dotnet/wpf · error · ArgumentException

' ' ContentType is not valid.

Error message

'{0}' ContentType is not valid.

What it means

GeneratePart validates the supplied ContentType after the writer check: a null or zero-length content type string is rejected with ArgumentException (SR.ReachPackaging_InvalidContentType) formatted with the invalid value. The library requires a well-formed, non-empty MIME content type for every package part.

Solutions

1. Pass a valid non-empty MIME type such as "application/vnd.ms-package.obfuscated-opentype" or "application/vnd.ms-printing.printticket".
2. Validate contentType.ToString().Length > 0 before calling GeneratePart.
3. Fix the configuration/mapping that produced an empty content type string.
4. If the content type is unknown, use one of the constants exposed by XpsS0Markup instead of constructing a bare ContentType.

Example fix

// before
var ct = new ContentType(config["PartContentType"] ?? "");
var part = xpsManager.GeneratePart(partUri, ct, comp);
// after
string ctString = config["PartContentType"];
if (string.IsNullOrEmpty(ctString)) throw new ArgumentException("PartContentType must be a non-empty MIME type");
var part = xpsManager.GeneratePart(partUri, new ContentType(ctString), comp);

if (contentType is null) throw new ArgumentNullException(nameof(contentType));
if (string.IsNullOrWhiteSpace(contentType.ToString())) throw new ArgumentException("Content type must be non-empty", nameof(contentType));

bool IsValidContentType(ContentType ct) => ct != null && !string.IsNullOrEmpty(ct.ToString());

try { return xpsManager.GeneratePart(partUri, contentType, option); }
catch (ArgumentException ex) when (ex.ParamName == nameof(contentType)) { /* fix/derive a valid MIME type */ throw new ArgumentException($"Invalid content type: {contentType}", ex); }

When it happens

Trigger: Calling GeneratePart (directly or via GenerateUniquePart/printTicketPart paths) with a contentType whose ToString() yields an empty string, e.g. new ContentType("") or a ContentType constructed from an empty/null-derived string.

Common situations: Building content types dynamically from config or file extension mapping where the mapping returns an empty string; copying ContentType values from data that lost its value; typo causing an empty constant to be passed.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• String passed for type 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/325ab99d75ea0279. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Packaging/XpsManager.cs:300

        /// Generate a unique Part for the content type and add it to the package.
        /// Adding to any relationships or selector/sequence markup is not done.
        /// </summary>
        public
        PackagePart
        GeneratePart(
            ContentType contentType,
            Uri      	partUri
            )
       {
            ObjectDisposedException.ThrowIf(_metroPackage is null, typeof(XpsManager));
            if (!IsWriter)
            {
                throw new XpsPackagingException(SR.ReachPackaging_OnlyWriters);
            }
            ArgumentNullException.ThrowIfNull(contentType);
            if (0 == contentType.ToString().Length)
            {
                throw new ArgumentException(SR.Format(SR.ReachPackaging_InvalidContentType, contentType), nameof(contentType));
            }
            
            //Do not compress image Content Types
            CompressionOption compressionOption = _compressionOption;

            if (contentType.AreTypeAndSubTypeEqual(XpsS0Markup.JpgContentType) ||
                contentType.AreTypeAndSubTypeEqual(XpsS0Markup.PngContentType) ||
                contentType.AreTypeAndSubTypeEqual(XpsS0Markup.TifContentType) ||
                contentType.AreTypeAndSubTypeEqual(XpsS0Markup.WdpContentType))
            {
                compressionOption = CompressionOption.NotCompressed;
            }

            PackagePart metroPart = _metroPackage.CreatePart(partUri,
                                                             contentType.ToString(),
                                                             compressionOption);

            //

View on GitHub (pinned to 81131a70a4)