Overlay data from JSON string to existing object instance

Realize - JsonConvert.PopulateObject(string,object) will NOT work for collections.

Even with PreserveReferencesHandling = Objects/Arrays/All and an IReferenceResolver. JSON.NET will not update items in collections. Instead, it will duplicate your collection items.

JSON.NET only uses its ("ref") Preserve Reference identifiers to reuse references read within the serialized JSON. JSON.NET will not reuse instances in existing nested object graphs. We attempted by adding an ID property to all our objects, but JSON.NET IReferenceResolver does not provide the facilities to find & match existing references within collections.

Our solution will be to deserialize JSON into a new object instance and map properties across the 2 instances using either Fasterflect or AutoMapper.

After poking around the source code (so much easier than reading the documentation, eh?) JSON.NET does exactly what I want already:

JsonConvert.PopulateObject(string, object)

See Json.NET: Populate an Object

I came across this post, and thought I would share my solution for dealing with arrays, as I couldn't find a fully worked up example anywhere. In order for this sample to work, the target array must implement IEnumerable and IList, and the target array objects must implement IEquatable(Of JToken). The implementation of IEquatable(Of JToken) is where you put your logic to determine whether the deserializer should act on an existing item or create a new one. The example also removes any items from the target that are not in the json. I haven't added a disposal check on the removed items, but trivial to do.

The new PopulateObject Call:

Private Sub PopulateObject(value As String, target As Object)

    'set up default converter
    Dim converter As ReconcileEnumerationConverter = New ReconcileEnumerationConverter

    JsonConvert.DefaultSettings = Function()
                                      Return New JsonSerializerSettings With {.Converters = {converter}}
                                  End Function

    'for some reason populate object won't call converter on root
    'so force the issue if our root is an array
    If converter.CanConvert(target.GetType) Then
        Dim array As JArray = JArray.Parse(value)
        converter.ReadJson(array.CreateReader, target.GetType, target, Nothing)
    Else
        JsonConvert.PopulateObject(value, target)
    End If

End Sub

The converter:

Public Class ReconcileEnumerationConverter : Inherits JsonConverter

    Public Overrides Function CanConvert(objectType As Type) As Boolean
        'check to ensure our target type has the necessary interfaces
        Return GetType(IList).IsAssignableFrom(objectType) AndAlso GetType(IEnumerable(Of IEquatable(Of JToken))).IsAssignableFrom(objectType)
    End Function

    Public Overrides ReadOnly Property CanWrite As Boolean
        Get
            Return False
        End Get
    End Property

    Public Overrides Function ReadJson(reader As JsonReader, objectType As Type, existingValue As Object, serializer As JsonSerializer) As Object

        Dim array As JArray = JArray.ReadFrom(reader)

        'cast the existing items
        Dim existingItems As IEnumerable(Of IEquatable(Of JToken)) = CType(existingValue, IEnumerable(Of IEquatable(Of JToken)))
        'copy the existing items for reconcilliation (removal) purposes
        Dim unvisitedItems As IList = existingItems.ToList 'start with full list, and remove as we go
        'iterate each item in the json array
        For Each j As JToken In array.Children
            'look for existing
            Dim existingitem As Object = existingItems.FirstOrDefault(Function(x) x.Equals(j))
            If existingitem IsNot Nothing Then 'found an existing item, update it
                JsonSerializer.CreateDefault.Populate(j.CreateReader, existingitem)
                unvisitedItems.Remove(existingitem)
            Else 'create a new one
                Dim newItem As Object = JsonSerializer.CreateDefault.Deserialize(j.CreateReader)
                CType(existingItems, IList).Add(newItem)
            End If
        Next
        'remove any items not visited
        For Each item As Object In unvisitedItems
            CType(existingItems, IList).Remove(item)
        Next
        Return existingItems

    End Function

    Public Overrides Sub WriteJson(writer As JsonWriter, value As Object, serializer As JsonSerializer)
        Throw New NotImplementedException
    End Sub

End Class

And a sample implementation of IEquatable(of JToken), keyed on an integer 'Id' field:

Public Shadows Function Equals(other As JToken) As Boolean Implements IEquatable(Of JToken).Equals
    Dim idProperty As JProperty = other.Children.FirstOrDefault(Function(x) CType(x, JProperty).Name = "Id")
    If idProperty IsNot Nothing AndAlso CType(idProperty.Value, JValue).Value = Id Then
        Return True
    Else
        Return False
    End If
End Function

Note that JsonConvert.PopulateObject

JsonConvert.PopulateObject(json, item, new JsonSerializerSettings());

Simply calls jsonSerializer.Populate (see here)

        string json = "{ 'someJson':true }";

        var jsonSerializer = new JsonSerializer();

        jsonSerializer.Populate(new StringReader(json), item);

So if you need to repeatedly convert a thousand objects, you may get better performance this route, so that a new JsonSerializer is not instantiated every time.