DataContractJsonSerializer can't serialize contract with ExtensionDataObject

[sivukhin]

Description

If you use DataContractJsonSerializer with ExtensionDataObject and try to serialize data when extension data contains a nested complex object with the single property then the serializer throws an exception because the deserializer incorrectly decides that property of a nested object has the type of array but this assumption breaks serializer invariants.

Consider the following data contract:

[DataContract]
public class ContractGeneric : IExtensibleDataObject
{
    public ExtensionDataObject ExtensionData { get; set; }
}

If you try to deserialize JSON {"item":{"id":0}} to the type ContractGeneric and after that immediately serialize this data back to JSON then you get the exception:

Unhandled exception. System.Runtime.Serialization.SerializationException: 
There was an error serializing the object of type DataContractJsonSerializeExample.ContractGeneric. 
Encountered unexpected element local name 'id' for item in collection. 'item' is the only valid local name for elements in a collection.

You can find a repository with the full example here: https://github.com/Umqra/DataContractJsonSerializeExample
Also, there is a docker image with a console application that reproduces the bug: https://hub.docker.com/r/sivukhin/data-contract-json-serializer-example

Configuration

Runtime: .NET 5.0 but problem also reproduced in netcoreapp2.0
Operation System: Windows
Architecture: x64

Problem

Probably the cause of the problem lies in the automatic type detection for ExtensionDataObject content, more precisely inconsistency in behaviour of deserializer for JSON may happen at the following line:

runtime/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/XmlObjectSerializerReadContext.cs

Line 969 in 57bfe47

if (elementName != null && couldBeCollectionData)

Here XmlObjectSerializerReadContext tries to determine type of element and in case when object contains only single property the algorithm decide that this is an array (which is wrong!).

[sivukhin]

Should I mention someone to draw attention to the issue?

[StephenMolloy]

Some more information. I can reproduce this exception on .Net 6, but not on .Net 4.8.