Strange behaviour of .NET binary serialization on Dictionary<Key, Value>

Question

I encountered a, at least to my expectations, strange behavior in the binary serialization of .NET.

All items of a Dictionary that are loaded are added to their parent AFTER the OnDeserialization callback. In contrast List does the other way. This can be really annoying in real world repository code, for example when you need to add some delegates to dictionary items. Please check the example code and watch the asserts.

Is it normal behaviour?

[Serializable]
public class Data : IDeserializationCallback
{
    public List<string> List { get; set; }

    public Dictionary<string, string> Dictionary { get; set; }

    public Data()
    {
        Dictionary = new Dictionary<string, string> { { "hello", "hello" }, { "CU", "CU" } };
        List = new List<string> { "hello", "CU" };
    }

    public static Data Load(string filename)
    {
        using (Stream stream = File.OpenRead(filename))
        {
            Data result = (Data)new BinaryFormatter().Deserialize(stream);
            TestsLengthsOfDataStructures(result);

            return result;
        }
    }

    public void Save(string fileName)
    {
        using (Stream stream = File.Create(fileName))
        {
            new BinaryFormatter().Serialize(stream, this);
        }
    }

    public void OnDeserialization(object sender)
    {
        TestsLengthsOfDataStructures(this);
    }

    private static void TestsLengthsOfDataStructures(Data data)
    {
        Debug.Assert(data.List.Count == 2, "List");
        Debug.Assert(data.Dictionary.Count == 2, "Dictionary");
    }
}

Answer 1

Yes, you've discovered an annoying quirk in Dictionary<TKey, TValue> deserialization. You can get around it by manually calling the dictionary's OnDeserialization() method:

public void OnDeserialization(object sender)
{
    Dictionary.OnDeserialization(this);
    TestsLengthsOfDataStructures(this);
}

Incidentally, you can also use the [OnDeserialized] attribute rather than IDeserializationCallback:

[OnDeserialized]
public void OnDeserialization(StreamingContext context)
{
    Dictionary.OnDeserialization(this);
    TestsLengthsOfDataStructures(this);
}