如何清除ASP.net DropDownList显示以前选择的项目

Question

请帮助我如何恢复它。我使用了ddlHassection.Items.Clear（）;但它不能正常工作。

using (var context = new SMAPPDBEntities())
{
     var addDept = new SMDepartment
     {
          DepartmentCode = txtDeptCode.Text,
          DepartmentName = txtDeptName.Text,
          UnitID= Convert.ToInt32( ddlUnitName.SelectedValue),
          Description = txtDeptDescription.Text,
          DepartmentHead = Convert.ToInt32(ddlDeptHeadName.SelectedValue),
          HasSection = Convert.ToBoolean(ddlHasSection.SelectedValue),
          Tag= txtDeptTag.Text
      };
      try
      {
           context.SMDepartment.Add(addDept);
           context.SaveChanges();
       }
       catch (Exception ex)
       {
           ex.ToString();
       }
}

Answer 1

protected void DropDownList1_SelectedIndexChanged1(object sender, EventArgs e)
{
  DropDownList1.Items.Clear();
}

or

DDL1.ClearSelection();