c# - How to use ?? with return?

Question

I want to return Todo object whenever there is such an object, otherwise return NotFound(). I also want to use only a single line consisting of return and ??. The following produces error at compile time. How to solve it?

public async Task<ActionResult<Todo>> GetTodo(long id)
{
    return await _context.Todos.FindAsync(id) ?? NotFound();
}

Attempt 1

Attempt 2

Attempt 3

MarTim's attempt passes the compiler,

public async Task<IActionResult> GetTodo(long id)
{
    return (IActionResult)(await _context.Todos.FindAsync(id)) ?? NotFound();
}

but generates runtime errors:

Answer 1

Let's simplify it a bit:

public async Task<ActionResult<Todo>> GetTodo(long id)
{
    Todo todo = await _context.Todos.FindAsync(id);
    return todo ?? NotFound();
}

The problem here is that todo and NotFound() have very different types. todo is a Todo, but NotFound() returns a NotFoundResult. These two are inherently incompatible, and the compiler doesn't know that it should convert both of them to an ActionResult<Todo>.

If you want to keep the method's return type as ActionResult<Todo>, you can write:

public async Task<ActionResult<Todo>> GetTodo(long id)
{
    return (await _context.Todos.FindAsync(id)) ?? new ActionResult<Todo>(NotFound());
}

Alteratively, you can use a helper method:

public static ActionResult<T> ResultOrNotFound<T>(T item) where T : class
{
    return item != null ? new ActionResult<T>(item) : new ActionResult<T>(NotFound());
}

Then you can write:

public async Task<ActionResult<Todo>> GetTodo(long id)
{
    return ResultOrNotFound(await _context.Todos.FindAsync(id));
}

---

Alternatively, if you're OK with returning an IActionResult instead, then you can do:

public static IActionResult OkOrNull(object item)
{
    return item != null ? Ok(item) : null;
}

Then you can write:

public async Task<IActionResult<Todo>> GetTodo(long id)
{
    return OkOrNull(await _context.Todos.FindAsync(id)) ?? NotFound();
}

You can make this look a bit nicer if you want, with another overload:

public static async Task<IActionResult> OkOrNull<T>(Task<T> task) where T : class
{
    T item = await task;
    return item != null ? Ok(item) : null;
}

Meaning you can write:

public async Task<IActionResult<Todo>> GetTodo(long id)
{
    return await OkOrNull(_context.Todos.FindAsync(id)) ?? NotFound();
}