Binding JSON object to list in ASP.NET Core

Question

I'm currently migrating an API from ASP.NET Web API to ASP.NET Core. My old API had an endpoint accepting a list as part of the body:

public IHttpActionResult Post(MyModel model)
{
    ...
}

public class MyModel
{
    public List<SomeObject> SomeObjects { get; set; }
}

I have some consumers of this API that was able to post the following JSON:

POST /post
{
    "someObjects": {}
}

Of course, this should have been:

POST /post
{
    "someObjects": []
}

but ASP.NET Web API model binding accepted this (setting someObjects to null I guess`).

When doing the same with ASP.NET Core, I get the following error when posting the same JSON to the API:

Cannot deserialize the current JSON object (e.g. {\"name\":\"value\"}) into type 'System.Collections.Generic.List`1[SomeObject]'

Again, I totally understand why the error is there. I need to be backwards compatible, so can anyone help? I guess I need a custom model binder or something to allow the empty object to be converted into null or an empty list?

Don't know the answer but maybe I can point you in the direction of setting your own JsonSerializerSettings newtonsoft.com/json/help/html/… — Anton Toshik
– Anton Toshik, Commented Sep 12, 2019 at 7:35
setting someObjects to null I guess` is this the actual current behavior? — Jota.Toledo
– Jota.Toledo, Commented Sep 12, 2019 at 7:47
No, the current behavior is that the request fails with the error I posted in the question. — ThomasArdal
– ThomasArdal, Commented Sep 12, 2019 at 7:50
I mean the "legacy" behavior; was the property set to null on your ASPNET WebApi? — Jota.Toledo
– Jota.Toledo, Commented Sep 12, 2019 at 8:05
I don't remember. The issue here isn't ASP.NET Web API though. I simply want to allow {} as a valid value for List<SomeObject. If it's null or an empty list in ASP.NET Core, I can handle that in the new version. — ThomasArdal
– ThomasArdal, Commented Sep 12, 2019 at 8:22

ThomasArdal · Accepted Answer · 2019-09-12 10:02:29Z

1

This is not a clean solution but you can write a custom JsonConverter for List<SomeObject> and return null or an empty list when not a list:

public class CustomConverter : JsonConverter<List<SomeObject>>
{
    public override List<SomeObject> ReadJson(JsonReader reader, Type objectType, List<SomeObject> existingValue, bool hasExistingValue, JsonSerializer serializer)
    {
        JToken token = JToken.Load(reader);
        if (token.Type == JTokenType.Array)
        {
            return token.ToObject<List<SomeObject>>();
        }
        return null;
    }

    public override void WriteJson(JsonWriter writer, List<SomeObject> value, JsonSerializer serializer)
    {
        serializer.Serialize(writer, value);
    }
}

public class MyModel
{
    [JsonConverter(typeof(CustomConverter))]
    public List<SomeObject> SomeObjects { get; set; }
}

edited Sep 12, 2019 at 10:02

ThomasArdal

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answered Sep 12, 2019 at 8:57

Kahbazi

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2 Comments

ThomasArdal Over a year ago

I have fixed the issue using your code but implemented the ReadJson method without exceptions. With your permission, I will update your code and mark it as the answer?

Kahbazi Over a year ago

Sure, I was just too lazy to implement the best solution :)

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