给DropDownList绑定数据时，须考虑空值的情况

DropDownListApproverId.DataSource = ds;
   DropDownListApproverId.DataTextField="name";
   DropDownListApproverId.DataValueField="employeeId";
   DropDownListApproverId.DataBind();
   DropDownListApproverId.Items.Add(new ListItem("未指定",""));

若不人工添加一个空值备选项，万一被赋予空值时将出错。如AssessorId为空值

DropDownListAssessorId.SelectedValue=task.AssessorId.Trim();