将 JSON 字符串中的数据覆盖到现有对象实例

I want to deserialize a JSON string which does not necessarily contain data for every member, e.g:

public class MyStructure
{
   public string Field1;
   public string Field2;
}

Suppose I have an instance:

Field1: "data1"
Field2: "data2"

and I deserialize a string:

{ "Field1": "newdata1" }个

结果应该是

Field1: "newdata1"
Field2: "data2"

Framework JavascriptSerializer and JSON.NET both return new objects in their deserialize methods, so the only way I can think of doing this directly would be to compare the deserialized object with the existing one using reflection which seems like a lot of unnecessary overhead. Ideally, some software would have a method in which I passed an existing instance of an object, and only those members which existed in the string would get updated. The point here is that I would like to be able to pass only data which has changed to the server, and update an existing object.

Is this possible using either of these tools, and if not, any suggestions on how to approach the problem?