Specifying a JSON enum string converter for a list property

[RenderMichael]

I have a property which is an enumeration of enums (in my case it is a HashSet<T> but this minimal repro is representative), and I am unsure how to specify the JSON enum string converter:

using System.Text.Json;
using System.Text.Json.Serialization;

using NUnit.Framework;

const string json = """
{
    "values": ["One", "Two", "Three"]
}
""";

HasNumbers? hasNumbers = JsonSerializer.Deserialize<HasNumbers>(json, NumberSerializerContext.Default.HasNumbers);
Assert.That(hasNumbers, Is.Not.Null);
Number[]? numbers = hasNumbers.Values;

Assert.That(numbers, Is.Not.Null);
Assert.That(numbers, Has.Length.EqualTo(3));
Assert.Multiple(() =>
{
    Assert.That(numbers[0], Is.EqualTo(Number.One));
    Assert.That(numbers[1], Is.EqualTo(Number.Two));
    Assert.That(numbers[2], Is.EqualTo(Number.Three));
});

[JsonSourceGenerationOptions(
// UseStringEnumConverter = true // this approach does not work
)]
[JsonSerializable(typeof(HasNumbers))]
public sealed partial class NumberSerializerContext : JsonSerializerContext;

public class HasNumbers
{
    // [JsonConverter(typeof(JsonStringEnumConverter<Number>))] // this does not work either
    public Number[]? Values { get; set; }
}

public enum Number
{
    One,
    Two,
    Three,
}

Is there any way to do what I am doing? Preferably in an AOT-friendly way.

[elgonzo]

Setting UseStringEnumConverter = true in the [JsonSourceGenerationOptions] attribute is the correct approach. However, this is not the only setting you need to take care of here.

Note that Values != values. To make deserialization work as desired, you also need to enable case-insensitive property name handling:

[JsonSourceGenerationOptions(
  UseStringEnumConverter = true,
  PropertyNameCaseInsensitive = true
)]
[JsonSerializable(typeof(HasNumbers))]
public sealed partial class NumberSerializerContext : JsonSerializerContext;

[RenderMichael]

Thanks for the tip, that got it working. Careless mistake on my part.

However, using the context-wide string enum converter property was a last-ditch attempt on my part. Is there a more localized way of pulling this off, ideally on the Values property itself?

[RenderMichael]

The Number enum (or rather, the actual enum I'm working with outside the minimal repro) is from a dependency, outside of my control.

[elgonzo]

Is there a more localized way of pulling this off, ideally on the Values property itself?

Depends on whether you want the desired behavior specifically only on the Values property, or specifically on the Numbers enum type regardless where it is being used in the object graph being (de)serialized.

If the behavior should be specific to the Values property (with other properties of the same enum type being unaffected), currently you would need to write a custom JsonConverter that then executes the StringEnumConverter on the collection items, similar to this implementation: #54189 (comment). This converter would then be applied to the Values property using the [JsonConverter] attribute. There are open issues concerning the inability to easily specify JsonConverters for collections items (such as this: #54189, for example), but as far as i am aware addressing this has been repeatedly pushed into the future by the team... :-(

If the StringEnumConverter should be applied to the Numbers type wherever it is being used, the solution is much simpler. Change the [JsonSourceGenerationOptions] annotation to:

[JsonSourceGenerationOptions(
  Converters = new[] { typeof(JsonStringEnumConverter<Number>) },
  PropertyNameCaseInsensitive = true
)]
[JsonSerializable(typeof(HasNumbers))]
public sealed partial class NumberSerializerContext : JsonSerializerContext;