我有這個下拉串列，這個加載了我從存盤程序中獲得的資料，你可以看到加載是正確的，但是當我在除錯中更改選定的值時，選定的值不會改變，它停留在第一個加載值，在這種情況下為 1。我能做什么？

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Dim adaptador As New SqlDataAdapter
    Dim datos As New DataTable
    Dim ord As SqlDataReader


    Conexiones.AbrirConexion()
    Conexiones.Cnn.Open()

    Dim cmd As SqlCommand = New SqlCommand("sp_devolver_empresas", Conexiones.Cnn)
    cmd.CommandType = CommandType.StoredProcedure

    ord = cmd.ExecuteReader

    datos.Load(ord)

    cbEmpresas.DataSource = datos
    cbEmpresas.DataTextField = "Nombre"
    cbEmpresas.DataValueField = "Identificador"
    cbEmpresas.DataBind()

    Conexiones.Cnn.Close()

End Sub

uj5u.com熱心網友回復：

對于您構建和寫入到時間盡頭的每個網頁？

您必須始終將加載和設定代碼放在回發檢查中。

每個按鈕，每個下拉串列，以及每次單擊按鈕或執行任何操作時，頁面加載總是會觸發并運行。因此，如果您更改一個值，您將丟失該值，因為頁面設定和加載說控制元件（在這種情況下是您的下拉串列）將觸發每次 - 從而覆寫。

在最后 100 個網頁中，我寫的第一件事是檢查回發。

因此，您的代碼應如下所示：

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Not IsPostBack Then
        LoadData
    End If

End Sub


Sub LoadData()
    Dim datos = New DataTable
    Using onexiones.Cnn
        Using cmd As SqlCommand = New SqlCommand("sp_devolver_empresas", onexiones.Cnn)
            cmd.CommandType = CommandType.StoredProcedure
            onexiones.Cnn.Open()
            datos.Load(cmd.ExecuteReader())
        End Using
    End Using

    cbEmpresas.DataTextField = "Nombre"
    cbEmpresas.DataValueField = "Identificador"
    cbEmpresas.DataSource = datos
    cbEmpresas.DataBind()

End Sub

所以，只有在“真正的”第一頁加載時，您才想加載您的網格、下拉串列等。記住，即使是一個簡單的按鈕拖放到頁面上，您也有一個點擊事件？（頁面加載事件每次都會觸發 - 并且每次）。所以再一次，您需要 IsPostBack 測驗 - 我非常迫切地想找到我的任何網頁在加載事件中沒有這些所有重要的代碼存根。

因此，請記住上述規則 - 始終設定您的設定代碼以在第一次運行時運行 - 而不是每次在網頁上出現按鈕或其他任何內容時。因此，每次您在該頁面上執行任何操作時，頁面加載（與桌面不同）都會觸發。

Also, note how I setup the data text, and value field BEFORE we give the drop down list a data source. In fact, I would suggest you put the data text, and data value field in the markup and NOT code (since then multiple different code routines can use that setting - not hard coded in code).

eg do this:

    <asp:DropDownList ID="cbEmpresas" runat="server"
        DataTextField = "Nombre"
        DataValueField = "Identificador" >
    </asp:DropDownList>

So, now your code becomes :

    cbEmpresas.DataSource = datos
    cbEmpresas.DataBind()

I suppsoe some out of habt like to set the text data settings for the dropdown list in code, but then again, in code you might want more then one palce that loads up the combo box - and thus you now have multiple places for those two settings. And better yet, you can even use the property sheet during design time to make the settings. (this is a "minor" issue, but I just perfer those settings in the markup as opposed to writing them in code for text/data settings).

At the end of the day? Just remember that golden rule: have that "test" for postback so you have a REAL first page load event code inside that if/then. In fact, you really can't even build a fnctional working web page if you break this rule. As noted, I count about 1-2 of my web pages out of 100's tht don't have that not postback code stub. It is quite much as important say compared to humans having to breathe air to function.

And why such a long post about this simple issue? Well on a near daily bases, both c# and vb.net questions are posted about how some combo box, grid view or just about anything else is "broken" and not working. And the answer 9 out of 10 times?

The developer forgot to use a Not IsPostBack code stub in their page load code.

If I was teaching asp.net, this would be the first lesson - ALWAYS use and consider the requirement for the Not IsPost back stub. You need that in 99% of your web pages. Or at the very least in ANY web page in which you have code to run on page load to setup grids, dropdowns, repeaters - don't matter what, you will NEED that code stub. So important, I often wonder if they should have create a even called first page load? It would have eliminated the daily posts and questions on SO that fail as a result of not heeding the above simple advice.

Edit: Now how to get selected value in code

Ok, now that we fixed up the code to setup the dropdown list, we now want to get in our code the value when the user makes a choice in the dropdown list.

You of course have code to fill the dropdown, BUT we also have to set autopostback = true for this dropdown.

所以，說要加載的代碼：

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Not IsPostBack Then
        LoadData
    End If

End Sub


Sub LoadData()

    Using conn As New SqlConnection(My.Settings.TEST4)
        Dim strSQL As String =
            "SELECT ID, HotelName from tblHotels ORDER BY HotelName"

        Using cmdSQL As SqlCommand = New SqlCommand(strSQL, conn)
            conn.Open()
            Dim rstData As New DataTable
            rstData.Load(cmdSQL.ExecuteReader)
            cboHotels.DataSource = rstData
            cboHotels.DataBind()
        End Using

    End Using

End Sub

現在選定的索引更改事件：

因此，我們的除錯輸出將如下所示：

輸出：

combo selected value = 82
combo selected Text = Canadian Rocky Mountain Resorts

如前所述，確保在屬性表或標記中設定自動回發為真。因此，標記中的上述組合框是這樣的：

    <asp:DropDownList ID="cboHotels" runat="server"
        DataValueField="ID"
        DataTextField="HotelName" AutoPostBack="True">
    </asp:DropDownList>

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