抛出异常的 IEnumerable 在 ASP.NET 核心 Web ZDB93474238D1043CACE1 中的 OkResult 传递时返回无效的 Json

Question

Start up a new ASP.NET Core Web API project in Visual Studio.

Now change the ValuesController.Get() action to be this:

// GET api/values
[HttpGet]
public ActionResult<IEnumerable<string>> Get()
{
    var results = new[] {"hi"}.Select<string, string>(x => throw new ArgumentException());
    return Ok(results);
}

When you hit this you get a 200 response with a response body of just [

Now if you now change it to enumerate the erroring Select before passing to Ok() it will raise an error correctly and return 500 with details of the error as expected in the default ASP.NET API template:

var results = new[] {"hi"}.Select<string, string>(x => throw new ArgumentException());
results.ToList();
return Ok(results);

What is happening here?

Answer 1

In the first example, the status code and response headers are sent to the client before the JSON serialisation starts. Giving the JSON serialiser an IEnumerable<string> results in deferred creation of the collection, which ends up being performed during the serialisation process. This means that the 200 status code and response headers are sent before the exception is thrown. Because the status code has already been sent, it cannot be changed from 200. The response cannot be "undone" either - it's a forward-only stream of data.

In the second example, the collection is created before the JSON serialisation process runs. In this case, the exception is thrown before the call to return Ok(results) can be reached. Whatever you're using for global exception handling catches this exception and writes out a response with a status code of 500.