即使DropDownList禁用了viestate，SelectedValue仍应返回一个值

2 回答

• 1

  没有选定的值，因为：

  1）下拉列表中没有项目，因为您只是第一次将数据绑定到它 . 通常这是一种很好的做法，但是当您关闭ViewState时，您必须每次都为控件显式重新创建数据 .

  2）您正在页面生命周期中将请求字典值应用于控件的点之后（即，在将ViewState恢复到控件期间）绑定数据 . 已发布的值存在于Request字典中，但由于没有项目在下拉列表中，它无法真正做到这一点 . 即使ViewState已关闭，您的作者也是正确的 - 将在生命周期中通常应用ViewState的点处应用已发布的值 .

  如果您要在Init（）中为列表重新创建数据，那么将填充下拉列表以显示要应用的已发布值，将应用该下拉列表，并且在您到达时，所选值将可用加载（） . 希望这很清楚 . 一些工作代码如下：

  public partial class _Default : System.Web.UI.Page
  {
      protected void Page_Load(object sender, EventArgs e)
      {
          if (IsPostBack)
          {
              Label1.Text = DropDownList1.SelectedValue;
          }
      }
  
      protected void Page_Init(object sender, EventArgs e)
      {
          string[] number = { "first", "second", "third" };
          DropDownList1.DataSource = number;
          DropDownList1.DataBind();
      }
  }
  

  回复于 2024-05-05T23:25:07+08:00
• 17

  这是一个更智能的DropDownList实现 . 不是巨魔或任何东西，但我不敢相信有些限制（阅读：愚蠢）一些ASP.Net控件....

  Public Class DropDownList
      Inherits System.Web.UI.WebControls.DropDownList
  
      Private _SelectedIndex As Integer? = -1
      Private _SelectedValue As String = Nothing
      Private _StateFromClient As Boolean = False
      Private _StateFromLocal As Boolean = False
  
      Protected Overrides Sub OnInit(ByVal e As EventArgs)
          MyBase.OnInit(e)
          Page.RegisterRequiresControlState(Me)
      End Sub
  
      Public Overrides Property SelectedIndex() As Integer
          Get
              If Not _StateFromLocal Then
                  Me.LoadStateFromClient()
  
                  If _StateFromClient Then
                      Return _SelectedIndex
                  End If
              End If
  
              Return MyBase.SelectedIndex
          End Get
          Set(ByVal value As Integer)
              _StateFromLocal = True
              MyBase.SelectedIndex = value
          End Set
      End Property
  
      Public Overrides Property SelectedValue() As String
          Get
              If Not _StateFromLocal Then
                  LoadStateFromClient()
  
                  If _StateFromClient Then
                      Return _SelectedValue
                  End If
              End If
  
              Return MyBase.SelectedValue
          End Get
          Set(ByVal value As String)
              _StateFromLocal = True
              MyBase.SelectedValue = value
          End Set
      End Property
  
      Private Sub LoadStateFromClient()
          If _StateFromClient Then Return
          If _StateFromLocal Then Return
          If Me.IsViewStateEnabled Then Return
          If Not Me.Page.IsPostBack Then Return
  
          If Not _SelectedIndex.HasValue Then
              Throw New Exception("ControlState has not yet been loaded and so state does not exist.")
          End If
          _SelectedValue = Me.Page.Request.Form(Me.UniqueID)
          _StateFromClient = True
      End Sub
  
      Protected Overrides Sub PerformSelect()
          ' Called when DataBound() is called, which can affect the Selected* property values
          _StateFromLocal = True
          MyBase.PerformSelect()
      End Sub
  
      Protected Overrides Function SaveControlState() As Object
          Dim state As Object = MyBase.SaveControlState()
  
          If Me.SelectedIndex >= 0 Then
              If state IsNot Nothing Then
                  Return New Pair(state, Me.SelectedIndex)
              Else
                  Return Me.SelectedIndex
              End If
          Else
              Return state
          End If
      End Function
  
      Protected Overrides Sub LoadControlState(ByVal state As Object)
          If state IsNot Nothing Then
              Dim p As Pair = TryCast(state, Pair)
              If p IsNot Nothing Then
                  MyBase.LoadControlState(p.First)
                  _SelectedIndex = CInt(p.Second)
              Else
                  If (TypeOf (state) Is Integer) Then
                      _SelectedIndex = CInt(state)
                  Else
                      MyBase.LoadControlState(state)
                  End If
              End If
          End If
          MyBase.SelectedIndex = _SelectedIndex
      End Sub
  
  End Class
  

  回复于 2024-05-05T23:25:07+08:00