[Feature Request] Add __typename to generated classes

[kaioduarte]

It would be great if the generated classes had the __typename field, which is needed when dealing with unions.

[gao-artur]

Alternatively, there is a proposal (2) in the #126. For example, for the union called Assignee with two entries: Team and Employee generate an empty interface IAssignee and "implement" it on the Team and Employee classes. Then, the user can use pattern matching to find out what the real type behind the union. For example

public interface IAssignee { }

public class Team : IAssignee
{
    public string Name { get; set; }
}

public class Employee : IAssignee
{
    public string FullName { get; set; }
}

Usage:

var assignee = QueryAssignee();
var greeting = assignee switch
{
    Team team => $"Hello {team.Name}!",
    Employee employee => $"Hello {employee.FullName}!",
    _ => throw new ArgumentException($"Unexpected assignee type {assignee.GetType().Name}", nameof(assignee))
};

[Husqvik]

I think it should deserialize into appropriate type just as interfaces do.

[Husqvik]

I'll implement it during this week.