从包装器类访问会话对象变量

我一直在使用这里引用的优秀帖子:如何访问会话变量从任何类在ASP.NET?

但是，我想在会话中存储一个自定义对象类型，但是我在构造对象会话类时遇到了麻烦。在会话中，我希望存储SchoolID和Grade对的列表。SchoolID-Grade对象如下所示:

[Serializable]
public class SchoolGrade
{
    public string SchoolID { get; set; }
    public string Grade { get; set; }
    public SchoolGrade(string SchoolID, string Grade)
    {
        this.SchoolID = SchoolID;
        this.Grade = Grade;
    }
}

会话包装器如下，我在构造函数上得到一个"空引用"错误。

public class EventSchoolsAndGradesSession
{
    #region " Properties "
    public List<SchoolGrade> schoolgrade  { get; set; }
    #endregion
    #region " Constructors "
    private EventSchoolsAndGradesSession()
    {
        schoolgrade.Add(new SchoolGrade("0", "0"));
    }
    #endregion
    #region " Methods "
    public static EventSchoolsAndGradesSession Current
    {
        get
        {
            EventSchoolsAndGradesSession session = (EventSchoolsAndGradesSession)HttpContext.Current.Session["_EventStudentSchoolAndGrade_"];
            if (session == null)
            {
                session = new EventSchoolsAndGradesSession();
                HttpContext.Current.Session["_EventStudentSchoolAndGrade_"] = session;
            }
            return session;
        }
    }
    #endregion
}

我的目标是能够在会话中添加和删除特定的SchoolID和Grade对。例如，这将获取会话中存在的pair，将它们添加到临时列表中，向临时列表添加新pair，然后将所有pair保存回会话:

    protected void AddAllButton_Click(object sender, EventArgs e)
    {
        List<SchoolGrade> selected = new List<SchoolGrade>();
        var current = EventSchoolsAndGradesSession.Current.schoolgrade.ToList();
        if (current != null)
        {
            foreach (SchoolGrade s in current)
            {
                selected.Add(new SchoolGrade(s.SchoolID,
                                                    s.Grade));
            }
        }            
        selected.Add(new SchoolGrade(SchoolsDropDownList.SelectedValue,
                                            GradeDropDownList.SelectedValue));
        EventSchoolsAndGradesSession.Current.schoolgrade = selected.ToList();
    }

谁能指出我做错了什么吗?

#region " Constructors "
private EventSchoolsAndGradesSession()
{
    schoolgrade = new List<SchoolGrade>();
    schoolgrade.Add(new SchoolGrade("0", "0"));
}