dotnet/wpf · error · InvalidOperationException

SR.PrevoiusPartialPageContentOutstanding

Error message

SR.PrevoiusPartialPageContentOutstanding

What it means

This InvalidOperationException is thrown when AddChild on FixedDocument is given a new partial PageContent while a previously added partial page has not yet finished initializing. FixedDocument supports only one outstanding partially-loaded page at a time because it must attach an Initialized handler and logical parent to exactly one pending page.

Solutions

1. Wait for the previous partial page's Initialized event before adding the next PageContent
2. Add fully initialized PageContent objects instead of partial ones
3. Ensure no duplicate AddChild calls for the same pending page
4. Catch InvalidOperationException and defer the add via a queue drained on page load

Example fix

// before
foreach (var pc in pageContents) fixedDocument.AddChild(pc); // throws if a partial page is pending
// after
void AddNext(IEnumerator<PageContent> e) {
    if (!e.MoveNext()) return;
    ((IAddChild)fixedDocument).AddChild(e.Current);
    Dispatcher.CurrentDispatcher.BeginInvoke(() => AddNext(e), System.Windows.Threading.DispatcherPriority.Loaded);
}

bool canAdd = fixedDocument._pages.All(p => p.IsInitialized); // check via public surface: only add PageContent once previous ones report Initialized

try { ((IAddChild)fixedDocument).AddChild(pageContent); } catch (InvalidOperationException) { pendingPages.Enqueue(pageContent); }

When it happens

Trigger: Adding multiple asynchronous/partial PageContent entries in quick succession (e.g. from async XPS loading) before the prior partial page's Initialized event fires.

Common situations: Async XPS document parsing or streaming scenarios; adding several page contents before UI/binder events complete; double-adding the same page content.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• Cannot countersign an unsigned package.
• Cannot read Page properties because it is not in a tree…
• Cannot sign read-only file.
• Exception of type 'System.InvalidOperationException' was…
• Page can have only one child.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/083d6c6790a33f03. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/FixedDocument.cs:154

                throw new ArgumentException(SR.Format(SR.UnexpectedParameterType, value.GetType(), typeof(PageContent)), nameof(value));
            }

            if (fp.IsInitialized)
            {
                _pages.Add(fp);
            }
            else
            {
                DocumentsTrace.FixedFormat.FixedDocument.Trace($"Page {_pages.Count} Deferred");
                if (_partialPage == null)
                {
                    _partialPage = fp;
                    _partialPage.ChangeLogicalParent(this);
                    _partialPage.Initialized += new EventHandler(OnPageLoaded);
                }
                else
                {
                    throw new InvalidOperationException(SR.PrevoiusPartialPageContentOutstanding);
                }
            }
        }

        ///<summary>
        /// Called when text appears under the tag in markup
        ///</summary>
        ///<param name="text">
        /// Text to Add to the Object
        ///</param>
        /// <ExternalAPI/>
        void IAddChild.AddText(string text)
        {
            XamlSerializerUtil.ThrowIfNonWhiteSpaceInAddText(text, this);
        }
        #endregion

        #region IUriContext

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