我正在开发一个winforms应用程序，我有一个从数据库绑定的组合框，每个项都有一个名称和一个值：

// New item class
 public class themeS
 {
     public string Name { get; set; }
     public string Value { get; set; }
     public override string ToString() { return this.Name; }
 }

 // Binding ComboBox Event
 using (DbEntities db = new DbEntities())
 {
    comboBox2.Items.Clear();
    IEnumerable tem  = from t in db.Themes where t.idCategorie == 1  select t;
    foreach (Themes Tem in tem)
    {
        comboBox2.Items.Add(new themeS { Value = Tem.idTheme.ToString(), Name= Tem.nomTheme });
    }
 }

现在我想检索组合框的选定项的值：

string curentIdTem = comboBox2.SelectedValue.ToString();

comboBox2.SelectedValue的返回值总是'NULL'，有人能帮忙吗？