Modify existing object with new partial JSON data using Json.NET

You want JsonSerializer.Populate() or its static wrapper method JsonConvert.PopulateObject():

Populates the JSON values onto the target object.

For instance, here it is updating an instance of your Calendar class:

public static class TestPopulate{    public static void Test()    {        var calendar = new Calendar        {            Id = 42,            CoffeeProvider = "Espresso2000",            Meetings = new[]            {                new Meeting                {                    Location = "Room1",                    From = DateTimeOffset.Parse("2014-01-01T00:00:00Z"),                    To = DateTimeOffset.Parse("2014-01-01T01:00:00Z")                },                new Meeting                {                    Location = "Room2",                    From = DateTimeOffset.Parse("2014-01-01T02:00:00Z"),                    To = DateTimeOffset.Parse("2014-01-01T03:00:00Z")                },            }        };        var patch = @"{    'coffeeprovider': null,    'meetings': [        {            'location': 'Room3',            'from': '2014-01-01T04:00:00Z',            'to': '2014-01-01T05:00:00Z'        }    ]}";        Patch(calendar, patch);        Debug.WriteLine(JsonConvert.SerializeObject(calendar, Formatting.Indented));    }    public static void Patch<T>(T obj, string patch)    {        var serializer = new JsonSerializer();        using (var reader = new StringReader(patch))        {            serializer.Populate(reader, obj);        }    }}

And the debug output produced is:

{  "id": 42,  "coffeeprovider": null,  "meetings": [    {      "location": "Room3",      "from": "2014-01-01T04:00:00+00:00",      "to": "2014-01-01T05:00:00+00:00"    }  ]}

Update

If you want to copy first, you could do:

    public static T CopyPatch<T>(T obj, string patch)    {        var serializer = new JsonSerializer();        var json = JsonConvert.SerializeObject(obj);        var copy = JsonConvert.DeserializeObject<T>(json);        using (var reader = new StringReader(patch))        {            serializer.Populate(reader, copy);        }        return copy;    }