Cannot have multiple items selected in a DropDownList but debugger steps through code perfectly

Question

The subject error appears when I refresh the page (databinding is done on page load). Stepping through the page's code reveals no errors and no page is referenced in the stack trace:

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

When last this happened, I had this image tag on the master page:

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

Modifying this to remove the <% %> (<img src="/Images/footerLogo.png" />) block fixed the error, but I don't have any such tags on the master page this time (or any page that links to the page in question). Here's my code:

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

As stated, this code executes perfectly, so I'm really at a loss as to what the problem is here.

I've tried Cleaning and Rebuilding the Solution, but it still hasn't fixed anything, so any insights are welcome!

Solution 1

Input from Cory made me decide to go back and check that every DropDownList had only 1 selection specified and that's when I found the problem.

In getReservation() above:

        ' 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

And then later:

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

This was just bad copy/paste, so for people viewing this answer, make 100% sure that you're only selecting ONCE

As a side note, I'm told that the following is better practice:

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

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

Solution 2

While any ListControl is being rendered, a flag is set if it encounters more than one selected item. If the flag is true, the abstract method VerifyMultiSelect is called. DropDownList's implementation of VerifyMultiSelect throws an exception if it encounters multiple selected items, so the only possible reason you're seeing this exception is because you have multiple ListItems with Selected set to true within one of your DropDownLists.

I would carefully examine the results of your SQL to determine if there's a case where more than one item in the same DropDownList is having its Selected property set to true. There are three lines of code in the getReservation() function in your sample above that alter this property.

Perhaps it's the way you handle IsPostBack. There's a previously selected item stored in ViewState for that control, and you're trying to select another value along with it after post-back.

EDIT: OP found a solution.

Solution 3

I had a similar issue when reworking someone elses old code. For me it was a view state issue that was addressed simply by calling

myDropDown.ClearSelection(); 

They were pulling the datasource for the dropdown from an enum without a key and thus using FindText to make a selection. If the dropdown selected item was changed however they re-ran the statement on PostBack with the FindText without first calling

myDropDown.ClearSelection();

which as per Corys reply meant two items - the one just selected plus the one already selected in viewstate - were now selected and an exception was therefore thrown.

Not sure how it ever worked but it wasn't exposed until some minor change was made to code. So if anyone is struggling to find duplicate lines of code as per OPs answer, then you might also want to consider view state mechanics on post back.

Comments

[Ortund]

I don't want multi-select otherwise I'd have used something other than a DropDownList. Does it look like that's what I'm going for? Because if yes, then that's the problem...

[Cᴏʀʏ]

@Ortund: Sorry, I misinterpreted your question.