动态创建的DropDownList在Postback上丢失了ListItems

2018-06-14 22:06:13

我有一个包含一些动态创建的控件（TextBox和DropDownList）的页面。当回发发生时，TextBoxes保留它们的值，但DropDownLists失去它们的ListItems。这很令人困惑，因为页面级别的DropDownList也会保留它的ListItems。任何人都可以看到下面的代码有什么问题吗？

任何帮助解决这个问题将不胜感激。

<%@ Page Language="VB"%>

<script runat="server">
  Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs)
    If Not Page.IsPostBack Then
      ddlFixed.Items.Add(New ListItem("12", "13"))
      ddlFixed.Items.Add(New ListItem("14", "15"))
    End If
    Dim i As Integer
    For i = 0 To 3
      Dim ddl As New DropDownList
      ddl.ID = "ddlPage" & i
      ddl.EnableViewState = True
      If Not Page.IsPostBack Then
        ddl.Items.Add(New ListItem("12", "13"))
        ddl.Items.Add(New ListItem("14", "15"))
      End If
      pnlDynamic.Controls.Add(ddl)
      Dim txtBx As New TextBox
      txtBx.ID = "txtPage" & i
      If Not Page.IsPostBack Then
        txtBx.Text = "Preset"
      End If
      pnlDynamic.Controls.Add(txtBx)
    Next
  End Sub
</script>


<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
  <title></title>
</head>
<body>
  <form id="form1" runat="server" enableviewstate="true">
  <div>
    <br />
    <br />
    <a href="Default.aspx">Get-Request</a>
    <br />
    <br />
    <asp:Panel runat="server" ID="pnlDynamic" EnableViewState="true" />    
    <br />
    <br />
    <asp:DropDownList runat="server" ID="ddlFixed" />
    <asp:Button runat="server" ID="btn" Text="Postback"/>
  </div>
  </form>
</body>
</html>

我找到了可以让我在回传时保持视图状态的解决方案。这是调用ItemCollection的TrackViewState。

CType(ddl.Items, IStateManager).TrackViewState()

感谢大家的帮助。

这是工作解决方案：

<%@ Page Language="VB"%>

<script runat="server">
  Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs)
    If Not Page.IsPostBack Then
      ddlFixed.Items.Add(New ListItem("12", "13"))
      ddlFixed.Items.Add(New ListItem("14", "15"))
    End If
    Dim i As Integer
    For i = 0 To 3
      Dim ddl As New DropDownList
      ddl.ID = "ddlPage" & i
      CType(ddl.Items, IStateManager).TrackViewState()
      If Not Page.IsPostBack Then
        ddl.Items.Add(New ListItem("12", "13"))
        ddl.Items.Add(New ListItem("14", "15"))
      End If
      pnlDynamic.Controls.Add(ddl)
      Dim txtBx As New TextBox
      txtBx.ID = "txtPage" & i
      If Not Page.IsPostBack Then
        txtBx.Text = "Preset"
      End If
      pnlDynamic.Controls.Add(txtBx)
    Next
  End Sub
</script>


<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
  <title></title>
</head>
<body>
  <form id="form1" runat="server" >
  <div>
    <br />
    <br />
    <a href="Default.aspx">Get-Request</a>
    <br />
    <br />
    <asp:Panel runat="server" ID="pnlDynamic" />    
    <br />
    <br />
    <asp:DropDownList runat="server" ID="ddlFixed" />
    <asp:Button runat="server" ID="btn" Text="Postback"/>
  </div>
  </form>
</body>
</html>

当viewstate被启用时，DDL将维护它的项目和选择。文本框将保持其内容，无论视图状态如何，因为生成的html会在回发中写回服务器。

我发现你的代码和标记中包含了所有这些视图状态语句。

几件事情之一正在发生：

控件上的ViewState被禁用

控制层面上方的ViewState被禁用（页面，窗体，面板）

您正在页面周期中动态地添加控件，以便从viewstate中加载选择。在INIT中加载应该没问题。

也许你需要为这些DropDownLists显式激活ViewState ...

编辑：这是我的意思是我最后的评论...

    <script runat="server">
      Protected Sub Page_Init(ByVal sender As Object, ByVal e As System.EventArgs)
        If Not Page.IsPostBack Then
          ddlFixed.Items.Add(New ListItem("12", "13"))
          ddlFixed.Items.Add(New ListItem("14", "15"))

          Dim i As Integer
          For i = 0 To 3
            Dim ddl As New DropDownList
            ddl.ID = "ddlPage" & i
            ddl.EnableViewState = True
            ddl.Items.Add(New ListItem("12", "13"))
            ddl.Items.Add(New ListItem("14", "15"))

            pnlDynamic.Controls.Add(ddl)
            Dim txtBx As New TextBox
            txtBx.ID = "txtPage" & i
            txtBx.Text = "Preset"
            pnlDynamic.Controls.Add(txtBx)
          Next
        End If
      End Sub
   </script>

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