XML Deserialization of collection property with code defaults

You are correct that many serializers (though not all) work this way. Json.NET does, its JsonConverter.ReadJson method actually has an Object existingValue for exactly this situation.

I don't know of any documents where these sorts of implementation details are spelled out. The easiest way to determine whether a serializer uses pre-allocated collections when present rather than unconditionally allocating and then setting one itself is to actually test it by using an ObservableCollection<T> and attaching debug listeners when it is changed:

[Serializable]
[DataContract]
public class TestConfiguration
{
    [DataMember]
    public String Name { get { return mName; } set { mName = value; } }

    private String mName = "Pete Sebeck";

    [DataMember]
    public ObservableCollection<String> Associates
    {
        get
        {
            Debug.WriteLine(mAssociates == null ? "Associates gotten, null value" : "Associates gotten, count = " + mAssociates.Count.ToString());
            return mAssociates;
        }
        set
        {
            Debug.WriteLine(value == null ? "Associates set to a null value" : "Associates set, count = " + value.Count.ToString());
            RemoveListeners(mAssociates);
            mAssociates = AddListeners(value);
        }
    }

    private ObservableCollection<String> mAssociates = AddListeners(new ObservableCollection<string>() { "Jon", "Natalie" });

    public override String ToString()
    {
        StringBuilder buffer = new StringBuilder();
        buffer.AppendLine(String.Format("Name: {0}", Name));
        buffer.AppendLine("Associates:");
        foreach (String associate in mAssociates)
        {
            buffer.AppendLine(String.Format("\t{0}", associate));
        }
        return buffer.ToString();
    }

    static ObservableCollection<String> AddListeners(ObservableCollection<String> list)
    {
        if (list != null)
        {
            list.CollectionChanged -= list_CollectionChanged; // In case it was already there.
            list.CollectionChanged += list_CollectionChanged;
        }
        return list;
    }

    static ObservableCollection<String> RemoveListeners(ObservableCollection<String> list)
    {
        if (list != null)
        {
            list.CollectionChanged -= list_CollectionChanged; // In case it was already there.
        }
        return list;
    }

    public static ValueWrapper<bool> ShowDebugInformation = new ValueWrapper<bool>(false);

    static void list_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (!ShowDebugInformation)
            return;
        switch (e.Action)
        {
            case NotifyCollectionChangedAction.Add:
                Debug.WriteLine(string.Format("Added {0} items", e.NewItems.Count));
                break;
            case NotifyCollectionChangedAction.Move:
                Debug.WriteLine("Moved items");
                break;
            case NotifyCollectionChangedAction.Remove:
                Debug.WriteLine(string.Format("Removed {0} items", e.OldItems.Count));
                break;
            case NotifyCollectionChangedAction.Replace:
                Debug.WriteLine("Replaced items");
                break;
            case NotifyCollectionChangedAction.Reset:
                Debug.WriteLine("Reset collection");
                break;
        }
    }
}

public static class TestTestConfiguration
{
    public static void Test()
    {
        var test = new TestConfiguration();

        Debug.WriteLine("\nTesting Xmlserializer...");
        var xml = XmlSerializationHelper.GetXml(test);
        using (new SetValue<bool>(TestConfiguration.ShowDebugInformation, true))
        {
            var testFromXml = XmlSerializationHelper.LoadFromXML<TestConfiguration>(xml);
            Debug.WriteLine("XmlSerializer result: " + testFromXml.ToString());
        }

        Debug.WriteLine("\nTesting Json.NET...");
        var json = JsonConvert.SerializeObject(test, Formatting.Indented);
        using (new SetValue<bool>(TestConfiguration.ShowDebugInformation, true))
        {
            var testFromJson = JsonConvert.DeserializeObject<TestConfiguration>(json);
            Debug.WriteLine("Json.NET result: " + testFromJson.ToString());
        }

        Debug.WriteLine("\nTesting DataContractSerializer...");
        var contractXml = DataContractSerializerHelper.GetXml(test);
        using (new SetValue<bool>(TestConfiguration.ShowDebugInformation, true))
        {
            var testFromContractXml = DataContractSerializerHelper.LoadFromXML<TestConfiguration>(contractXml);
            Debug.WriteLine("DataContractSerializer result: " + testFromContractXml.ToString());
        }

        Debug.WriteLine("\nTesting BinaryFormatter...");
        var binary = BinaryFormatterHelper.ToBase64String(test);
        using (new SetValue<bool>(TestConfiguration.ShowDebugInformation, true))
        {
            var testFromBinary = BinaryFormatterHelper.FromBase64String<TestConfiguration>(binary);
            Debug.WriteLine("BinaryFormatter result: " + testFromBinary.ToString());
        }

        Debug.WriteLine("\nTesting JavaScriptSerializer...");
        var javaScript = new JavaScriptSerializer().Serialize(test);
        using (new SetValue<bool>(TestConfiguration.ShowDebugInformation, true))
        {
            var testFromJavaScript = new JavaScriptSerializer().Deserialize<TestConfiguration>(javaScript);
            Debug.WriteLine("JavaScriptSerializer result: " + testFromJavaScript.ToString());
        }
    }
}

I ran the test above, and found:

1. XmlSerializer and Json.NET use the pre-existing collection if present. (In Json.NET this can be controlled by setting JsonSerializerSettings.ObjectCreationHandling to Replace)
2. JavaScriptSerializer, BinaryFormatter and DataContractSerializer do not, and always allocate the collection themselves. For the latter two this is not surprising as both do not call default constructors and instead simply allocate empty memory directly.

I don't know why the serializers in case 1 behave this way. Perhaps their authors were concerned that the containing class might want to internally use a subclass of the collection being deserialized, or attach observers to observable collections as I have done, and so decided to honor that design?

One note - for all serializers (except, maybe, BinaryFormatter, about which I am unsure), if a collection property is declared specifically as an array then the serializer will allocate the array itself and set the array after it is fully populated. This means that arrays can always be used as proxy collections during serialization.

By using a proxy array, you can guarantee that your collection is overwritten during deserialization:

    [IgnoreDataMember]
    [XmlIgnore]
    [ScriptIgnore]
    public ObservableCollection<String> { get; set; } // Or List<string> or etc.

    [XmlArray("Associates")]
    [DataMember(Name="Associates")]
    public string[] AssociateArray
    {
        get
        {
            return (Associates == null ? null : Associates.ToArray());
        }
        set
        {
            if (Associates == null)
                Associates = new ObservableCollection<string>();
            Associates.Clear();
            if (value != null)
                foreach (var item in value)
                    Associates.Add(item);
        }
    }

Now the collection comes back with only the previously serialized members with all 5 serializers.