Initialize private readonly fields after Deserializing

Question

I need to initialize private readonly field after Deserialization. I have folowing DataContract:

[DataContract]
public class Item
{
    public Item()
    {
        // Constructor not called at Deserialization 
        // because of FormatterServices.GetUninitializedObject is used
        // so field will not be initialized by constructor at Deserialization
        _privateReadonlyField = new object();
    }

    // Initialization will not be called at Deserialization (same reason as for constructor)
    private readonly object _privateReadonlyField = new object();

    [DataMember]
    public string SomeSerializableProperty { get; set; }

    [OnDeserializing]
    public void OnDeserializing(StreamingContext context)
    {
        // With this line code even not compiles, since readonly fields can be initialized only in constructor
        _privateReadonlyField = new object();
    }
}

All what I need, that after Deserialization _privateReadonlyField is not null.

Any suggestions about this - is it possible at all? Or I need to remove "readonly" key, which is not a good option.

Answer 1

Serialization is able to read in values for read-only fields because it uses reflection, which ignores accessibility rules. It can be argued that the following is, therefore, justified as part of the serialization process, even though I would recommend strongly against it in almost any other circumstance:

private readonly Doodad _oldField;

[OptionalField(VersionAdded = 2)]
private readonly Widget _newField;

[OnDeserialized]
private void OnDeserialized(StreamingContext context)
{
    if (_oldField != null && _newField == null)
    {
        var field = GetType().GetField("_newField",
            System.Reflection.BindingFlags.Instance |
            System.Reflection.BindingFlags.DeclaredOnly |
            System.Reflection.BindingFlags.NonPublic);
        field.SetValue(this, new Widget(_oldField));
    }
}

Answer 2

Any field declared as private readonly can be instantiated in the same line where it was declared or inside a constructor. Once that is done it cannot be changed.

From MSDN:

The readonly keyword is a modifier that you can use on fields. When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class.

That means that you will have to remove readonly keyword to get it to work.