部分更新期间ASP.NET Web API中的显式vs.隐式null

Question

I have been experimenting with Web API 2 for the past couple of weeks and I'm trying to figure out the best way to handle a particular scenario during partial updates.

The API will allow the user to do partial updates via POST. By design, any properties that are not supplied in the request body are assumed to be unchanged and no changes will be made to those properties.

The challenge, however, is trying to determine when a user has explicitly set a property value to null. When the request body is pulled in via the controller, any absent properties are set as null automatically as the JSON request body is deserialized. As such, I lose my ability to determine if the property value is null explicitly (ie the user is trying to set the property value as null, in which case I want to update the property to null) or if the property value is null implicitly (ie the property value was set to null during deserilaization, in which case I want to ignore it).

I am looking for any ideas of how I might approach this situation.

Answer 1

I have played around with this a bit and I have come up with a way to work around this. Maybe there's another way to address the issue but this seems to be a reasonable approach:

In Web API, the request body can only be read once. Therefore, if you have the controller pull the object in for you, the object will be deserialized and you will lose access to the original request content and will not be able to determine if something was explicitly null or absent. This is demonstrated by something like this:

[Route("cars"), HttpPost]
public IHttpActionResult AddCar(Car car)
{
Car newCar = Car.Add(car);
return Created<Car>("/cars/" + newCar.car_id, newCar);
}

If you want to be able to examine exactly what was sent in, you don't have the controller pull in the object for you, instead, you pull the request body in manually, thus preserving your ability to see exactly what was sent in, like this:

[Route("cars"), HttpPost]
public IHttpActionResult AddCar()
{
HttpContent content = Request.Content;
string carJson = content.ReadAsStringAsync().Result;

// You now have the original JSON and can examine it before deserialization

}

It certainly adds a few more steps, but at least in my case where I needed to be able to see exactly what was POSTed, this provides a solution that seems to work.