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Thread: Problem getting selected value from DropDownList using ArrayList as DataSource

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  1. #1
    Join Date
    May 2003
    Posts
    599

    Question Problem getting selected value from DropDownList using ArrayList as DataSource

    I'm having a problem getting the selected value from a DropDownList that uses an ArrayList as a DataSource. When I select a year and click on the submit button, I get the following error message:

    Object reference not set to an instance of an object.

    Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

    I'm not understanding why this isn't working. The years (1900 - 1993) are getting populated into the DropDownList without a problem, but on the btnSubmit_Click job gets submitted, I get that error. And and all help would be appreciated. Hopefully it's something simple that I'm missing.

    Here's the code I'm using:

    VB.NET CODE-BEHIND:
    Code:
    'Assign date variables
        Dim dtCurrDate As DateTime = DateTime.Now 'Assign current date
        Dim intCurrYear As Integer = DateTime.Now.Year 'Assign current year
        Dim intVotingYear As Integer = intCurrYear - 18 'Assign voting year
    
    Sub Page_Load(ByVal Sender As Object, ByVal E As EventArgs) Handles Me.Load
    
        If Not Page.IsPostBack Then 'loads with page
    
                'Declare an array for the dynamic dropdownlist
                Dim colArrayList As New System.Collections.ArrayList()
    
                'Declare loop variables
                Dim i As Integer = 1900
                Dim intDisplayYear As Integer = i
    
                'Loop through years
                For i = 1900 To intVotingYear
                    colArrayList.Add(i)
                    colArrayList.TrimToSize()
                    colArrayList.Sort() 'Sort years
                    colArrayList.Reverse() 'Reverse sort/descending order
                Next
    
                'Bind data
                ddlYear.DataSource = colArrayList
                ddlYear.DataBind()
    
                'Add a new listitem to the beginning of the dropdownlist
                ddlYear.Items.Insert(0, New ListItem("- Year -", "")) 'Text, value
                ddlYear.SelectedIndex = ddlYear.Items.IndexOf(ddlYear.Items.FindByText(Session("SelectedYear")))
    
            End If
    
    End Sub
    
    Sub btnSubmit_Click(ByVal Sender As Object, ByVal E As EventArgs)
    
        'Declare form variables
        Dim strFormYear As String = ddlYear.SelectedItem.Text '<<<<-----THIS IS WHERE I GET THE ERROR
    
        'NOTE: I've also tried changing the above line to:
        'Dim strFormYear As String = ddlYear.SelectedItem.Value
        '...but that resulted in the same error.
    
        'Declare session variables
        Session("SelectedYear") = dob_year_form
    
        If Page.IsValid Then
            Try...
            ...
        End If
    
    End Sub
    ASP.NET PAGE:
    Code:
    <form id="form1" method="post" runat="server">
    
        <asp:DropDownList id="ddlYear" runat="server"></asp:DropDownList>
    
        <asp:Button ID="btnSubmit" Text="Submit" OnClick="btnSubmit_Click" runat="server" />
    
    </form>
    KWilliams
    -----------------------
    It's the end of the world as we know it...and I feel fine

  2. #2
    Join Date
    May 2003
    Posts
    599

    resolved

    I thought that posting my solution might help others, so here it goes:

    After getting some advice from another forum user, I was able to solve the problem by taking a step back. I did that by using a simple example of the ArrayList at W3Schools at http://www.w3schools.com/aspnet/aspnet_arraylist.asp to compare it to my code. The issue ended up being in the page declaration. I had EnableViewState="false", and I changed it to EnableViewState="true". Once I did that, and I placed the initial code within an "If Not Page.IsPostBack Then...", it worked as it should.
    KWilliams
    -----------------------
    It's the end of the world as we know it...and I feel fine

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