using System;

using System.Threading.Tasks;

using System.Threading.Tasks;

using System.Collections.Generic;

using System.Linq;

​

namespace Program

{

    public class Program

    {

        public static void onlyUnassignedIssues<TBacklogState>(Issue<Unassigned, TBacklogState> a)where TBacklogState : IBacklogState

        {

        }

​

        public static void Main(string[] args)

        {

            Issue<Unassigned, InBacklog> unassigned = new Issue<Unassigned, InBacklog>();

            onlyUnassignedIssues(unassigned);

            Issue<Assigned, InBacklog> assigned = unassigned.assign(new Unassigned());

            onlyUnassignedIssues(assigned); // I cannot pass assigned issue

            Issue<Assigned, InBacklog> assigned2 = assigned.assign(new Assigned()); // I cannot assign issue again

            Issue<Unassigned, Todo> todo = unassigned.addToSprint(new InBacklog()); // I can first move it to sprint

            Issue<Assigned, Todo> assignedTodo = todo.assign(new Unassigned()); // And then assign

            Issue<Assigned, Todo> movedToSprintAgain = assignedTodo.addToSprint(new Todo()); // But I cannot move it to sprint again

        }

    }

​

    public interface IAssignedState

    {

    }

​

    public class Assigned : IAssignedState

    {

        Assigned()

        {

        }

    }

​

    public class Unassigned : IAssignedState

    {

        public Unassigned()

        {

        }

    }

​

    public interface IAssignable<T, U>

        where T : IAssignedState

    {

        U assign(T t);

    }

​

    public interface IBacklogState

    {

    }

​

    public class InBacklog : IBacklogState

    {

        public InBacklog()

        {

        }

    }

​

    public class Todo : IBacklogState

    {

        Todo()

        {

        }

    }

​

    public interface ITodoable<T, U>

        where T : IBacklogState

    {

        U addToSprint(T t);

    }

​

    public class Issue<TAssignableState, TBacklogState> : IAssignable<TAssignableState, Issue<Assigned, TBacklogState>>, ITodoable<TBacklogState, Issue<TAssignableState, Todo>> where TAssignableState : IAssignedState where TBacklogState : IBacklogState

    {

        public Issue<Assigned, TBacklogState> assign(TAssignableState t)

        {

            return new Issue<Assigned, TBacklogState>();

        }

​

        public Issue<TAssignableState, Todo> addToSprint(TBacklogState t)

        {

            return new Issue<TAssignableState, Todo>();

        }

    }

}