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Dropdown SelectedIndexChanged event doesn't know selected value when called from another function

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User673640389 posted

My form has an asp dropdownlist that is populated from a sqldatasource. It has an "selectedindexchanged" event that populates another dropdown list based on the value selected in the first dropdown. That works all fine and dandy.

However, in the page load event, I'm evaluating url parameters. The URL parameters populate various fields, including this dropdownlist. From the page load event, I call the selectedindexchanged event. But when that event is called from the page load event, the dropdown lists "selectedvalue" is null...even though the dropdown list appears to have been set correctly based on the URL parameter. So the end result, is that my 2nd dropdown list does not populate with the first one is populated by a URL parameter.

I used a "message box" to display the value of the first drop down in the selectedindexchange method, which is how I know the backend does not know the value of the dropdown when it's populated by the URL parameter and run from the page load event.

Code (with some things masked out):

Protected Sub Exception_dl_SelectedIndexChanged(sender As Object, e As EventArgs)

Dim sqldatasource_reason As New SqlDataSource
reason_dl.DataSource = sqldatasource_reason
reason_dl.DataTextField = "description"
reason_dl.DataValueField = "ID"

sqldatasource_reason.ConnectionString = connectionstring_transbsddb

sqldatasource_reason.SelectParameters.Add("exceptionid", exception_dl.SelectedValue)
sqldatasource_reason.SelectCommand = "Select ID, Description FROM ***** where id In (Select reasonid from **** where Active='Y' and exceptionid = @exceptionid) "

reason_dl.DataBind()
end sub

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

empid = Request.QueryString("empid")
If empid <> "" Then
empid_txt.Text = empid
End If

exceptiontype = Request.QueryString("extype")
If exceptiontype >= 1 Then
exception_dl.SelectedIndex = exceptiontype
End If

exceptiondate = Request.QueryString("exdate")
If exceptiondate <> "" Then
exceptiondate_txt.Value = exceptiondate
End If

Exception_dl_SelectedIndexChanged(New Object(), New EventArgs())

End Sub

Thursday, December 20, 2018 9:05 PM

Answers

User-1174608757 posted

Hi AaronR_

According to your description,I have made a sample here. I guess that you bind exception_dl in aspx such as using sqldatasource control but not in code behind .The key to your issue is that the bind event of exception_dl happened after page_load event
If you just bind exception_dl in aspx ,the reason_dl couldn't directly get the selected value of exception_dl in code behind when first load the page.
Similarly, the selected index of exception_dl shows -1 means the value is null.It can't get the value.since it hasn't bound the data in page_load.

Here are two demos about the situations.I hope it can help you.

(Bind in aspx)dropdownlist.aspx:

<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            <asp:DropDownList ID="D1" runat="server" OnSelectedIndexChanged="D1_SelectedIndexChanged"
                AutoPostBack="True" DataSourceID="SqlDataSource1" DataTextField="fruit" DataValueField="fruit" >
                              
            </asp:DropDownList>
            <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:mssqlserver %>" SelectCommand="SELECT [fruit] FROM [db]"></asp:SqlDataSource>
            <asp:DropDownList ID="D2" runat="server"></asp:DropDownList>
        </div>
    </form>
</body>

dropdownlist.aspx.vb:

Public Class dropdownlist
    Inherits System.Web.UI.Page
    Public Shared Function ExecuteDataTable(ByVal sql As String, ParamArray pms As SqlParameter()) As DataTable
        Dim conStr As String = ConfigurationManager.ConnectionStrings("mssqlserver").ConnectionString
        Dim dt As DataTable = New DataTable()

        Using adapter As SqlDataAdapter = New SqlDataAdapter(sql, conStr)

            If pms IsNot Nothing Then
                adapter.SelectCommand.Parameters.AddRange(pms)
            End If
            adapter.Fill(dt)
        End Using

        Return dt
    End Function

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not IsPostBack Then
            D1.SelectedIndex = 1
            Response.Write(D1.SelectedIndex)
            D1_SelectedIndexChanged(New Object, New EventArgs)
        End If
    End Sub
    Protected Sub D1_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
        Dim s As String = D1.SelectedValue

        Dim sql As String = "select color from db where fruit=@fruit"
        D2.DataSource = ExecuteDataTable(sql, New SqlParameter("@fruit", s))
        D2.DataValueField = "color"
        D2.DataBind()
    End Sub

End Class

It shows as below:

(Bind in code behind)dropdownlist.aspx:

<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            <asp:DropDownList ID="D1" runat="server" OnSelectedIndexChanged="D1_SelectedIndexChanged" AutoPostBack="true"></asp:DropDownList>
            <asp:DropDownList ID="D2" runat="server"></asp:DropDownList>
        </div>
    </form>
</body>

aspx.vb:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not IsPostBack Then
            Dim sql As String = "select fruit from db "
            D1.DataSource = ExecuteDataTable(sql)
            D1.DataValueField = "fruit"
            D1.DataBind()
            D1.SelectedIndex = 1
            Response.Write(D1.SelectedIndex)
            D1_SelectedIndexChanged(New Object, New EventArgs)
        End If
    End Sub
    Protected Sub D1_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
        Dim s As String = D1.SelectedValue
        Dim sql As String = "select color from db where fruit=@fruit"
        D2.DataSource = ExecuteDataTable(sql, New SqlParameter("@fruit", s))
        D2.DataValueField = "color"
        D2.DataBind()
    End Sub

It shows:

Best Regards

Wei Zhang

Marked as answer by Anonymous Thursday, October 7, 2021 12:00 AM

Friday, December 21, 2018 8:09 AM

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User673640389 posted

I think I"m running into some kind of order of operations issue.

In the page load event, I set the selected index value of the drop down to 1, and the dropdown shows the correct associated text value. Then I put a MessageBox.Show(exception_dl.SelectedIndex) after that, and it say it reports -1.

When I run this code, the message box actually appears first showing a value of -1, then the dropdown list index gets set to 1. So the bolded line is setting the selected index value to 1, and the next line which should be showing the newly set index of 1 is instead showing -1. Which would explain why the code that is supposed to set the 2nd box doesn't work.

If exceptiontype >= 1 Then
exception_dl.SelectedIndex = exceptiontype <----sets value to 1
MessageBox.Show(exception_dl.SelectedIndex) <----shows -1

End if

Thursday, December 20, 2018 10:36 PM