Question

我对.net相对较新，我正在尝试根据下拉框的值启用/禁用文本框。（用户选择5个位置，只显示5个（10井，2个5个）框。）

通过阅读此处和本地其他地方的主题，我认为FindControl是可行的方法（在页面上找到控件然后根据需要显示或不显示），但是下面的代码在txtBox上生成NullReferenceException.Visible =是的，因为，txtBox是“没什么”。

        Dim txtBox As TextBox = New TextBox()
        Dim textboxName As String

...

        For LoopCounter = 1 To 10
            If LoopCounter < ddNumberOfPlaces.SelectedValue + 1 Then
                textboxName = "txtGuestName" + CStr(LoopCounter)
                txtBox = CType(FindControl(textboxName), TextBox)
                txtBox.Visible = True
                txtBox = CType(FindControl("txtGuestOrganisation" + CStr(LoopCounter)), TextBox)
                txtBox.Visible = True
            Else
                txtBox = CType(FindControl("txtGuestName" + CStr(LoopCounter)), TextBox)
                txtBox.Visible = False
                txtBox = CType(FindControl("txtGuestOrganisation" + CStr(LoopCounter)), TextBox)
                txtBox.Visible = False
            End If
        Next

那我做错了什么？这是实现这个的最好方法吗？

先谢谢。

P.S。 textboxName（在异常点）包含有效的控件ID。

Answer 1

如果您使用母版页，则FindControl不起作用。 FindControl仅返回顶级控件。它没有递归地找到控件。您可以使用以下函数递归查找控件。不要调用FindControl，而是调用此函数。

     Private Function FindControlRecursive(ByVal rootControl As Control, ByVal controlID As String) As Control
        If (rootControl.ID = controlID) Then
            FindControlRecursive = rootControl
        Else
            For Each controlToSearch As Control In rootControl.Controls

                Dim controlToReturn As Control = FindControlRecursive(controlToSearch, controlID)
                If (controlToReturn IsNot Nothing) Then
                    FindControlRecursive = controlToReturn
                End If

            Next
            FindControlRecursive = Nothing
        End If
    End Function

感谢。你的功能似乎总是返回“Nothing”所以我稍微修改了它。

Private Function FindControlRecursive(ByVal rootControl As Control, ByVal controlID As String) As Control

    Dim controlToReturn As Control = Nothing

    If (rootControl.ID = controlID) Then
        controlToReturn = rootControl
    Else
        For Each controlToSearch As Control In rootControl.Controls
            If (controlToReturn Is Nothing) Then
                controlToReturn = FindControlRecursive(controlToSearch, controlID)
            End If
        Next
    End If
    FindControlRecursive = controlToReturn
End Function

以编程方式访问webcontrols（通过FindControl）

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