为什么 ASP.NET Core 操作可以返回 `ActionResult <IEnumerable<ZapResult> &gt;` 返回任何对象的 `BadRequest()`？

Question

I wonder why an ASP.NET Core action returning an ActionResult<IEnumerable<ZapResult>> may return a BadRequest(scan) , where scan is an object of a custom user-defined class?

In this case, I don't see any sensible way of converting ZapScan to IEnumerable<ZapResult> .

Indeed, both Ok() and BadRequest() return objects derived from ActionResult (not ActionResult<T> ), but in either case everything works fine and JSON is returned to the client:

[HttpGet, FormatFilter]
public ActionResult<IEnumerable<ZapResult>> OnGet([FromQuery] ZapScan scan)
{
    return HandleRequest(scan);
}

private ActionResult<IEnumerable<ZapResult>> HandleRequest(ZapScan scan)
{
    if (ValidateScan(scan)) {
        IEnumerable<ZapResult> zapResults = _zapDispatcher.Dispatch(scan);
        return Ok(zapResults);
    }
    else
    {
        var clientIp = HttpContext.Request.HttpContext.Connection.RemoteIpAddress;
        var requestUrl = UriHelper.GetDisplayUrl(HttpContext.Request);
        _logger.LogError("{0}: Invalid request: [{1}] {2} ({3})", MethodBase.GetCurrentMethod().Name, clientIp, requestUrl, scan);
        return BadRequest(scan);
    }
} 

To me it seems like anything can be converted to ActionResult<T> , and that such return type is completely generic?

Answer 1

BadRequestResult (returned by BadRequest() ) derives from ActionResult .
ActionResult<T> is implicitly convertible to ActionResult (via the IConvertToActionResult interface it implements).

Therefore the return types are compatible which is why they can be used interchangeably.

BadRequestResult has a constructor that takes any object and serializes it to a string that is output as-is to the client.