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

            SqlCommand command = new SqlCommand("insert into User_Skill (id_skill, id_user, level) values (@SkillID, @UserID, @Level)");
            command.Parameters.Add(new SqlParameter("SkillID", user_skill.SkillID));
            command.Parameters.Add(new SqlParameter("UserID", user_skill.UserID));
            command.Parameters.Add(new SqlParameter("Level", user_skill.Level));

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

            SqlCommand command = new SqlCommand("update User_Skill set id_skill=@SkillID, id_user=@UserID, level=@Level where id_user_skill = @UserSkillID");
            command.Parameters.Add(new SqlParameter("UserSkillID", user_skill.UserSkillID));
            command.Parameters.Add(new SqlParameter("SkillID", user_skill.SkillID));
            command.Parameters.Add(new SqlParameter("UserID", user_skill.UserID));
            command.Parameters.Add(new SqlParameter("Level", user_skill.Level));

            command.ExecuteNonQuery();
        }