dotnet/wpf · error · ArgumentException

SR.ReachSerialization_ExpectedFixedDocument

Error message

SR.ReachSerialization_ExpectedFixedDocument

What it means

NgcSerializer.SerializeObject (the FixedDocument overload) casts the serializedObject to FixedDocument and, when the cast yields null, throws ArgumentException(SR.ReachSerialization_ExpectedFixedDocument). This serializer entry point is typed for FixedDocument only.

Solutions

Pass a FixedDocument instance to this SerializeObject overload.
Use the FixedDocumentSequence serializer for sequences and the FixedPage serializer for pages.
Add an 'is FixedDocument' guard at the call site to fail with a clearer message.

Example fix

// before
fixedDocSerializer.SerializeObject(page); // page is FixedPage
// after
if (page is FixedDocument doc) fixedDocSerializer.SerializeObject(doc);
else pageSerializer.SerializeObject(page);

Defensive patterns

Strategy: type-guard

Validate before calling

if (obj is not FixedDocument)
    throw new ArgumentException("FixedDocument serializer requires a FixedDocument instance");

Type guard

bool IsFixedDocument(object o) => o is FixedDocument;

Try / catch

try { serializer.SerializeObject(obj); }
catch (ArgumentException ex) when (ex.Message.Contains("FixedDocument")) { /* route to the correct serializer for obj's actual type */ }

Prevention

Match serializer overloads to object types explicitly
Avoid passing object-typed variables to typed serializer overloads
Add compile-time overloads instead of runtime dispatch where possible

When it happens

Trigger: Calling this SerializeObject overload with anything other than a FixedDocument instance (null is separately guarded) — e.g. a FixedPage, FixedDocumentSequence, or custom type.

Common situations: Invoking the wrong NgcSerializer for the document level (using the FixedDocument serializer for a sequence or a page); callers binding object variables typed as object.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/6058955471a50a43. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/ReachFramework/Serialization/manager/NGCSerializer.cs:87

            ArgumentNullException.ThrowIfNull(manager);
        }

        /// <summary>
        ///
        /// </summary>
        public
        override
        void
        SerializeObject(
            Object serializedObject
            )
        {
            ArgumentNullException.ThrowIfNull(serializedObject);
            FixedDocument fd = serializedObject as FixedDocument;
            if( fd == null )
            {

               throw new ArgumentException(SR.ReachSerialization_ExpectedFixedDocument);
            }
            NgcSerializationManager ngcManager = SerializationManager as NgcSerializationManager;

            ngcManager.StartDocument(fd,true);

            ReachSerializer serializer = ngcManager.GetSerializer(fd.Pages);
            serializer.SerializeObject(fd.Pages);

            ngcManager.EndDocument();
        }


        /// <summary>
        /// The method is called once the object data is discovered at that
        /// point of the serialization process.
        /// </summary>
        /// <param name="serializableObjectContext">
        /// The context of the object to be serialized at this time.

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