant.Assert(package != null);

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