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

            SqlCommand command = new SqlCommand("insert into Project_Skill (id_skill, id_project, level) values (@SkillID, @ProjectID, @Level)");
            command.Parameters.Add(new SqlParameter("SkillID", project_skill.SkillID));
            command.Parameters.Add(new SqlParameter("ProjectID", project_skill.ProjectID));
            command.Parameters.Add(new SqlParameter("Level", project_skill.Level));

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

            SqlCommand command = new SqlCommand("update Project_Skill set id_skill=@SkillID, id_project=@ProjectID, level=@Level where id_project_skill = @ProjectSkillID");
            command.Parameters.Add(new SqlParameter("ProjectSkillID", project_skill.ProjectSkillID));
            command.Parameters.Add(new SqlParameter("SkillID", project_skill.SkillID));
            command.Parameters.Add(new SqlParameter("ProjectID", project_skill.ProjectID));
            command.Parameters.Add(new SqlParameter("Level", project_skill.Level));

            command.ExecuteNonQuery();
        }