dotnet/wpf · error

The child of a DocumentViewer must implement the…

Error message

The child of a DocumentViewer must implement the IDocumentPaginatorSource interface.

What it means

The child passed to DocumentViewerBase via IAddChild.AddChild must implement IDocumentPaginatorSource (as FlowDocument-hosting sources do). Any other object type throws ArgumentException because the viewer can only paginate and render such sources.

Solutions

1. Pass an object implementing IDocumentPaginatorSource (e.g. assign a FlowDocument via the Document property)
2. Use a ContentControl/ScrollViewer-based layout if you need to host arbitrary visuals instead of a DocumentViewer
3. Set the viewer's Document property in code rather than AddChild with a non-document object

Example fix

// before
documentViewer.AddChild(new StackPanel());
// after
documentViewer.Document = new FlowDocument(new Paragraph(new Run("Hello")));

if (value is IDocumentPaginatorSource doc) { viewer.Document = doc; } else { /* route to another container */ }

static bool IsValidViewerChild(object o) => o is System.Windows.Documents.IDocumentPaginatorSource;

try { ((IAddChild)viewer).AddChild(value); }
catch (ArgumentException) { /* value is not an IDocumentPaginatorSource */ }

When it happens

Trigger: Placing an arbitrary element inside <DocumentViewer> in XAML (parsed through IAddChild), or calling documentViewer.AddChild(someControl) in code with a non-document object.

Common situations: XAML typos nesting a Grid/StackPanel directly in a DocumentViewer, or misunderstanding that DocumentViewer hosts documents, not arbitrary content.

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

• ' ' is not a Visual or Visual3D.
• 0x80070057
• Animation_AnimationTimelineTypeMismatch
• Cannot have empty collection of DocumentPageView objects.
• DocumentViewer element can have only one child.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/adcbd3cfd6539a69. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/Primitives/DocumentViewerBase.cs:1707

        /// <summary>
        /// Called to add the object as a Child.
        /// </summary>
        /// <param name="value">Object to add as a child.</param>
        /// <remarks>DocumentViewerBase only supports a single child of type IDocumentPaginatorSource.</remarks>
        void IAddChild.AddChild(Object value)
        {
            ArgumentNullException.ThrowIfNull(value);
            // Check if Content has already been set.
            if (this.Document != null)
            {
                throw new InvalidOperationException(SR.DocumentViewerCanHaveOnlyOneChild);
            }
            // Only IDocumentPaginatorSource is a valid content.
            IDocumentPaginatorSource document = value as IDocumentPaginatorSource;
            if (document == null)
            {
                throw new ArgumentException(SR.DocumentViewerChildMustImplementIDocumentPaginatorSource, nameof(value));
            }
            this.Document = document;
        }

        /// <summary>
        /// Called when text appears under the tag in markup
        /// </summary>
        /// <param name="text">Text to add to the Object.</param>
        /// <remarks>DocumentViewer does not support Text children.</remarks>
        void IAddChild.AddText(string text)
        {
            XamlSerializerUtil.ThrowIfNonWhiteSpaceInAddText(text, this);
        }

        #endregion IAddChild

        //-------------------------------------------------------------------
        //

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