不能在 DropDownList 中选择多个项目，但调试器完美地逐步执行代码

Question

刷新页面时出现主题错误（数据绑定在页面加载时完成）。单步执行页面代码没有发现任何错误，并且堆栈跟踪中没有引用任何页面：

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Web.HttpException: Cannot have multiple items selected in a DropDownList.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:
[HttpException (0x80004005): Cannot have multiple items selected in a DropDownList.]
   System.Web.UI.WebControls.DropDownList.VerifyMultiSelect() +106
   System.Web.UI.WebControls.ListControl.RenderContents(HtmlTextWriter writer) +161
   System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer) +32
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +208
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +8
   System.Web.UI.WebControls.WebControl.RenderContents(HtmlTextWriter writer) +10
   System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer) +32
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +208
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +8
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +208
   System.Web.UI.UpdatePanel.RenderChildren(HtmlTextWriter writer) +256
   System.Web.UI.UpdatePanel.Render(HtmlTextWriter writer) +37
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +208
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +8
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +208
   System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer) +173
   System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) +31
   System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output) +53
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
   System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer) +40
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +208
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +8
   System.Web.UI.Control.Render(HtmlTextWriter writer) +10
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +208
   System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +8
   System.Web.UI.Page.Render(HtmlTextWriter writer) +29
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +100
   System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3060

上次发生这种情况时，我在母版页上有这个图像标签：

<img src="<%=Page.ResoveUrl("~/Images/footerLogo.png") %>" />

修改它以删除 <% %> (<img src="/Images/footerLogo.png" />) 块修复了错误，但这次我在母版页上没有任何此类标签（或任何链接到相关页面的页面）。这是我的代码：

Imports MySql.Data
Imports MySql.Data.MySqlClient

Partial Class maintreservation
    Inherits System.Web.UI.Page

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

        If Not IsPostBack Then

            getClients()
            getDrivers()
            getVehicles()

            pnlDriven.Visible = False
            pnlSelfDriven.Visible = False

            If Not String.IsNullOrEmpty(Session("resid")) AndAlso Session("resid") IsNot Nothing Then

                getReservation()

            End If

        End If
    End Sub

    Protected Sub getDrivers()
        ddlDriver.ClearSelection()

        Dim db As New Database

        Dim sql As String = "select driverid, drivername from drivers order by drivername"

        Dim dr As MySqlDataReader = db.GetReader(sql)

        If dr.Read Then

            ddlDriver.DataSource = dr
            ddlDriver.DataTextField = "drivername"
            ddlDriver.DataValueField = "driverid"
            ddlDriver.DataBind()

        End If
        dr.Close()

        ddlDriver.Items.Insert(0, New ListItem("Select a Driver", 0))
    End Sub

    Protected Sub getVehicles()
        ddlVehicle.ClearSelection()

        Dim db As New Database

        Dim sql As String = "select veh.vehicleid, veh.licence_plate, veh.model, vm.make from vehicles veh " & _
            "inner join vehicle_manufacturers vm on veh.make = vm.makeid order by veh.make"

        Dim dr As MySqlDataReader = db.GetReader(sql)

        While dr.Read

            ddlVehicle.Items.Add(New ListItem(dr("make") & " " & dr("model") & " " & " (" & dr("licence_plate") & ")", dr("vehicleid")))
        End While
        dr.Close()

        ddlVehicle.Items.Insert(0, New ListItem("Select a Vehicle", 0))
    End Sub

    Protected Sub getClients()
        ddlClient.ClearSelection()

        Dim db As New Database

        Dim sql As String = "select clientid, clientname, contact from clients order by clientname"

        Dim dr As MySqlDataReader = db.GetReader(sql)

        If dr.Read Then

            ddlClient.DataSource = dr
            ddlClient.DataTextField = "clientname"
            ddlClient.DataValueField = "clientid"
            ddlClient.DataBind()

        End If
        dr.Close()

        ddlClient.Items.Insert(0, New ListItem("Select an Agent", 0))
        getClientContact()
    End Sub

    Protected Sub getClientContact()

        Dim db As New Database

        Dim sql As String = "select contact, vat_no from clients where clientid = " & ddlClient.SelectedItem.Value

        Dim dr As MySqlDataReader = db.GetReader(sql)

        If dr.Read Then

            If Not IsDBNull(dr("contact")) Then
                lblClientContact.Text = dr("contact")
            Else
                lblClientContact.Text = ""
            End If
            If Not IsDBNull(dr("vat_no")) Then
                txtVatNo.Text = dr("vat_no")
            Else
                txtVatNo.Text = ""
            End If
        End If
        dr.Close()
    End Sub

    Protected Sub ddlClient_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ddlClient.SelectedIndexChanged

        getClientContact()

    End Sub

    Protected Sub getReservation()

        Dim db As New Database

        Dim sql As String = _
            "select * from reservations res " & _
            "inner join reservationbody trip on res.reservationid = trip.reservationid " & _
            "inner join vehicles vehicle on trip.vehicleid = vehicle.vehicleid " & _
            "left outer join drivers driver on trip.driverid = driver.driverid " & _
            "where res.reservationid = " & Session("resid")

        Dim dr As MySqlDataReader = db.GetReader(sql)

        If dr.Read Then

            ' GET CLIENT
            ' -------------------------------------------------------------
            ddlClient.ClearSelection()
            ddlClient.Items.FindByValue(dr("clientid")).Selected = True
            getClientContact()

