ASP.Net 5 MVC 6 - how to return invalid JSON message on POST using FromBody?

Question

I am creating an ASP.Net 5 application with MVC 6, using .Net 4.5.1. I have a POST method that uses a FromBody parameter to get the object automatically.

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Answer 1

The default JsonInputFormatter will in fact return a null model upon encountering an error - but it will populate ModelState with all exceptions.

So you have access to all encountered errors by digging into ModelState:

[HttpPost]
public IActionResult Insert([FromBody]Agent agent)
{
    if (!ModelState.IsValid)
    {
        var errors = ModelState
            .SelectMany(x => x.Value.Errors, (y, z) => z.Exception.Message);

        return BadRequest(errors);
    }

    // Model is valid, do stuff.
}

Output of above is an array of all exception messages, e.g.:

[
    "After parsing a value an unexpected character was encountered: l. Path 'Name', line 2, position 20",
    "Another exception message..."
]

JsonInputFormatter - Source