How to make ASP.NET Core bind partially to valid data?

Question

Let's say I have this model:

public enum State
{
    Valid = 1,
    Invalid = 2
}

public class Person
{
    public string Name { get; set; }
    
    public State state { get; set; }
}

And this controller action:

[HttpPost]
public object SavePerson([FromBody] Person person)
{
    return person;
}

If I send this JSON, everything works just fine:

{
    "name": "John",
    "state": 1
}

However, if I change the "state": 1 to an invalid enumeration like "state": "" or "state": "1", then the person parameter would be null.

In other words, if I send a JSON that is partially valid, ASP.NET Core ignores all fields.

How can I configure ASP.NET Core to at least extract the valid fields from the body?

Does this answer your question? POST null because property type does not match? — wei
– wei, Commented Dec 2, 2022 at 8:23
@wei, that is Newtonsoft.Json. We're using System.Text.Json. — Ali EXE
– Ali EXE, Commented Dec 2, 2022 at 8:26
seems like System.Text.Json doesn't have error handler. github.com/dotnet/runtime/issues/38049, github.com/dotnet/runtime/issues/44390 — wei
– wei, Commented Dec 2, 2022 at 9:24
System.Text.Json may not do what you want. You can use Newtonsoft.Json or try a custom formatter. — Chen
– Chen, Commented Dec 27, 2022 at 6:36

Ziv Weissman · Accepted Answer · 2022-12-02 09:08:38Z

1

You need to handle deserialization exception.

This code will put a default value each time it will encounter a problem in a field but the json string itself must be a valid json string.

    static void Main(string[] args)
    {
        //This should be run on startup code (there are other ways to do that as well - you can put this settings only on controllers etc...)
        JsonConvert.DefaultSettings = () => new JsonSerializerSettings
        {
            Error = HandleDeserializationError
        };

        var jsonStr = "{\"name\": \"John\",\"state\": \"\" }";
        var person = JsonConvert.DeserializeObject<Person>(jsonStr);         

        Console.WriteLine(JsonConvert.SerializeObject(person)); //{"Name":"John","state":0}
    }

    public static void HandleDeserializationError(object sender, ErrorEventArgs args)
    {
        var error = args.ErrorContext.Error.Message;
        args.ErrorContext.Handled = true;
    }

For WebAPI you should have a Startup.cs file looks like this:

    public void ConfigureServices(IServiceCollection services)
    {
        //look for services.AddControllers() -> or add it if it does not exist
        services.AddControllers()
            .AddMvcOptions(options =>
            {
               //... might have options here
            })
               //this is what you need to add
            .AddNewtonsoftJson(options =>
            {
                options.SerializerSettings.Error = HandleDeserializationError
            });
    }

edited Dec 2, 2022 at 9:08

answered Dec 2, 2022 at 8:28

Ziv Weissman

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3 Comments

Ali EXE Over a year ago

Thank you so much. Can you please explain how can I handle this in ASP.NET Core pipeline? Because I already knew that I had to handle it myself somewhere. I don't know where?

Ziv Weissman Over a year ago

@AliEXE see edit - if it is a webAPI .net core app

Ali EXE Over a year ago

Awesome. Yet we do not use Newtonsoft.Json. We use System.Text.Json. Thank you for helping us.

Nileksh Dhimer · Accepted Answer · 2022-12-02 08:41:32Z

0

Try this nullable operator:

public State? state { get; set; }

answered Dec 2, 2022 at 8:41

Nileksh Dhimer

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1 Comment

Ali EXE Over a year ago

Thank you so much. But this is not what I'm searching for. I don't want to change all of my models forcefully. I prefer to configure ASP.NET Core.

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