System.Text.Json serialization against an anonymous object

Question

I'm working on an ASP .Net Core 3.1 application, porting part of the code from another using 2.2. So far, I'd like to switch from the NewtonSoft JSON serialization library to the new one, System.Text.Json, but I have some trouble.

Consider a function to serve a HTTP-GET service with this returning type:

 [HttpGet("myservice")] public async Task<ActionResult<object>> GetDataAsync( ...

Then, the final part could be depicted as follows:

 var items = new List<IMyInterface>(); int totalCount = ... int min = ... int max = ... return new ActionResult<object>(new { totalCount, min, max, items });

However, it doesn't work: the items collection is serialized by its declared type (IMyInterface), instead of the actual type(s). I read here that this is an expected behavior, although not so intuitive to me.

My question is: is there any convenient yet reliable way to leverage the new serializer even dealing with anonymous objects? I would avoid to create a specific object every time I can compose the result inline.

UPDATE:

doing this it seems to work, but it looks really ugly:

 return new ActionResult<object>(new { totalCount, min, max, items = items.Cast<object>() });

have you considered casting items to object within your new ActionResult object (before serializing) — Jawad
– Jawad, Commented Jan 3, 2020 at 16:14
You could have a look at these two (1, 2) issues on GitHub. They might help you out. But it seems like a deliberate "feature" with polymorphic deserialization. — fredrik
– fredrik, Commented Jan 3, 2020 at 16:14
@Jawad I updated the post doing a cast on each item: it works but looks quite ugly. By casting the entire collection it doesn't work, though. — Mario Vernari
– Mario Vernari, Commented Jan 3, 2020 at 16:21

Jawad · Accepted Answer · 2020-01-03 16:49:07Z

DotNetFiddler

If you want to serialize the objects, why not initialize them as objects? Is there a requirement to create it strong typed?

 public static void Test() { var items = new List<object>() { new Class1 { Foo = "foo1", Bar1 = "Bar1" }, new Class2 { Foo = "foo1", Bar2 = "Bar2" } }; int totalCount = 1; int min = 2; int max = 3; var root = new { totalCount, min, max, items, }; var json = JsonSerializer.Serialize<object>(root, new JsonSerializerOptions { WriteIndented = true, }); Console.WriteLine(json); }

If you create the items as List<object>, you dont have to change or do anything. This might be a cleaner way instead of casting each of them to object at time of creating the object.

That's another way to surround the problem. In my case, many of these collections are generated by Linq'ed expressions, thus I must apply a Cast<object>(). Still I can't understand why the serialization is fine by using object: why not using the runtime type all the times?
I know newtonsoft json.net works fine... System.text.json has issues.

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