空合并运算符不接受不同类型

Question

I started out, returning a custom class:

[HttpGet()]
public ActionResult<Round> Get(string id) =>
    this._roundsService.Get(id);

The Get method in the rounds service can return null and that gets converted to HTTP 204 No Content. I was wondering how to return 404 when I get null:

[HttpGet()]
public ActionResult<Round> Get(string id) =>
    this._roundsService.Get(id) ?? NotFound();

Apperantly this doesn't work and gives me a CS0019 error: Operator '??' cannot be applied to operands of type 'Round' and 'NotFoundResult' Operator '??' cannot be applied to operands of type 'Round' and 'NotFoundResult'

I'm open to other one-liners that would return the desired object if not null and 404 if null.

I'm using C# 8.0 with the netcoreapp3.0 framework. I haven't enabled the nullable feature. Could that be causing the problem?

Just in case, here's the method in the service class:

public Round Get(string id) =>
    this._rounds.Find(round => round.Id == id).FirstOrDefault();

Answer 1

When you call NotFound() , you are creating a NotFoundResult . Your method has a return type of ActionResult<Round> but NotFoundResult doesn't actually inherit from ActionResult<Round> , so you cannot return the NotFoundResult object directly.

When you do type return NotFound() then what actually happens is that the compiler will use the implicit operator ActionResult<T> (ActionResult) to transform the NotFoundResult into a ActionResult<Round> .

This works fine when you just return the value directly but it will not work when used within a ternary conditional or null-coalescing expression. Instead, you would have to do the conversion yourself:

public ActionResult<Round> Get(string id) =>
    this._roundsService.Get(id) ?? new ActionResult<Round>(NotFound());

Because the constructor of ActionResult<T> accepts any ActionResult , you can just pass the NotFoundResult to it to make sure that it gets converted properly.

Of course, you can also split this up again and have the compiler do the conversion for you:

public ActionResult<Round> Get(string id)
{
    var result = this._roundsService.Get(id);
    if (result != null)
        return result;
    return NotFound();
}

Answer 2

Of course it cannot do that. It is only shorthand for a if . I think the equivalent to what you wrote would be around this:

ActionResult<Round> result = this._roundsService.Get(id);
if(result == null)
  result = NotFound();
return result;

And at this point, the compiler is really confused why you try to assign the "NotFound()" return value to a ActionResult variable.