Setting the initial value of a property when using DataContractSerializer

Question

If I am serializing and later deserializing a class using DataContractSerializer how can I control the initial values of properties that were not serialized?

Consider the Person class below. Its data contract is set to serialize the FirstName and LastName properties but not the IsNew property. I want IsNew to initialize to TRUE whether a new Person is being instantiate as a new instance or being deserialized from a file.

This is easy to do through the constructor, but as I understand it DataContractSerializer does not call the constructor as they could require parameters.

[DataContract(Name="Person")]
public class Person 
{
    [DataMember(Name="FirstName")]
    public string FirstName { get; set; }

    [DataMember(Name = "LastName")]
    public string LastName { get; set; }

    public bool IsNew { get; set; }

    public Person(string first, string last)
    {
        this.FirstName = first;
        this.LastName = last;
        this.IsNew = true;
    }
}

Answer 1

You can use a serialization callback. Add the following method to your Person class:

[OnDeserialized]
void OnDeserialized(StreamingContext context)
{
    this.IsNew = true;
}

Another option is to remove the [DataContract] and [DataMember] attributes. In this case DCSerializer will call your constructor when it deserializes.

Answer 2

Actually the correct way of doing it is by using the OnDeserializing attribute (notice the "ing" suffix). The method marked with this attribute is called before the member values are deserialized.