using System;

using System.Text.Json;

using System.Text.Json.Serialization;

​

[JsonPolymorphic(TypeDiscriminatorPropertyName = "IsTrue")]

[JsonDerivedType(typeof(MyJson1), "true")]

[JsonDerivedType(typeof(MyJson2), "false")]

public abstract record MyAbstractJson();

​

public record MyJson1(string SomeProperty) : MyAbstractJson();

public record MyJson2() : MyAbstractJson();

​

public class Program

{

    public static void Main()

    {

        var json = """

            {

                "IsTrue": true

                "SomeProperty":"SomeValue"

            }

        """;

        var obj = JsonSerializer.Deserialize<MyAbstractJson>(json);

        var casted = obj as MyJson1;

        Console.WriteLine(casted?.SomeProperty);

    }

}