reified-enums.cs

abstract class State
{
	public abstract bool IsFinalState { get; }
	public abstract bool IsValidTransition(State next);
	public abstract override string ToString();

	public static State NotStarted { get; } = new NotStartedT();
	public static State Running { get; } = new RunningT();
	public static State Errored { get; } = new ErroredT();
	public static State Completed { get; } = new CompletedT();

	private abstract class FinalState : State
	{
		public override bool IsFinalState => true;
		public override bool IsValidTransition(State next) => false;
	}

	private sealed class ErroredT : FinalState
	{
		public override string ToString() => "Errored";
	}

	private sealed class CompletedT : FinalState
	{
		public override string ToString() => "Completed";
	}

	private sealed class RunningT : State
	{
		public override bool IsFinalState => false;
		public override bool IsValidTransition(State next)
			=> next == Completed || next == Errored;
		public override string ToString() => "Running";
	}

	private sealed class NotStartedT : State
	{
		public override bool IsFinalState => false;
		public override bool IsValidTransition(State next)
			=> next == Running || next == Errored;
		public override string ToString() => "Not Started";
	}
}