Optional<T> with [FromBody]

[sps-campbellwray]

I just found this issue discussing the limitations of .NET Model Binders and Optional<T> #213 when using query string parameters.

I am running into the same issue with [FromBody] when trying to convert the JSON body of a request into an object with a series of Optional<T> properties.

public class TestModel
{
    [JsonConverter(typeof(OptionalConverterFactory))]
    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
    public Optional<string> Test { get; set; }
}

When providing the following test value:

{
  "test": "Hello World!"
}

I get the same System.ArgumentException: The type 'System.String&' may not be used as a type argument. exception.

Is there any way to make this work? It seems crazy that I can do this

string s = "{ \"test\": \"Hello World!\" }";
TestModel model = JsonSerializer.Deserialize<TestModel>(s);

with two lines of code, but as soon as there is a ModelBinder in the way everything breaks.

[sakno]

Do you have standalone example of this exception? It was fixed by marking ValueRef property with JsonIgnore attribute. Seems like the model binder ignores it for some reason.

[sps-campbellwray]

Hi @sakno, I'm not sure I totally understand what you are asking.

I was using this on an API controller, so something like:

[HttpPost]
public async Task<IActionResult> PostValueAsync([FromBody] TestModel model)
{
    //exception thrown by ModelBinder before we reach this point
    ...
}

When POSTing the example body I provided in my original question I was getting a System.ArgumentException

[sakno]

The only way to fix it is to write custom model binder as discussed in #213. Also, ASP.NET Core minimal API has no issues since it doesn't use model binding at all:

app.MapPost("/hello", (TestModel model) => Results.Ok(model.Test.ToString()));

The example above works without any problems.

[sps-campbellwray]

Thanks for the help @sakno

I'm not sure what the .NET/.NEXT team intends to do with this Optional type, but it feels like it is almost perfect for use in API controller design, it is unfortunate that it doesn't work for even the simplest of use cases.

If this only works in small/beginner projects using minimal APIs, or projects belonging to large teams who are willing to write their own model binding logic, that feels like is a pretty big gap in the middle for teams using traditional controller-based APIs, who need a simple solution that works out of the box.

If it is helpful I can create an issue to track adding Model Binder support by default, otherwise I will close this discussion as I am looking for something a little easier to implement.

[sakno]

small/beginner projects using minimal APIs

ASP.NET Core minimal API is not positioned for small/beginner projects. it's for microservices. In most of the cases, it's not clear why do you need complex model binding layer just for REST endpoints where only serialization/deserialization required.

[sps-campbellwray]

I didn't initially realise that minimal APIs didn't use model binding. Even if that approach were to work, I guess it's more a case of not wanting to rewrite our existing services that use Controller based APIs to use minimal APIs, and the code organisation headache that would come with that (10s or 100s of endpoints which are currently abstracted away in controllers being added to Program.cs).

[sakno]

After an experiment with custom model binder I can conclude that there is no way to provide proper support for Optional<T> in MVC. I tried to play with IModelBinder interface implementation, but without success. The infrastructure of ASP.NET Core MVC should ignore the properties returning the value by reference (managed pointer).