public static void Add(IContext context, ITaskSkill task_skill)
        {
            Context ctx = context as Context;
            if (ctx == null)
                throw new Exception(typeof(Context).FullName + " expected.");

            SqlCommand command = new SqlCommand("insert into Task_Skill (id_skill, id_task, level) values (@SkillID, @TaskID, @Level)");
            command.Parameters.Add(new SqlParameter("SkillID", task_skill.SkillID));
            command.Parameters.Add(new SqlParameter("TaskID", task_skill.TaskID));
            command.Parameters.Add(new SqlParameter("Level", task_skill.Level));

            command.ExecuteNonQuery();
        }
        public static void Update(IContext context, ITaskSkill task_skill)
        {
            Context ctx = context as Context;
            if (ctx == null)
                throw new Exception(typeof(Context).FullName + " expected.");

            SqlCommand command = new SqlCommand("update Task_Skill set id_skill=@SkillID, id_task=@TaskID, level=@Level where id_task_skill = @TaskSkillID");
            command.Parameters.Add(new SqlParameter("TaskSkillID", task_skill.TaskSkillID));
            command.Parameters.Add(new SqlParameter("SkillID", task_skill.SkillID));
            command.Parameters.Add(new SqlParameter("TaskID", task_skill.TaskID));
            command.Parameters.Add(new SqlParameter("Level", task_skill.Level));

            command.ExecuteNonQuery();
        }