Problem with a gridview, paging and "object reference not set" error

Question

I'm stuck with the following problem. I'm trying to implement a basic GridView paged result set, which connects to an Oracle database. By itself, the GridView, and the paged results, work fine. The problem comes when I try to put it in page layout class that we have at work.

We have ClassA, which inherits from Page, and is a corporate standard. Then I have ClassB, which inherits from ClassA and which includes application-specific code. The page that the GridView is on inherits from ClassB. This all seems to work fine in other pages, and I don't think it's the source of the problem, but I thought I'd mention it.

What happens is that the first time the page with the GridView loads, everything looks normal. The query runs and the first 10 records are displayed, with the numbers for paging below. When I click on "2" or any of the other pages, I get the "yellow screen of death" with the following message: "Object reference not set to an instance of an object". The object being referred to in that error line is "Me", the Page object (ASP.pagename_aspx in the debugger). I don't believe that the exact line it fails on is that important, because I've switched the order of a few statements around and it just fails on the earliest one.

I've traced through with the debugger and it looks normal, only that on Page 1 it works fine, and Page 2 it fails.

I have implemented the PageIndexChanging event (again, it works by itself if I remove inheritance from ClassB. Also, if I try inheriting directly from ClassA (bypassing ClassB entirely), I still get the problem.

Any ideas? Thanks.

Answer 1

I ran into a similar situation where the base (ClassA in your example) had variables that were set up to handle all the paging and sorting bits, and the GridView was wired up to events that used those variables. Not setting the proper base class variables in my page caused the exact same sort of error.

Answer 2

When I've had similar problems in the past, it has usually been a databinding problem (not calling DataBind() at the right time so when it tries to look at the next page the DataSource is null).

Answer 3

I agree with @DotNetDaddy in that you need to make sure you set the datasource on post-back, as this is almost certainly the reason for the "fun" yellow screen of death. The below is a very simple example that shows sorting and paging for a GridView in .NET 2.0+

The below is the exact markup required for this gridview to work correctly w/ my vb code

<asp:GridView ID="gridSuppliers" EnableViewState="false" runat="server" OnPageIndexChanging="gridSuppliers_PageIndexChanging" AutoGenerateColumns="false" AllowPaging="true" AllowSorting="true" CssClass="datatable" CellPadding="0" CellSpacing="0" BorderWidth="0" GridLines="None">...</asp:GridView>

Next is the code-behind file w/ the required sorting/paging implementation for a collection based databind

Partial Public Class _Default
    Inherits System.Web.UI.Page
    Implements ISupplierView

    Private presenter As SupplierPresenter

    Protected Overrides Sub OnInit(ByVal e As System.EventArgs)
        MyBase.OnInit(e)
        presenter = New SupplierPresenter(Me)
    End Sub

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

    Protected Sub gridSuppliers_PageIndexChanging(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewPageEventArgs) Handles gridSuppliers.PageIndexChanging
        gridSuppliers.PageIndex = e.NewPageIndex
        presenter.PopulateSupplierList()
    End Sub

    Private Sub gridSuppliers_Sorting(ByVal sender As Object, ByVal e As GridViewSortEventArgs) Handles gridSuppliers.Sorting
        If DirectCast(ViewState("PreviousSortExpression"), String) = e.SortExpression Then
            If DirectCast(ViewState("PreviousSortDirection"), String) = "Ascending" Then
                e.SortDirection = System.Web.UI.WebControls.SortDirection.Descending
                ViewState("PreviousSortDirection") = "Descending"
            Else
                e.SortDirection = System.Web.UI.WebControls.SortDirection.Ascending
                ViewState("PreviousSortDirection") = "Ascending"
            End If
        Else
            e.SortDirection = System.Web.UI.WebControls.SortDirection.Ascending
            ViewState("PreviousSortDirection") = "Ascending"
        End If
        ViewState("PreviousSortExpression") = e.SortExpression

