DropDownList SelectedIndex = 0

Question

我寫了一個簡單的程序，它連接數據庫，並從兩個表讀取數據，並將它們寫入兩個DropDownLists（DDL）。 DDL具有像父母和孩子一樣的結構。這是我的代碼：

protected void Page_Load(object sender, EventArgs e) 
{ 
    FillControls(); 
    if (!IsPostBack) 
    { 
     ddl_SelectedIndexChanged((DropDownList)PlaceHolder1.FindControl("ddlTurCountry"), null); 
    } 
    else 
    { 
     if (((DropDownList)PlaceHolder1.FindControl("ddlTurCountry")).SelectedIndex == 0) ddl_SelectedIndexChanged((DropDownList)PlaceHolder1.FindControl("ddlTurCountry"), null); //without this string dropDownList doesn't call ddl_SelectedIndexChanged() when it's SelectedIndex=0 
    } 
} 

private void FillControls() 
{ 
    Panel panel = new Panel(); 
    Label lbl = new Label(); 
    lbl.Text = "Country: "; 
    panel.Controls.Add(lbl); 
    DropDownList ddl = new DropDownList(); 
    ddl.ID = "ddlTurCountry"; 
    ddl.DataTextField = "CountryName"; 
    ddl.DataValueField = "CountryName"; 
    ddl.SelectedIndexChanged += new EventHandler(ddl_SelectedIndexChanged); 
    ddl.AutoPostBack = true; 
    panel.Controls.Add(ddl); 
    panel.Controls.Add(new LiteralControl("<br />")); 

    lbl = new Label(); 
    lbl.Text = "Kurort: "; 
    panel.Controls.Add(lbl); 
    ddl = new DropDownList(); 
    ddl.ID = "ddlTurKurort"; 
    ddl.DataTextField = "KurortName"; 
    ddl.DataValueField = "KurortName";    
    panel.Controls.Add(ddl); 
    PlaceHolder1.Controls.Add(panel); 

    FillDDL("ddlTurCountry", "SELECT CountryName from Countries"); 
} 

private void ddl_SelectedIndexChanged(object sender, EventArgs e) 
{ 
    if (((DropDownList)sender).ID == "ddlTurCountry") 
    { 
     FillDDL("ddlTurKurort", "SELECT Kurorts.KurortName FROM Kurorts INNER JOIN Countries ON Countries.idCountry = Kurorts.idCountry WHERE (Countries.CountryName = N'" + ((DropDownList)PlaceHolder1.FindControl("ddlTurCountry")).SelectedValue + "')"); 
    } 
} 

private void FillDDL(string ddlID, string SQLCommand) 
{ 
    ((DropDownList)PlaceHolder1.FindControl(ddlID)).DataSource = GetDataFromDB(SQLCommand); 
    ((DropDownList)PlaceHolder1.FindControl(ddlID)).DataBind(); 
} 

private DataTable GetDataFromDB(string SQLCommand) 
{ 
    DataTable dt = new DataTable(); 
    string sqlcnString = @"Data Source=.\SQLEXPRESS;Initial Catalog=turs;Integrated Security=True;Pooling=False"; 
    SqlConnection sqlcn = new SqlConnection(sqlcnString); 
    if (sqlcn.State.ToString() == "Closed") sqlcn.Open(); 
    SqlDataAdapter dataAdapter = new SqlDataAdapter(SQLCommand, sqlcn); 
    dataAdapter.Fill(dt); 
    sqlcn.Close(); 
    return dt; 
} 

此代碼無法正常工作。當我運行這個程序時，它啓動得很好，兩個DDL填充數據。然後，我開始在第一個DDL中更改Value，它也可以工作，但只有在不選擇第一個Value時纔有效。

如果我選擇第一個值，第二個DDL不會填充。當我用調試器看時，我發現當第一個DDL的SelectedIndex等於零時，方法ddl_SelectedIndexChanged不適用。在這種情況下，我加入這一行：

if (((DropDownList)PlaceHolder1.FindControl("ddlTurCountry")).SelectedIndex == 0) 
    ddl_SelectedIndexChanged((DropDownList)PlaceHolder1.FindControl("ddlTurCountry"), null); 

到Page_Load，但我不喜歡這樣，我不明白爲什麼它不工作時selectedIndex=0。


解決

protected void Page_Init(object sender, EventArgs e) 
    { 
     FillControls(); 
    } 

    protected void Page_Load(object sender, EventArgs e) 
    { 
     if (!IsPostBack) ddl_SelectedIndexChanged((DropDownList)PlaceHolder1.FindControl("ddlTurCountry"), null); 
    } 

Answer 1

你填充下拉每次您的列表頁面刷新。不要這樣做。 只有調用FillControls（）如果！Page.Postback

protected void Page_Load(object sender, EventArgs e) 
{ 

    if (!IsPostBack) 
    { 
     FillControls(); 
     //Not sure why you have this here, probably not needed 
     ddl_SelectedIndexChanged((DropDownList)PlaceHolder1.FindControl("ddlTurCountry"), null); 
    } 
    else 
    { 

    } 
}