How can I serialize a property in an inherited ICollection class?

Question

I have a class that presented like

public class ItemCollection<T> : ICollection<T> {
    public ItemCollection() {
        Items = new List<T>();
    }
    public List<T> Items { get; set; }
    ...
}

Now it will be serialized into:

{
    "Property": [{...}]
}

But I want the result is like:

{
    "Property": {"Items": [{...}]}
}

Sorry for the missing information of this question.

I now stuck in serialization when using System.Text.Json.

In Newtonsoft.Json, I use [JsonObject] to annotate this class so it can serialization correctly into json with "Items": value, but I don't know how to serialize the Items property using System.Text.Json.

I have some classes inherited this class and the inheritances will be as properties in other classes.

Solution:

Thank you for every one that answered this question, I have found a solution to solve this. I create a ConverterFactory to resolve the needed types to create the converters. In the converter, I create new JsonObject and use Reflection to create the properties in the type, after this, I serialize the JsonObject so I can get the correct result.

When you call `JsonSerializer.Serialize` then it will call the `GetEnumerator` method of the `ItemCollection` that's why there is no `Items` there. — Peter Csala, Mar 31 '22 at 08:05

score 0 · Answer 1 · answered Mar 31 '22 at 08:00

Thank you for your question, but I did not found any difference between the serialization of Newtonsoft and System.Text.Json.

Perhabs you can provide an example in code to show us the problem directly. With your provided information I got with following code example the same results

static void Main(string[] args)
    {
        ItemCollection<Person> list = new ItemCollection<Person> {new Person(){ FirstName = "FirstName", Name = "Name"}};
        string jsonString = JsonSerializer.Serialize(list);
        Console.WriteLine(jsonString);


        string jsonString2 =Newtonsoft.Json.JsonConvert.SerializeObject(list);
        Console.WriteLine(jsonString2);

        Console.ReadLine();
    }

[{"Name":"Name","FirstName":"FirstName"}]

Based on the provided sample json it seems like the OP is serializing a wrapper class, not an `ItemCollection` instance directly. Something like this: `class Wrapper { public ItemCollection Property { get; set; } }` — Peter Csala, Mar 31 '22 at 08:11

score 0 · Answer 2 · answered Mar 31 '22 at 13:16

There is no built-in attribute corresponding to Newtonsoft's JsonObjectAttribute that will force a collection to be serialized as a JSON object.¹. And there is no public equivalent to IContractResolver that can be overridden to customize serialization metadata. Thus you will need to create a custom JsonConverter to serialize the properties of your ItemCollection<T>, such as the following:

[System.Text.Json.Serialization.JsonConverter(typeof(ItemCollectionJsonConverter))]
public partial class ItemCollection<T> : ICollection<T> {
    internal ItemCollection(List<T> items) { // I added this for use by the converter
        Items = items ?? throw new ArgumentNullException();
    }
    public ItemCollection() {
        Items = new List<T>();
    }
    public List<T> Items { get; set; }
    // Remainder omitted
}

public class ItemCollectionJsonConverter : JsonConverterFactory
{
    public override bool CanConvert(Type typeToConvert) => GetItemCollectionValueType(typeToConvert) != null;

    public override JsonConverter CreateConverter(Type type, JsonSerializerOptions options)
        => (JsonConverter)Activator.CreateInstance(typeof(ItemCollectionJsonConverterInner<>).MakeGenericType(GetItemCollectionValueType(type)!))!;

    static Type? GetItemCollectionValueType(Type type) =>
        (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(ItemCollection<>)) ? type.GetGenericArguments()[0] : null;

    class ItemCollectionJsonConverterInner<T> : JsonConverter<ItemCollection<T>>
    {
        class ItemCollectionDTO
        {
            public List<T>? Items { get; set; }
            // Add other properties here
        }
    
        public override ItemCollection<T> Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
            // TODO: Decide whether to throw on null
            new ItemCollection<T>((JsonSerializer.Deserialize<ItemCollectionDTO>(ref reader, options)?.Items) ?? throw new JsonException());
        
        public override void Write(Utf8JsonWriter writer, ItemCollection<T> value, JsonSerializerOptions options) =>
            JsonSerializer.Serialize(writer, new ItemCollectionDTO { Items = value.Items }, options);
    }
}

Notes:

I added the converter directly to ItemCollection<T> via attributes, but you could add it to JsonSerializerOptions.Converters if you prefer.
Many serializers will not serialize collection properties. See e.g. XmlSerializer doesn't serialize everything in my class for another.
Adding properties to collections isn't really a recommended practice; see Why not inherit from List<T>? for a discussion why.

Demo fiddle here.

¹ The list of all System.Text.Json serialization attributes is documented here.

How can I serialize a property in an inherited ICollection class?

2 Answers2

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