打印在标签上的数据库下拉列表值

Question

我正在使用 ASP.net 来制作下拉菜单。下拉菜单链接到数据库，一切正常。如果在下拉列表中选择了一个值，则必须填充与数据库中的下拉值匹配的标签。 希望我很清楚，这是我到目前为止所拥有的：

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        OleDbConnection conn = new OleDbConnection();
        conn.Open();
        conn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0; Data Source=" + Server.MapPath(@"\App_Data") + @"\JipEnJanneke.mdb";


        OleDbCommand cmd1 = new OleDbCommand("Select (Prijs, Jaartal, ISBN) from JipEnJanneke where Titel = @Titel", conn);
        cmd1.Parameters.AddWithValue("@Titel", DropDownList1.SelectedValue.ToString());
        OleDbDataReader rd = cmd1.ExecuteReader();
        while (rd.Read())
        {
            lbl_Prijs.Text = rd["Prijs"].ToString();
            lbl_Jaar.Text = rd["Jaartal"].ToString();
            lbl_Isbn.Text = rd["ISBN"].ToString();
        }
        conn.Close();

不幸的是，这使标签为空。如果我将该函数添加到我的 page_load 中，标签确实会被填充，但由于某种原因，只能填充下拉列表中的第一个值。此处的类似帖子建议将其推到 selectindexchanged 中，但这对我来说是空的。有人有想法吗？

这是我现在的 page_load 事件

        OleDbConnection conn = new OleDbConnection();
        conn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0; Data Source=" + Server.MapPath(@"\App_Data") + @"\JipEnJanneke.mdb";
        //conn.ConnectionString = "Provider=Microsoft.JET.OLEDB.4.0; Data Source=" + Server.MapPath(@"\App_Data") + @"\JipEnJanneke.mdb";

        lblConnectionFeedback.Text = "";
        try
        {
            conn.Open();
            lblConnectionFeedback.Text += "Connection is: " + conn.State.ToString();

            // HIER QUERY
            OleDbCommand cmd = new OleDbCommand();
            cmd.Connection = conn;
            cmd.CommandText = "SELECT * FROM Boeken";

            OleDbDataReader rd = cmd.ExecuteReader();

            DropDownList1.DataSource = rd;
            DropDownList1.DataTextField = "Titel";
            DropDownList1.DataValueField = "Titel";
            DropDownList1.DataBind();
            rd.Close();

            conn.Close();

        }
        catch (Exception exc)
        {
            lblConnectionFeedback.Text = exc.Message;
        }
        finally
        {
            conn.Close();
            lblConnectionFeedback.Text += "<br />Connection is: " + conn.State.ToString();
        }

Answer 1

在您的加载事件和 dropdown_selectedindexchanged 中的代码中，检查页面的 IsPostBack。例如：

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
if (IsPostBack){
        OleDbConnection conn = new OleDbConnection();
        conn.Open();
        conn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0; Data Source=" + Server.MapPath(@"\App_Data") + @"\JipEnJanneke.mdb";


        OleDbCommand cmd1 = new OleDbCommand("Select (Prijs, Jaartal, ISBN) from JipEnJanneke where Titel = @Titel", conn);
        cmd1.Parameters.AddWithValue("@Titel", DropDownList1.SelectedValue.ToString());
        OleDbDataReader rd = cmd1.ExecuteReader();
        while (rd.Read())
        {
            lbl_Prijs.Text = rd["Prijs"].ToString();
            lbl_Jaar.Text = rd["Jaartal"].ToString();
            lbl_Isbn.Text = rd["ISBN"].ToString();
        }
        conn.Close();
}
}

加载：

if (!IsPostBack){
   OleDbConnection conn = new OleDbConnection();
        conn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0; Data Source=" + Server.MapPath(@"\App_Data") + @"\JipEnJanneke.mdb";
        //conn.ConnectionString = "Provider=Microsoft.JET.OLEDB.4.0; Data Source=" + Server.MapPath(@"\App_Data") + @"\JipEnJanneke.mdb";

        lblConnectionFeedback.Text = "";
        try
        {
            conn.Open();
            lblConnectionFeedback.Text += "Connection is: " + conn.State.ToString();

            // HIER QUERY
            OleDbCommand cmd = new OleDbCommand();
            cmd.Connection = conn;
            cmd.CommandText = "SELECT * FROM Boeken";

            OleDbDataReader rd = cmd.ExecuteReader();

            DropDownList1.DataSource = rd;
            DropDownList1.DataTextField = "Titel";
            DropDownList1.DataValueField = "Titel";
            DropDownList1.DataBind();
            rd.Close();

            conn.Close();

        }
        catch (Exception exc)
        {
            lblConnectionFeedback.Text = exc.Message;
        }
        finally
        {
            conn.Close();
            lblConnectionFeedback.Text += "<br />Connection is: " + conn.State.ToString();
        }
}

Answer 2

请检查 DropDownList1 的 AutoPostBack 属性是否设置为 True。