启用DropDownList以返回默认值

Question

我正在尝试将默认值返回到我的2下拉列表中。我已经使用我的SQLserver数据绑定了我的DDLpolice，同时通过项属性添加了DDLocation。只要管理员选择了某个位置，它就会显示可用的选项。

以下是2下拉列表

1. lblocation
2. DDLpolice

我在各自的DDL中有以下这些值

• lblocation

  1. 选择位置（默认值）
  2. 选择TownA（项目值）
  3. 选择TownB（项目值）

• DDLpolice（选择：TownA）

  1. 选择主管（默认值）
  2. 选择123（项目值）
  3. 选择124（项目值）

• DDpolice（选择：TownB）

  1. 选择位置（默认值）
  2. 选择126（项目值）
  3. 选择127（项目值）

在我的数据库中，各自的Town都有各自的警察局

• A镇
  1. 123
  2. 124
• B镇
  1. 126
  2. 127

每当管理员选择一个特定的caseID，一个位置和一个警察时，它会自动更新数据库，这是完全正常的。因此，逻辑上它应该“刷新”DDL而不显示所选的policeID。然而，DDL的价值仍然存在。 我添加了一个loadgrid，以刷新我的gridview信息。我在DDL上添加了一个Item.Clear（），它完全清除了我的数据DDL。

public partial class AssignTask : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {


        if (!IsPostBack)
        {
            LoadGrid();
        }

    }

    private void LoadGrid()
    {

        SqlConnection conn = new SqlConnection();
        conn.ConnectionString = "Data Source =localhost;" +
            "Initial Catalog = project; Integrated Security = SSPI";
        conn.Open();

        DataSet ds = new DataSet();

        SqlDataAdapter da = new SqlDataAdapter("SELECT memberreportid, typeofcrime, location, crdatetime, citizenreport, image1, image2, image3, image4, image5 FROM MemberReport where handle='unhandled' AND caseprogress='inprogress'", conn);
        da.Fill(ds);

        GWCase.DataSource = ds;
        GWCase.DataBind();

        conn.Close();

    }

    protected void GWCase_SelectedIndexChanged(object sender, EventArgs e)
    {

        lblCID.Text = GWCase.SelectedRow.Cells[1].Text;

    }

    protected void lblocation_SelectedIndexChanged(object sender, EventArgs e)
    {
        using (var connAdd = new SqlConnection("Data Source = localhost; Initial Catalog = project; Integrated Security= SSPI"))
        {
            connAdd.Open();


            var sql = "Select policeid from LoginRegisterPolice where location='" + lblocation.SelectedItem.Text + "' AND status='available' AND handle='offcase'";
            using (var cmdAdd = new SqlDataAdapter(sql, connAdd))
            {
                DataSet ds2 = new DataSet();
                cmdAdd.Fill(ds2);

                DDLpolice.DataSource = ds2;
                DDLpolice.DataTextField = "policeid";
                DDLpolice.DataValueField = "policeid";
                DDLpolice.DataBind();
            }      
            connAdd.Close();
        }

    }

    protected void btnAssign_Click(object sender, EventArgs e)
    {

        if (lblCID.Text.Equals(""))
        {
            lblmsg.Text = "Please select one case";
            return;
        }
        else if (lblocation.SelectedItem.Text.Equals("Select Location"))
        {
            lblmsg.Text = "Please select a location";
            return;
        }
        else if (DDLpolice.SelectedItem.Text.Equals("Select Officer"))
        {
            lblmsg.Text = "Please select an officer";
            return;

        }
        else
        {
            lblmsg.Text = "The case has been successfully assigned.";

        }



        using (var connAdd = new SqlConnection("Data Source = localhost; Initial Catalog = project; Integrated Security= SSPI"))
        {
            connAdd.Open();
            var sql = "Update AdminAssign Set assign ='" + DDLpolice.SelectedItem.Text + "' where memberreportid='" + lblCID.Text + "'";
            using (var cmdAdd = new SqlCommand(sql, connAdd))
            {
                cmdAdd.ExecuteNonQuery();
            }

            sql = "Insert into PoliceReport (memberreportid, policeid) values ('" + lblCID.Text.Trim() + "','" + DDLpolice.SelectedItem.Text + "')";
            using (var cmdAdd = new SqlCommand(sql, connAdd))
            {
                cmdAdd.ExecuteNonQuery();
            }


            sql = "Update LoginRegisterPolice Set handle = '" + lblCID.Text + "' where policeid='" + DDLpolice.SelectedItem.Text + "'";
            using (var cmdAdd = new SqlCommand(sql, connAdd))
            {
                cmdAdd.ExecuteNonQuery();
            }

            sql = "Update MemberReport Set handle = 'handled' where memberreportid='" + lblCID.Text + "'";
            using (var cmdAdd = new SqlCommand(sql, connAdd))
            {
                cmdAdd.ExecuteNonQuery();
            }
            connAdd.Close();
        }
        LoadGrid();


        lblCID.Text = "";
        lblocation.Items.Clear();
        DDLpolice.Items.Clear(); 
    }

    protected void DDLpolice_SelectedIndexChanged1(object sender, EventArgs e)
    {

    }

}
}  

因此，我想知道如何恢复我的DDLocation，默认值和项值以及DDLpolice默认值。