WCF is used to transfer data between clients and a server. Old DTO:

[Serializable]
public class TestClass
{
    private string firstProperty;

    [XmlAttribute]
    public string FirstProperty
    {
        get => firstProperty;
        set => firstProperty = value;
    }
}

Clients will keep sending the old version. The class needs to be extended to look like this:

[Serializable]
public class TestClass
{
    private string firstProperty;
    private string secondProperty;
    
    [XmlAttribute]
    public string FirstProperty
    {
        get => firstProperty;
        set => firstProperty = value;
    }

    [XmlAttribute]
    public string SecondProperty
    {
        get => secondProperty;
        set => secondProperty = value;
    }
}

Serialization:

public static void SerializeDataContract<T>(T obj, string path)
{
    var serializer = new DataContractSerializer(typeof(T));
    var settings = new XmlWriterSettings { Indent = true };
    using (var writer = XmlWriter.Create(path, settings))
    {
        serializer.WriteObject(writer, obj);
    }
}

public static T DeserializeDataContract<T>(string path)
{
    var serializer = new DataContractSerializer(typeof(T));
    using (var s = File.OpenRead(path))
    {
        return (T) serializer.ReadObject(s);
    }
}

The server and some clients will use the new version. If I serialize the old version and deserialize it into the new version, the following error is thrown:

System.Runtime.Serialization.SerializationException: 'Error in line 1 position 129. 'EndElement' 'TestClass' from namespace 'http://schemas.datacontract.org/2004/07/DTOs' is not expected. Expecting element 'secondProperty'.'

The exception is usually thrown in the WCF layer but I extracted a minimal reproducible example. If I use the XmlSerializer the error is gone. But changing the serializer is not an option because the old clients will keep using the DataContractSerializer.

So I'm having a hard time getting this to work because of the XmlSerializer attributes in combination with the DataContractSerializer. Any advice?

CodePudding user response：

You can indicate that secondProperty is optional by marking it with [OptionalFieldAttribute]:

[Serializable]
public class TestClass
{
    private string firstProperty;

    [OptionalField]
    private string secondProperty;

    [XmlAttribute]
    public string FirstProperty
    {
        get => firstProperty;
        set => firstProperty = value;
    }

    [XmlAttribute]
    public string SecondProperty
    {
        get => secondProperty;
        set => secondProperty = value;
    }
}

When a type is marked with [Serializable] but not data contract attributes, the data contract serializer will serialize the public and private fields of instances of the type -- not the properties -- in a manner similar to BinaryFormatter. As such all fields are required to be present unless marked with [OptionalField].

For more see Types Supported by the Data Contract Serializer and Version tolerant serialization: Tolerance of missing data.

Notes:

• To mark the backing field of an auto-implemented property as optional, see NetDataContractSerializer Deserialization With New Property.

  That being said, I don't recommend using auto-properties with [Serializable] types since the serialization stream will contain the names of the secret backing fields. See .NET WebAPI Serialization k_BackingField Nastiness for details.

• You have marked your type with [XmlAttribute], but this attribute is ignored by DataContractSerializer. It only affects serialization by XmlSerializer.

Demo fiddle here.