Return an object along with a 409 Conflict error in a Web API 2 POST call backed by Entity Framework?

Question

I have a C# Entity Framework Web API 2 controller. Currently when an attempt is made via the POST method to create an object with the same text for the main text field, I return a 409 Conflict error as an StatusCode result to indicate the addition is considered a duplicate.

What I'd like to do is return the server side object that triggered the duplicate error too. So I need something akin to the Ok() method but a variant that returns a 409 Conflict error as the HTTP status code instead of an HTTP OK status code.

Is there such a thing? How can I do this? If I can make this work the client doesn't have to do a subsequent Get call to the server to get the existing object after receiving a 409 Conflict error.

Here's the current POST method:

    public IHttpActionResult PostCanonical(Canonical canonical)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        // Check for duplicate Canonical text for the same app name.
        if (db.IsDuplicateCanonical(canonical.AppName, canonical.Text))
        {
            // It's a duplicate.  Return an HTTP 409 Conflict error to let the client know.
            return StatusCode(HttpStatusCode.Conflict);
        }

        db.CanonicalSentences.Add(canonical);
        db.SaveChanges();

        return CreatedAtRoute("DefaultApi", new { id = canonical.ID }, canonical);
    }

Answer 1

You should return Content:

return Content(HttpStatusCode.Conflict, original);

Content is method on the ApiController class which will create a NegotiatedContentResult with the provided HttpStatusCode and content. There is no need to create your own extension method on the ApiController class like in the accepted answer.

Answer 2

Arrived here looking for help with ASP.NET Core HTTP 409 - this is related, just the newer approach to solving this same problem.

Conflict ActionResult

 return Conflict(new { message = $"An existing record with the id '{id}' was already found."});

Answer 3

EDIT: This solution is for WebApi prior v5, please see this answer if you are using v5 or above.

You could return a NegotiatedContentResult<T> that lets you specify the status code and an object to be put into the http message body.

Change your code to something like this:

public IHttpActionResult PostCanonical(Canonical canonical)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }

    // Check for duplicate Canonical text for the same app name.
    if (db.IsDuplicateCanonical(canonical.AppName, canonical.Text))
    {
        // It's a duplicate.  Return an HTTP 409 Conflict error to let the client know.
        var original = db.CanonicalSentences.First(c => c.ID == canonical.ID);
        return new NegotiatedContentResult<T>(HttpStatusCode.Conflict, original, this);
    }

    db.CanonicalSentences.Add(canonical);
    db.SaveChanges();

    return CreatedAtRoute("DefaultApi", new { id = canonical.ID }, canonical);
}

Or maybe wrap it an extension method like this:

public static class HttpActionResultExtensions {
    public static IHttpActionResult StatusCodeWithContent<T>(this ApiController @this, HttpStatusCode statusCode, T content) {
        return new NegotiatedContentResult<T>(statusCode, content, @this);
    }
}

And then use the extension like this:

public IHttpActionResult PostCanonical(Canonical canonical)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }

    // Check for duplicate Canonical text for the same app name.
    if (db.IsDuplicateCanonical(canonical.AppName, canonical.Text))
    {
        // It's a duplicate.  Return an HTTP 409 Conflict error to let the client know.
        var original = db.CanonicalSentences.First(c => c.ID == canonical.ID);
        return StatusCodeWithContent(HttpStatusCode.Conflict, original)
    }

    db.CanonicalSentences.Add(canonical);
    db.SaveChanges();

    return CreatedAtRoute("DefaultApi", new { id = canonical.ID }, canonical);
}