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我在下拉列表数据绑定事件期间手动添加了一些附加属性。但是，回发后该属性丢失。我发现DropDownList中ListItems属性的相关问题在回发时丢失了？我完全按照解决方案。

这就是我所拥有的

下拉列表数据绑定事件 *.aspx.vb，我在每个列表项中添加了属性“类”

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

    If Not IsPostBack Then
        ddlCountry.DataBind()
    End If
End Sub

Protected Sub ddlCountry_DataBound(ByVal sender As Object, ByVal e As System.EventArgs) Handles ddlCountry.DataBound
    Dim ddlCountryList As DropDownList = LocationBLL.GetDropDownList("Country", "C", "C", False, "")
    Dim lstPleaseSelect As ListItem = New ListItem("Please Select", "-1")
    Dim lstOthers As ListItem = New ListItem("Others", "0")

    lstPleaseSelect.Attributes("class") = "-1"
    lstOthers.Attributes("class") = "-1"

    ddlCountry.Items.Add(lstPleaseSelect)
    For Each lstItem As ListItem In ddlCountryList.Items
        lstItem.Attributes("class") = lstItem.Value
        ddlCountry.Items.Add(lstItem)
    Next
    ddlCountry.Items.Add(lstOthers)
End Sub

*.aspx

<%@ Register TagPrefix="msjNewControls" Namespace="NewControls"%>

*.aspx

<msjNewControls:NewDropDownList ID="ddlCountry" runat="server" AutoPostBack="False" >
</msjNewControls:NewDropDownList>

类文件 *.vb

Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Security.Permissions
Imports System.Linq
Imports System.Text
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls

Namespace NewControls
<DefaultProperty("Text")> _
<ToolboxData("<{0}:ServerControl1 runat=server></{0}:ServerControl1>")> _
Public Class NewDropDownList
    Inherits DropDownList

    <Bindable(True)> _
    <Category("Appearance")> _
    <DefaultValue("")> _
    <Localizable(True)> _
    Protected Overrides Function SaveViewState() As Object
        ' create object array for Item count + 1
        Dim allStates As Object() = New Object(Me.Items.Count) {}

        ' the +1 is to hold the base info
        Dim baseState As Object = MyBase.SaveViewState()
        allStates(0) = baseState

        Dim i As Int32 = 1
        ' now loop through and save each Style attribute for the List
        For Each li As ListItem In Me.Items
            Dim j As Int32 = 0
            Dim attributes As String()() = New String(li.Attributes.Count - 1)() {}
            For Each attribute As String In li.Attributes.Keys
                attributes(System.Math.Max(System.Threading.Interlocked.Increment(j), j - 1)) = New String() {attribute, li.Attributes(attribute)}
            Next
            allStates(System.Math.Max(System.Threading.Interlocked.Increment(i), i - 1)) = attributes
        Next
        Return allStates
    End Function

    Protected Overrides Sub LoadViewState(ByVal savedState As Object)
        If savedState IsNot Nothing Then
            Dim myState As Object() = DirectCast(savedState, Object())

            ' restore base first
            If myState(0) IsNot Nothing Then
                MyBase.LoadViewState(myState(0))
            End If

            Dim i As Int32 = 1
            For Each li As ListItem In Me.Items
                ' loop through and restore each style attribute
                For Each attribute As String() In DirectCast(myState(System.Math.Max(System.Threading.Interlocked.Increment(i), i - 1)), String()())
                    li.Attributes(attribute(0)) = attribute(1)
                Next
            Next
        End If
    End Sub
End Class
End Namespace

但是我仍然在我的类文件中收到以下错误。

Exception Details: System.InvalidCastException: Unable to cast object of type 'System.Collections.DictionaryEntry' to type 'System.Web.UI.WebControls.ListItem'.

Source Error: 

Line 34:                     attributes(System.Math.Max(System.Threading.Interlocked.Increment(j), j - 1)) = New String() {attribute, li.Attributes(attribute)}

Source File: D:\root\App_Code\BLL\CustomDropDownListBLL.vb    Line: 34

任何人都可以帮忙吗？

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1 回答 1

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我发现 ddl 中唯一保留的值是文本和值。因此，我编写了以下代码来保持用户标记为高优先级的颜色编码和粗体列表项。这在 SelectedIndexChanged 事件中对我有用。由于 HighPriority 标志存储在 SQL 中，我将 ListItem 文本的第一个字符设置为 ! 返回数据行时。Also, I found that when an item is selected that the Forecolor attribute still would not apply, so i used the bSetSelected flag to set the ForeColor to the appropriate color as needed.

    Private Sub ddlYourDropDownList_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ddlYourDropDownList.SelectedIndexChanged

    Try
        'do your processing here, then reset the attributes below

        'color code high priority requests in Red
        Dim li As ListItem, bSetSelected As Boolean = False
        For Each li In ddlYourDropDownList.Items
            'see if this is a High Priority item, SQL Proc places the ! at the start of the string
            If li.Text.Substring(0, 1) = "!" Then
                li.Attributes.Add("style", "color:Red; font-weight: bold;")
                'once we set the selected one, no need to change ForeColor anymore
                If Not bSetSelected Then
                    If li.Selected Then
                        ddlYourDropDownList.ForeColor = Drawing.Color.Red
                        bSetSelected = True
                    Else
                        ddlYourDropDownList.ForeColor = Drawing.Color.Black
                    End If
                End If
            Else
                li.Attributes.Add("style", "color:Black; font-weight: normal;")
            End If
        Next


    Catch ex As Exception
        lblMessage.Text = "Error: " & ex.Message
    End Try
End Sub
于 2019-07-16T12:47:23.630 回答