.NET Core Catch Json Deserialization Error In Middleware

Question

I'm certain I'm missing something patently obvious.

Is it possible to handle JSON deserialization errors in .NET Core's default middleware/deserializer? I need to be sure that a particular value is a JSON primitive and not an object/array. If it's not a primitive, I'd like to terminate the request and return the appropriate status code before it gets to the controller.

Controller:

[HttpPost]
public IActionResult Post([FromBody] List<MyType> myTypes)
{
    // Logic ....

    return Created(new Uri("some-location/", UriKind.Relative), someValue);
}

Request body:

[
    {
        "prop1": "some value",
        "prop2": "some value",
        "prop3": null
    },
    {
        "prop1": "some value",
        "prop2": "some value",
        "prop3": {
            "prop3-1": "some value"
        }
    }
]

The MyType constructor inspects the arguments passed in and throws an exception if there's a problem. prop3 in MyType is a dynamic field. I've verified that the exception is being thrown when the second item in the request body is deserialized.

This all feels pretty straightforward but I just can't find where to access the default deserializer.

I tried this and it does catch errors but only after the request has made it into the controller.

Is there a way to handle this in JSON annotations? I looked through the Newtonsoft docs but nothing jumped out.

Answer 1

Looking at the documentation here: https://learn.microsoft.com/en-us/aspnet/core/mvc/models/model-binding?view=aspnetcore-2.1 I think what you are looking for is the AddJsonOptions()

in Startup.cs:

services.AddMvc().AddJsonOptions(options =>
{
     options.SerializerSettings.MissingMemberHandling = Newtonsoft.Json.MissingMemberHandling.Error;
});

This will not throw an exception, but it will fail to bind, so myTypes becomes null.

Then you can just add a null check

[HttpPost]
public IActionResult Post([FromBody] List<MyType> myTypes)
{
    if(myTypes == null)
    {
        return BadRequest();
    }

    // Logic ....

    return Created(new Uri("some-location/", UriKind.Relative), someValue);
}