public abstract class AppUser
{
  public Guid Id { get; set; }
}

public class RegisteredUser : AppUser
{
  public void Apply(Registered registered)
  {
    Username = registered.Username;
  }
  
  // remove this projection when user is disabled
  internal bool ShouldDelete(Disabled _) => true;

  public string Username { get; set; }
}

public class DisabledUser : AppUser
{
  // start this projection when user is disabled
  public void Apply(Disabled disabled)
  {
     UserName = disabled.Username
  }

  public string Username { get; set; }
}

services.AddMarten(options =>
{
  options.Schema.For<AppUser>()
    .AddSubClass<RegisteredUser>()
    .AddSubClass<DisabledUser>();
});


// register user, then disable it as both events are on the same stream
var registered = new Registered(username);
var disabled = new Disabled(username);

// store events
session.Events.StartStream<AppUser>(streamId, registered, disabled);
await session.SaveChangesAsync();

var user = session.Load<AppUser>(streamId); // (AppUser is DisabledUser)-> true
var users = session.Query<AppUser>().ToList(); // Count -> 1
var registeredUser = session.Load<RegisteredUser>(streamId); // -> null
var disabledUser = session.Load<DisabledUser>(streamId); // -> not null