Creating a Generic JsonConverter for Interfaces in System.Text.Json

Introduction

I will introduce how to create a generic converter to serialize and deserialize classes that implement interfaces like the following using System.Text.Json.

public interface IEmployee
{
    int EmployeeId { get; }
}

public class Employee : IEmployee
{
    public Employee(int employeeId)
    {
        EmployeeId = employeeId;
    }

    public int EmployeeId { get; }
}

That being said, since serialization works out of the box, the main focus here is deserialization.

Implementing the Converter

Prepare a custom implementation of System.Text.Json.Serialization.JsonConverter.

public class InterfaceConverter<TInterface, TImplement>
    : JsonConverter<TInterface> where TImplement : TInterface
{
    public override TInterface Read(
        ref Utf8JsonReader reader,
        Type typeToConvert,
        JsonSerializerOptions options)
    {
        return (TInterface)JsonSerializer.Deserialize(ref reader, typeof(TImplement), options);
    }

    public override void Write(
        Utf8JsonWriter writer,
        TInterface value,
        JsonSerializerOptions options)
    {
        JsonSerializer.Serialize(writer, value, typeof(TImplement), options);
    }
}

Using the Converter

You can use it like this:

var employee = (IEmployee)new Employee(100);

var options = new JsonSerializerOptions
{
    Converters =
    {
        new InterfaceConverter<IEmployee, Employee>()
    }
};

var employeeString = JsonSerializer.Serialize(employee, options);
Console.WriteLine(employeeString);
var deserializedUnit = JsonSerializer.Deserialize<IEmployee>(employeeString, options);

That's it!