How ASP.NET Core APIs convert ActionResult<T> to T in action methods

Question

As an example look at below code which is a an API action:

[HttpGet("send")]
public ActionResult<string> Send()
{
    if (IsAuthorized())
    {
        return "Ok";
    }
    return Unauthorized(); // is of type UnauthorizedResult -> StatusCodeResult -> ActionResult -> IActionResult
}

My question is how this data conversion is happening here? How doesn't the compiler fail?

Answer 1

This is possible due to a language feature called operator overloading which allows for the creation of custom operators. ActionResult has such an implementation:

public sealed class ActionResult<TValue> : IConvertToActionResult
{
       public TValue Value { get; }

       public ActionResult(TValue value)
       {
            /* error checking code removed */
            Value = value;
       }

       public static implicit operator ActionResult<TValue>(TValue value)
       {
           return new ActionResult<TValue>(value);
       }
}

public static implicit operator I.e. this method provides the logic for TValue to be implicitly casted to type ActionResult. It's a very simple method that creates a new ActionResult with the value set to a public variable called Value. This method makes this legal:

ActionResult<int> result = 10; <-- // same as new ActionResult(10)

This essentially creates syntatic sugar for what you do in the Action methods to be legal.