C# .NET: Deserialize JSON array into IList<Interface> with System.Text.Json

Question

I'm removing Newtonsoft.Json from a project and replace it with an implementation of System.Text.Json.Serialization.

The old model uses a JsonArrayAttribute in combination with CustomCreationConverter<T>.

class JsonListItemConverter<TListItem>:
    CustomCreationConverter<TListItem>
{
    public override TListItem Create( Type objectType )
    {
        return ( TListItem ) FormatterServices.GetSafeUninitializedObject( typeof( TListItem ) );
    }
}

interface FooInterface
{
    string Value { get; set; }
}

class Foo:
    FooInterface
{
  public virtual string Value { get; set; }
}

interface BarInterface
{
    virtual IList<FooInterface> { get; set; }
}

class Bar:
    BarInterface
{
    [JsonArray( ItemConverterType = typeof( JsonListItemConverter<Foo> ) )]
    public virtual IList<FooInterface> { get; set; }
}

This still enables me to implement against interfaces but deserializes concrete classes.

I'm now looking for an equivalent in System.Net.Json to replace the now unkown JsonArrayAttribute.

I assume I'd use JsonConverter. I found out there're several converters in System.Text.Json.Serialization.Converters, like the ListOfTConverter<TCollection, TElement>. But they are all internal and they are for serializing only.

I'm looking for a solution to deserialize a JSON array into a strongly interfaced collection type like IList<FooInterface>.

Does System.Text.Json: How to apply a JsonConverter for a collection with a custom converter for the collection's items? answer your question? The answers provide converters that function similarly to the missing ItemConverterType functionality. — dbc
– dbc, Commented Sep 30, 2024 at 22:57

Khaja Hussain · Accepted Answer · 2024-10-01 00:40:43Z

You are trying to replace Newtonsoft.Json with System.Text.Json and need to deserialize a JSON array into an IList<FooInterface>. Since System.Text.Json doesn't support deserializing interfaces out of the box, you'll need to create a custom converter.

using System;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace DeserializeInterfaces
{    
    public interface FooInterface
    {
        string Value { get; set; }
    }

    public interface BarInterface
    {
        IList<FooInterface> Foos { get; set; }
    }
   
    public class Foo : FooInterface
    {
        public string Value { get; set; }
    }

    public class Bar : BarInterface
    {
        [JsonConverter(typeof(InterfaceListConverter<FooInterface, Foo>))]
        public IList<FooInterface> Foos { get; set; }
    }
   
    public class InterfaceListConverter<TInterface, TImplementation> : JsonConverter<IList<TInterface>>
        where TImplementation : TInterface, new()
    {
        public override IList<TInterface> Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {            
            var list = JsonSerializer.Deserialize<List<TImplementation>>(ref reader, options);
            return list != null ? new List<TInterface>(list) : null;
        }

        public override void Write(Utf8JsonWriter writer, IList<TInterface> value, JsonSerializerOptions options)    
    {           
            JsonSerializer.Serialize(writer, (List<TImplementation>)value, options);
        }
    }

 
    class Program
    {
        static void Main(string[] args)
        {
            var json = @"{ ""Foos"": [ { ""Value"": ""Item1"" }, { ""Value"": ""Item2"" } ] }";

            var bar = JsonSerializer.Deserialize<Bar>(json);

            foreach (var foo in bar.Foos)
            {
                Console.WriteLine(foo.Value);
            }         
        }
    }
}

Explanation:

System.Text.Json doesn't natively support deserializing to interface types. By creating a custom converter, you guide the deserialization process to use the concrete class while allowing your code to work with the interface. Some useful tools to visually check Complex JSON.

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