Returning error message from .NET Core Web API

Question

I have a .NET Core Web API controller to handle a POST request which takes a JSON ApiUser object and also returns an ApiUser object

[HttpPost]
public ApiUser Post([FromBody] ApiUser model) {
    return new ApiUser();
}

Let's say I want to return an error message to the client: "Email already exists." I don't know how best to do it in a simple manner.

I could add an "ErrorMsg" property to ApiUser (and every other class that I return through the API), but not sure if this is the correct way.

"Pre-Core", I'd throw a HttpResponseException, which is not supported in Core. This was nice because it would create a simple JSON with "Message" which I could then grab. (Goofy: I'd use status 202 Accepted as a flag to my client that we had a "known error".)

throw new HttpResponseException(request.CreateErrorResponse(HttpStatusCode.Accepted, msg));

(I think I'd prefer to deal with this inside the controller, instead of "filters" or "middleware". One concern with the latter two is that if I add razor pages, I assume I'd need to know if error came from api calls or from UI web calls.)

UPDATE #1

Seems I can do the following (return ActionResult< T>) to simulate what I did before with HttpResponseException. Still not sure if doing something like this or returning status 200 with adding ErrorMsg to ApiUser would be better.

[HttpPost]
public async Task<ActionResult<ApiUser>> Post([FromBody] ApiUser model) {
    ...
    if (emailExists) return StatusCode(400, "Email already exists.");
    else return user;
}

Similar/same questions: stackoverflow.com/questions/41464540/… stackoverflow.com/questions/47142142/… — Ernie Thomason
– Ernie Thomason, Commented May 21, 2020 at 9:40

LouraQ · Accepted Answer · 2020-05-22 01:37:53Z

6

You can use HttpResponseException in the core web api:

  [HttpPost]
        public ApiUser Post([FromBody] ApiUser model)
        {
            if (model.Email == "11")// this condition can be changed as your requirement.
            {
                var response = new HttpResponseMessage(HttpStatusCode.BadRequest);
                response.Content = new StringContent("Email already exists.");
                throw new HttpResponseException(response);
            }

            return new ApiUser();
        }

Here is the test result:

Update

You can also change return type as IActionResult to return different content as follow:

   [HttpPost]
        public IActionResult Post([FromBody] ApiUser model)
        {
            if (model.Email == "11")// this condition can be changed as your requirement.
            { 
                return BadRequest("Email already exists.");
            }

            return Ok(new ApiUser());
        }

The test result is the same as before.

edited May 22, 2020 at 1:37

answered May 21, 2020 at 8:18

LouraQ

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3 Comments

Ernie Thomason Over a year ago

It does seem I can still use "HttpResponseException" if I install a package called "Microsoft.AspNet.WebApi.Core", but this hasn't been updated since 2018. I'm not sure this is the best solution. Great video. I just installed Postman to help me test this.

LouraQ Over a year ago

@ErnieThomason, if you don't want to use HttpResponseException, you can change your return type to IActionResult , as the method you update in your question, but there are some details to change ,I have updated in my post, you can follow it.

Ernie Thomason Over a year ago

Thank you very much for your help. This gives me enough info on my options so I can move forward.

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