combox的这个功能有时候用来保存数据内容和对应的id，
combox的这两个代码只有在绑定数据源的时候才可以用。
请看代码演示：

  Department department = new Department();
  IList<DepartmentInfo> depts = department.GetDepartments();
 this.cmbDepartment.DisplayMember = "Department_Name";
 this.cmbDepartment.ValueMember = "Department_Id";
 this.cmbDepartment.DataSource = depts.ToList();

其他信息在下面的注明中，这里将展示的信息设置为Department_Name，保存的value设置为Department_Id，然后用datasource绑定数据源，即从数据库中返回的查询结果。
这样设置有利于取选中的项的value值，是一种很常见的做法。
注明：
DepartmentInfo是一个部门类，用来保存部门的信息，详细如下public class DepartmentInfo
{
public DepartmentInfo() { }

    public DepartmentInfo(string departmentId, string departmentName)
    {
        this.Department_Id = departmentId;
        this.Department_Name = departmentName;
    }

    [Column(Name = "Department_Id", IsPrimaryKey = true, DbType = "varchar(3)")]
    public string Department_Id { get; set; }

    [Column(Name = "Department_Name", DbType = "varchar(50)")]
    public string Department_Name { get; set; }
}epartment是一个操作类类，用来保存部门的信息的操作，详细如下：

 public class Department : IDepartment
    {
        public IList<DepartmentInfo> GetDepartments()
        {
            StudentMgmtDataContext db = new StudentMgmtDataContext();
            var departments = from d in db.Departments
                              select d;

            return departments.ToList();
        }
    }