dotnet/wpf · error · ArgumentException

String passed for type is not valid.

Error message

String passed for type is not valid.

What it means

GenerateUniquePart validates that the ContentType is not ContentType.Empty; passing an empty type throws ArgumentException with SR.ReachPackaging_InvalidType ("String passed for type is not valid."). Unlike GeneratePart this checks for the empty sentinel rather than merely zero length.

Solutions

1. Pass a concrete MIME ContentType, e.g. new ContentType("application/vnd.ms-printing.printticket") for print tickets or the image type for thumbnails.
2. Compare against ContentType.Empty before calling and substitute a proper type or abort early.
3. Fix upstream parsing that yields ContentType.Empty and handle the failure at the parse site.
4. Use the XpsS0Markup content type constants for known XPS part types.

Example fix

// before
ContentType ct = ContentType.Empty; // placeholder
xpsManager.GenerateUniquePart(ct);
// after
ContentType ct = new ContentType(XpsS0Markup.PrintTicketContentType);
if (ContentType.Empty.AreTypeAndSubTypeEqual(ct)) throw new ArgumentException("contentType must not be empty");
xpsManager.GenerateUniquePart(ct);

if (contentType is null || ContentType.Empty.AreTypeAndSubTypeEqual(contentType)) throw new ArgumentException("contentType must be a concrete, non-empty type", nameof(contentType));

bool IsConcreteType(ContentType ct) => ct != null && !ContentType.Empty.AreTypeAndSubTypeEqual(ct);

try { return xpsManager.GenerateUniquePart(contentType); }
catch (ArgumentException ex) when (ex.Message.Contains("type")) { /* substitute the correct XPS content type constant */ }

When it happens

Trigger: Calling GenerateUniquePart (or AddSignatureDefinitionPart, AddDocumentPropertiesPart, AddThumbnail that funnel into it) with contentType equal to ContentType.Empty, e.g. default-initialized or failed parse results.

Common situations: ContentType fields default-initialized and never assigned; a parsing step that returned ContentType.Empty on failure being passed through; refactored code that replaced a literal MIME string with ContentType.Empty placeholder.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• ' ' ContentType is not valid.
• " }} " element found. Expected fixed page element ( }} ).
• PackagePart URI does not correspond to a FixedDocument.
• PackagePart URI does not correspond to a FixedPage.
• SR.DigitalSignatureNoFixedDocumentSequence

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b626505cd8d14494. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Packaging/XpsManager.cs:350

        /// </param>
        /// <returns>
        /// Returns the newly created Metro Part.
        /// </returns>
        public
        PackagePart
        GenerateUniquePart(
            ContentType contentType
            )
        {
            ObjectDisposedException.ThrowIf(_metroPackage is null, typeof(XpsManager));
            if (!IsWriter)
            {
                throw new XpsPackagingException(SR.ReachPackaging_OnlyWriters);
            }
            ArgumentNullException.ThrowIfNull(contentType);
            if (ContentType.Empty.AreTypeAndSubTypeEqual(contentType))
            {
                throw new ArgumentException(SR.ReachPackaging_InvalidType);
            }

            //
            // Generate a unique part Uri
            //
            System.Uri partUri = GenerateUniqueUri(contentType);

            return GeneratePart( contentType, partUri);
       }

        public
        PrintTicket
        EnsurePrintTicket(
            Uri partUri
            )
        {
            PrintTicket printTicket = null;
            PackagePart printTicketPart = GetPrintTicketPart(partUri);

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