How to fail ASP.NET Web API deserialization when types do not match

Question

I am having a problem with silently failing deserialization in ASP.NET Web API (version 5.1.2). I would like the deserialization to raise an error instead but I am unable to find a configuration for it.

My specific (simplified) case is this. A client application (AngularJS) sends a HTTP POST request to the ASP.NET Web API backend. As a payload there are a bunch of strings:

["ABC100", "ABC200", "ABC300"]

However, the server is expecting a list of integers:

List<int> Ids { get; set; }

What ends up happening is that the deserialization fails, the Ids list will be empty and there are no errors.

Ids: []

Of course the mismatch needs to be fixed as well, but it seems obvious to me that the POST request should fail in this case. How can I make it the default?

JavaScript will always return an array, it doesn't understand List<T>. List is a C# thing. And you are expecting int, whereas the values send by client are not. — Arghya C
– Arghya C, Commented Nov 23, 2015 at 9:11
In your API method, you can do any kinds of checks you want on the data (although this is happening after the deserialization) and manually return an HTTP 500: asp.net/web-api/overview/error-handling/exception-handling — modal_dialog
– modal_dialog, Commented Nov 23, 2015 at 9:15
@modal_dialog of course I can make checks against the Ids list, but it is quite valid that it is empty. Just not in the case when the client sends data but the types do not match. — merrr
– merrr, Commented Nov 23, 2015 at 9:24
as a work around, create a custom media type formatter, handle the serialization/deserialization manually. — Amit Kumar Ghosh
– Amit Kumar Ghosh, Commented Nov 23, 2015 at 10:03

merrr · Accepted Answer · 2015-11-23 11:05:08Z

1

One solution to this problem seems to be checking the ModelState.IsValid property right at the start of the controller method:

[HttpPost]
[Route("Stuff/Ids/")]
public void PostStuff(List<int> Ids)
{
    if(!ModelState.IsValid)
        throw new Exception("ModelState is not valid.");

    // Carry on...
}

The ModelState.IsValid is indeed false in the case described by my question.

The check can be made global by creating an action filter out of it. Instructions for this can be found for example in this article: Model Validation in ASP.NET Web API

answered Nov 23, 2015 at 11:05

merrr

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