dotnet/wpf · error · ArgumentException

SR.Format(SR.UnexpectedParameterType, value.GetType()…

Error message

SR.Format(SR.UnexpectedParameterType, value.GetType(), typeof(DocumentReference))

What it means

DocumentSequence.AddChild implements IAddChild and only accepts DocumentReference children. If the value passed cannot be cast to DocumentReference, it throws ArgumentException with SR.UnexpectedParameterType. DocumentSequence markup/child collections must contain only <DocumentReference> elements.

Solutions

Only add DocumentReference instances to a DocumentSequence
In XAML, wrap each child FixedDocument in a <DocumentReference> element
If building programmatically, create a DocumentReference, set its Source (or Id/Document), then call AddChild
Move unrelated children (styles, resources) out of the FixedDocumentSequence element

Example fix

// before
fixedDocumentSequence.AddChild(fixedDocument);
// after
var docRef = new DocumentReference();
docRef.Document = fixedDocument;
fixedDocumentSequence.AddChild(docRef);

Defensive patterns

Strategy: type-guard

Type guard

static bool IsDocumentReference(object value) => value is DocumentReference;

Try / catch

if (docRef is DocumentReference r) fixedDocumentSequence.AddChild(r); else throw new ArgumentException(nameof(docRef));

Prevention

Only wrap FixedDocument children in DocumentReference in XAML
Never call IAddChild.AddChild directly with foreign objects
Keep styles/resources outside the FixedDocumentSequence children

When it happens

Trigger: Calling AddChild directly with a non-DocumentReference object; putting an element other than DocumentReference inside a FixedDocumentSequence in XAML so the parser calls AddChild with the wrong type.

Common situations: Hand-editing XAML and nesting a FixedDocument or PageContent directly under FixedDocumentSequence instead of DocumentReference; programmatic document construction adding pages directly to the sequence.

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/15d29ab85b3e7ca4. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/DocumentSequence.cs:101

        #region IAddChild

        ///<summary>
        /// Called to Add the object as a Child.
        ///</summary>
        ///<param name="value">
        /// Object to add as a child
        ///</param>
        void IAddChild.AddChild(Object value)
        {
            ArgumentNullException.ThrowIfNull(value);

//             Dispatcher.VerifyAccess();

            DocumentReference docRef = value as DocumentReference;

            if (docRef == null)
            {
                throw new ArgumentException(SR.Format(SR.UnexpectedParameterType, value.GetType(), typeof(DocumentReference)), nameof(value));
            }

            if (docRef.IsInitialized)
            {
                _references.Add(docRef);
            }
            else
            {
                DocumentsTrace.FixedDocumentSequence.Content.Trace($"Doc {_references.Count} Deferred");
                if (_partialRef == null)
                {
                    _partialRef = docRef;
                    _partialRef.Initialized += new EventHandler(_OnDocumentReferenceInitialized);
                }
                else
                {
                    throw new InvalidOperationException(SR.PrevoiusUninitializedDocumentReferenceOutstanding);
                }

View on GitHub (pinned to 81131a70a4)