Serialize objects implementing interface with System.Text.Json

Question

I have a master class which contains a generic collection. Elements in the collection are of diffetent types, and each implements an interface.

Master class:

public class MasterClass
{
    public ICollection<IElement> ElementCollection { get; set; }
}

Contract for the elements:

public interface IElement
{
    string Key { get; set; }
}

Two samples for the elements:

public class ElementA : IElement
{
    public string Key { get; set; }

    public string AValue { get; set; }
}

public class ElementB : IElement
{
    public string Key { get; set; }

    public string BValue { get; set; }
}

I need to serialize an instance of MasterClass object using the new System.Text.Json library in Json. Using the following code,

public string Serialize(MasterClass masterClass)
{
    var options = new JsonSerializerOptions
    {
        WriteIndented = true,
    };
    return JsonSerializer.Serialize(masterClass, options);
}

I get the follwing JSON:

{
    "ElementCollection":
    [
        {
            "Key": "myElementAKey1"
        },
        {
            "Key": "myElementAKey2"
        },
        {
            "Key": "myElementBKey1"
        }
    ]
}

instead of:

{
    "ElementCollection":
    [
        {
            "Key": "myElementAKey1",
            "AValue": "MyValueA-1"
        },
        {
            "Key": "myElementAKey2",
            "AValue": "MyValueA-2"
        },
        {
            "Key": "myElementBKey1",
            "AValue": "MyValueB-1"
        }
    ]
}

Which class (converter, writer, ...)should I implement to obtain the complete JSON ?

Thanks in advance for your help.

Answer 1

This works for me:

public class TypeMappingConverter<TType, TImplementation> : JsonConverter<TType>
  where TImplementation : TType
{
  [return: MaybeNull]
  public override TType Read(
    ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
      JsonSerializer.Deserialize<TImplementation>(ref reader, options);

  public override void Write(
    Utf8JsonWriter writer, TType value, JsonSerializerOptions options) =>
      JsonSerializer.Serialize(writer, (TImplementation)value!, options);
}

Usage:

var options =
   new JsonSerializerOptions 
   {
     Converters = 
     {
       new TypeMappingConverter<BaseType, ImplementationType>() 
     }
   };

JsonSerializer.Deserialize<Wrapper>(value, options);

---

Tests:

[Fact]
public void Should_serialize_references()
{
  // arrange
  var inputEntity = new Entity
  {
    References =
    {
      new Reference
      {
        MyProperty = "abcd"
      },
      new Reference
      {
        MyProperty = "abcd"
      }
    }
  };

  var options = new JsonSerializerOptions
  {
    WriteIndented = true,
    Converters =
    {
      new TypeMappingConverter<IReference, Reference>()
    }
  };

      var expectedOutput =
@"{
  ""References"": [
    {
      ""MyProperty"": ""abcd""
    },
    {
      ""MyProperty"": ""abcd""
    }
  ]
}";

  // act
  var actualOutput = JsonSerializer.Serialize(inputEntity, options);

  // assert
  Assert.Equal(expectedOutput, actualOutput);
}

[Fact]
public void Should_deserialize_references()
{
  // arrange

  var inputJson =
@"{
  ""References"": [
    {
      ""MyProperty"": ""abcd""
    },
    {
      ""MyProperty"": ""abcd""
    }
  ]
}";

  var expectedOutput = new Entity
  {
    References =
    {
      new Reference
      {
        MyProperty = "abcd"
      },
      new Reference
      {
        MyProperty = "abcd"
      }
    }
  };

  var options = new JsonSerializerOptions
  {
    WriteIndented = true
  };

  options.Converters.AddTypeMapping<IReference, Reference>();

  // act
  var actualOutput = JsonSerializer.Deserialize<Entity>(inputJson, options);

  // assert
  actualOutput
      .Should()
      .BeEquivalentTo(expectedOutput);
}


public class Entity
{
  HashSet<IReference>? _References;
  public ICollection<IReference> References
  {
    get => _References ??= new HashSet<IReference>();
    set => _References = value?.ToHashSet();
  }
}

public interface IReference
{
  public string? MyProperty { get; set; }
}

public class Reference : IReference
{
  public string? MyProperty { get; set; }
}

Answer 2

What you're looking for is called polymorphic serialization.

Here's Microsoft documentation article

Here's another question about it

According to documentation you just need to cast your interface to an object. For example:

public class TreeRow
{
    [JsonIgnore]
    public ICell[] Groups { get; set; } = new ICell[0];

    [JsonIgnore]
    public ICell[] Aggregates { get; set; } = new ICell[0];

    [JsonPropertyName("Groups")]
    public object[] JsonGroups => Groups;

    [JsonPropertyName("Aggregates")]
    public object[] JsonAggregates => Aggregates;


    public TreeRow[] Children { get; set; } = new TreeRow[0];
}