            ' GET VOUCHER
            ' -------------------------------------------------------------
            If IsDBNull("vouchernumber") Then
                txtVoucher.Text = ""
            Else
                txtVoucher.Text = dr("vouchernumber")
            End If

            ' GET DRIVER
            ' -------------------------------------------------------------
            If dr("driverid") = "0" Then
                ' Self Drive
                rblTrips.Items(1).Selected = True

                pnlDriven.Visible = False
                pnlSelfDriven.Visible = True

            Else
                ' Driven
                rblTrips.Items(0).Selected = True

                pnlSelfDriven.Visible = False
                pnlDriven.Visible = True

                ddlDriver.ClearSelection()
                ddlDriver.Items.FindByValue(dr("driverid")).Selected = True
            End If

            ' GET PASSENGER/S
            ' -------------------------------------------------------------
            If IsDBNull(dr("passengers")) Then
                txtPassengers.Text = ""
            Else
                txtPassengers.Text = dr("passengers")
            End If
            If IsDBNull(dr("passengercontact")) Then
                txtPassengerContact.Text = ""
            Else
                txtPassengerContact.Text = dr("passengercontact")
            End If

            ' GET COMMENTS
            ' -------------------------------------------------------------
            If IsDBNull(dr("comments")) Then
                txtComments.Text = ""
            Else
                txtComments.Text = dr("comments")
            End If

            ' GET VEHICLES
            ' -------------------------------------------------------------
            ddlVehicle.ClearSelection()
            ddlDriver.Items.FindByValue(dr("vehicleid")).Selected = True

            ' GET DATES AND TIMES
            ' -------------------------------------------------------------
            txtPickupDate.Text = Left(dr("res_date"), 10) ' USE DATE(trip.res_date) as res_date in the sql statement

            ucTimeIn.Time = dr("time_in")

            ucTimeOut.Time = dr("time_out")

        End If
        dr.Close()
    End Sub

    Protected Sub rblTrips_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles rblTrips.SelectedIndexChanged

        If rblTrips.SelectedItem.Value = "driven" Then

            pnlDriven.Visible = True
            pnlSelfDriven.Visible = False

        Else

            pnlSelfDriven.Visible = True
            pnlDriven.Visible = False

        End If
    End Sub

End Class

如上所述，这段代码执行得很好，所以我真的不知道问题出在哪里。

我已尝试清理和重建解决方案，但仍未解决任何问题，因此欢迎提供任何见解！

Answer 1

在呈现任何ListControl 时，如果遇到多个选定项，则会设置一个标志。如果标志为真，则调用抽象方法VerifyMultiSelect。 DropDownList 的 VerifyMultiSelect 的实现如果遇到多个选定项，则会引发异常，因此您看到此异常的唯一可能原因是因为您有多个 ListItems，其中 Selected 设置为 true您的DropDownLists 之一。

我会仔细检查您的 SQL 的结果，以确定是否存在同一 DropDownList 中的多个项目将其 Selected 属性设置为 true 的情况。上面示例中的 getReservation() 函数中有三行代码更改了此属性。

也许这就是您处理IsPostBack 的方式。在ViewState 中为该控件存储了一个先前选择的项目，您正在尝试在回发后选择另一个值。

编辑： OP 找到了 solution。

Answer 2

来自Cory 的输入让我决定返回并检查每个 DropDownList 是否只指定了 1 个选择，这就是我发现问题的时候。

在getReservation() 上面：

        ' GET DRIVER
        ' -------------------------------------------------------------
        If dr("driverid") = "0" Then
            ' Self Drive
            rblTrips.Items(1).Selected = True

            pnlDriven.Visible = False
            pnlSelfDriven.Visible = True

        Else
            ' Driven
            rblTrips.Items(0).Selected = True

            pnlSelfDriven.Visible = False
            pnlDriven.Visible = True

            ddlDriver.ClearSelection()
            ddlDriver.Items.FindByValue(dr("driverid")).Selected = True
        End If

然后：

        ' GET VEHICLES
        ' -------------------------------------------------------------
        ddlVehicle.ClearSelection()
        ddlDriver.Items.FindByValue(dr("vehicleid")).Selected = True

这只是糟糕的复制/粘贴，因此对于查看此答案的人，100% 确保您只选择 ONCE

作为旁注，我被告知以下是更好的做法：

' Do this:
ddlVehicles.SelectedValue = dr("vehicleid")

' Instead of this:
ddlVehicles.Items.FindByValue(dr("vehicleid").Selected = True

Answer 3

我在修改别人的旧代码时遇到了类似的问题。 对我来说，这是一个视图状态问题，只需调用

myDropDown.ClearSelection(); 

他们在没有键的情况下从枚举中提取下拉菜单的数据源，因此使用 FindText 进行选择。如果下拉所选项目已更改，但他们使用 FindText 重新运行 PostBack 上的语句，而无需先调用

myDropDown.ClearSelection();

根据 Corys 的回复，这意味着两个项目——一个刚刚选择的项目加上一个已经在视图状态中选择的项目——现在被选中，因此引发了一个异常。

不确定它是如何工作的，但直到对代码进行了一些小的更改后才公开。因此，如果有人根据 OP 的答案努力寻找重复的代码行，那么您可能还需要考虑在回发时查看状态机制。