        Dim gv As GridView = DirectCast(sender, GridView)
        If e.SortExpression.Length > 0 Then
            For Each field As DataControlField In gv.Columns
                If field.SortExpression = e.SortExpression Then
                    ViewState("PreviousHeaderIndex") = gv.Columns.IndexOf(field)
                    Exit For
                End If
            Next
        End If
        presenter.PopulateSupplierList()
    End Sub

#Region "ISupplierView Properties"
    Private ReadOnly Property PageIsPostBack() As Boolean Implements ISupplierView.PageIsPostBack
        Get
            Return Page.IsPostBack
        End Get
    End Property

    Private ReadOnly Property SortExpression() As String Implements ISupplierView.SortExpression
        Get
            If ViewState("PreviousSortExpression") Is Nothing Then
                ViewState("PreviousSortExpression") = "CompanyName"
            End If
            Return DirectCast(ViewState("PreviousSortExpression"), String)
        End Get
    End Property

    Public ReadOnly Property SortDirection() As String Implements Library.ISupplierView.SortDirection
        Get
            If ViewState("PreviousSortDirection") Is Nothing Then
                ViewState("PreviousSortDirection") = "Ascending"
            End If
            Return DirectCast(ViewState("PreviousSortDirection"), String)
        End Get
    End Property

    Public Property Suppliers() As System.Collections.Generic.List(Of Library.Supplier) Implements Library.ISupplierView.Suppliers
        Get
            Return DirectCast(gridSuppliers.DataSource(), List(Of Supplier))
        End Get
        Set(ByVal value As System.Collections.Generic.List(Of Library.Supplier))
            gridSuppliers.DataSource = value
            gridSuppliers.DataBind()
        End Set
    End Property
#End Region

End Class

And finally, the presenter class used in the code-behind

Public Class SupplierPresenter
    Private mView As ISupplierView
    Private mSupplierService As ISupplierService

    Public Sub New(ByVal View As ISupplierView)
        Me.New(View, New SupplierService())
    End Sub

    Public Sub New(ByVal View As ISupplierView, ByVal SupplierService As ISupplierService)
        mView = View
        mSupplierService = SupplierService
    End Sub

    Public Sub OnViewLoad()
        If mView.PageIsPostBack = False Then
            PopulateSupplierList()
        End If
    End Sub

    Public Sub PopulateSupplierList()
        Try
            Dim SupplierList As List(Of Supplier) = mSupplierService.GetSuppliers()
            SupplierList.Sort(New GenericComparer(Of Supplier)(mView.SortExpression, mView.SortDirection))
            mView.Suppliers = SupplierList
        Catch ex As Exception
            Throw ex
        End Try
    End Sub
End Class

**the class required to sort a generic collection that is referenced in the presenter

Imports System.Reflection
Imports System.Web.UI.WebControls

Public Class GenericComparer(Of T)
    Implements IComparer(Of T)

    Private mDirection As String
    Private mExpression As String

    Public Sub New(ByVal Expression As String, ByVal Direction As String)
        mExpression = Expression
        mDirection = Direction
    End Sub

    Public Function Compare(ByVal x As T, ByVal y As T) As Integer Implements System.Collections.Generic.IComparer(Of T).Compare
        Dim propertyInfo As PropertyInfo = GetType(T).GetProperty(mExpression)
        Dim obj1 As IComparable = DirectCast(propertyInfo.GetValue(x, Nothing), IComparable)
        Dim obj2 As IComparable = DirectCast(propertyInfo.GetValue(y, Nothing), IComparable)
        If mDirection = "Ascending" Then
            Return obj1.CompareTo(obj2)
        Else
            Return obj2.CompareTo(obj1)
        End If
    End Function
End Class

Answer 4

I lost my original unregistered login which I used to post this question.

Anyway, Harper Shelby's answer turned out to be correct. There was an unset variable in that base class (a custom object that is our corporate standard) that caused the problem (and no helpful error message).

If an admin, or someone with the right powers sees this, you could mark Harper's answer as the one, and close this. Thanks to everyone for their help.