Clear collections before adding items when populating existing objects

Question

I have an object which has several collection properties defined with public getter but private setter, In this case JsonConvert.PopulateObject adds the deseria

Answer 1

Okay, so after some trip to Json.NET sources I found the following solution by inheriting a custom contract resolver from DefaultContractResolver.

I needed to override the array contract creation to add a deserialization callback. At this point the callback receives the concrete collection instance, so we can manipulate it (in this case clear it).

As long as I can determine, it is safe to use, but feel free to warn about any drawbacks of this method.

Note: Am I the only one who feels that this should probably be the default behavior?

public class CollectionClearingContractResolver : DefaultContractResolver
{
    protected override JsonArrayContract CreateArrayContract(Type objectType)
    {
        var c = base.CreateArrayContract(objectType);
        c.OnDeserializingCallbacks.Add((obj, streamingContext) =>
        {
            var list = obj as IList;
            if (list != null && !list.IsReadOnly)
                list.Clear();
        });
        return c;
    }
}

...

public class Test {
    public List<int> List { get; private set; }
    public Test() {
        List = new List<int>();
    }
}  

...

var myObj = new Test();
myObj.List.AddRange(new[] {1,2,3});
var listReference = myObj.List;    

JsonConvert.PopulateObject("{ List: [4, 5, 6] }", myObj, 
    new JsonSerializerSettings {
        ContractResolver = new CollectionClearingContractResolver(),
    });

myObj.List.ShouldEqual(listReference); // didn't recreate list
myObj.List.Count.ShouldEqual(3);
myObj.List.SequenceEqual(new[] { 4, 5, 6}).ShouldBeTrue();