dotnet/wpf · error · ArgumentException

SR.FixedDocumentExpectsDependencyObject

Error message

SR.FixedDocumentExpectsDependencyObject

What it means

FixedDocument.GetObjectPosition throws ArgumentException with SR.FixedDocumentExpectsDependencyObject when the object passed to it is not a DependencyObject. Because WPF document content elements (PageContent, FixedPage, etc.) are DependencyObjects, the API refuses any plain CLR object that cannot participate in the dependency property system.

Solutions

  1. Pass the actual DependencyObject document element (e.g. PageContent containing the FixedPage) rather than the underlying data object.
  2. Guard before calling: if (!(o is DependencyObject)) resolve the corresponding element first.
  3. If you hold a data item, map it to its content element (e.g. via ItemsControl.ItemContainerGenerator-style lookup) before querying the position.

Example fix

// before
object position = fixedDocument.GetObjectPosition(myDataItem);

// after
if (myDataItem is DependencyObject element)
{
    object position = fixedDocument.GetObjectPosition(element);
}
Defensive patterns

Strategy: type-guard

Validate before calling

bool isDependencyObject = o is System.Windows.DependencyObject;

Type guard

static bool IsDependencyObject(object o) => o is System.Windows.DependencyObject;

Try / catch

try
{
    var position = fixedDocument.GetObjectPosition(element);
}
catch (ArgumentException ex)
{
    logger.LogWarning(ex, "GetObjectPosition requires a DependencyObject; got {Type}.", element?.GetType().Name);
}

Prevention

When it happens

Trigger: Calling GetObjectPosition (the inverse of GetContentPositionForElement) with a POCO, a string, or a non-WPF object instead of a document element such as PageContent or FixedPage.

Common situations: Passing view-model or data items instead of the actual visual/document elements; passing content loaded from a non-WPF pipeline; null-checking was fine but the object's type was wrong.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a0e00da9cdac4d39. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/FixedDocument.cs:444

                    asyncRequest.Cancelled = true;
                    asyncRequest.PageContent.GetPageRootAsyncCancel();
                }
            }
        }

        /// <summary>
        /// <see cref="DynamicDocumentPaginator.GetObjectPosition"/>
        /// </summary>
        /// <exception cref="ArgumentNullException">element is NULL.</exception>
        internal ContentPosition GetObjectPosition(Object o)
        {
            ArgumentNullException.ThrowIfNull(o);

            DependencyObject element = o as DependencyObject;

            if (element == null)
            {
                throw new ArgumentException(SR.FixedDocumentExpectsDependencyObject);
            }
            DocumentsTrace.FixedFormat.IDF.Trace($"IDF.GetContentPositionForElement({element})");
            // Make sure that the call is in the right context.
//             Dispatcher.VerifyAccess();


            // walk up the logical parent chain to find the containing page
            FixedPage fixedPage = null;
            int pageIndex = -1;
            if (element != this)
            {
                DependencyObject el = element;
                while (el != null)
                {
                    fixedPage = el as FixedPage;

                    if (fixedPage != null)
                    {

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