What causes "Container already has Starting Part." in dotnet/wpf?

Check whether a starting part already exists and reuse it instead of setting a new one.

dotnet/wpf · error · XpsPackagingException

Container already has Starting Part.

Error message

Container already has Starting Part.

What it means

The XpsManager.StartingPart setter throws XpsPackagingException(SR.ReachPackaging_AlreadyHasStartingPart) when the package is non-streaming and a starting part (FixedDocumentSequence root) has already been set. A fixed-format package may define only one starting part.

Solutions

Check whether a starting part already exists and reuse it instead of setting a new one.
Remove the existing starting part first if you truly intend to replace the document root.
For streaming scenarios enable streaming mode, which bypasses this restriction, or create a fresh package.

Example fix

// before
manager.StartingPart = newSeq; // may throw if one exists
// after
if (manager.StartingPart == null)
    manager.StartingPart = newSeq;
else
    // reuse existing root or replace via documented remove/rewrite path

Defensive patterns

Strategy: validation

Validate before calling

if (!streaming && xpsManager.StartingPart != null)
    throw new InvalidOperationException("Package already has a starting part");

Try / catch

try { manager.StartingPart = seq; }
catch (XpsPackagingException ex) when (ex.Message.Contains("Starting Part")) { /* reuse existing starting part */ }

Prevention

Set StartingPart only once during package initialization; guard idempotently.
Never append a second FixedDocumentSequence root to an existing non-streaming package.

When it happens

Trigger: Setting XpsManager.StartingPart (or creating a second document root) on a non-streaming package that already has a FixedDocumentSequence/starting part registered via GetXpsDocumentStartingPart.

Common situations: Appending a new FixedDocumentSequence to an existing XPS file opened in ReadWrite; re-running initialization code twice on the same package; mixing Write/ReadWrite flows on one package.

Understand the failure class

Background: "already exists" / EEXIST / FileAlreadyExistsException: what the 'file already exists' error means and how to fix it — this error's family across 37 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/7d989f68f8d9f17b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Packaging/XpsManager.cs:201

        /// <value>A Metro Part representing the root.</value>
        public PackagePart StartingPart
        {
            get
            {
                PackagePart part = null;
                if (null != _metroPackage)
                {
                    part = GetXpsDocumentStartingPart(_metroPackage);
                }

                return part;
            }
            set
            {
                ObjectDisposedException.ThrowIf(_metroPackage is null, typeof(XpsManager));
                if ( !Streaming && null != GetXpsDocumentStartingPart(_metroPackage))
                {
                    throw new XpsPackagingException(SR.ReachPackaging_AlreadyHasStartingPart);
                }

                SetXpsDocumentStartingPart(_metroPackage, value);
            }
        }

        /// <summary>
        /// Gets the XpsDocument that owns the XpsManager
        /// </summary>
        /// <value>A boolean flag.</value>
        public
        XpsDocument
        XpsDocument
        {
            set
            {
                _xpsDocument = value;
            }

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