Question

I am attempting to sort a gridview in an updatepanel. It compiles, but I get an Object reverence not set to an instance of an object error. I am trying to follow this guide, but in vb

default.aspx

 <ajx:UpdatePanel ID="ajaxpanel" runat="server">   
    <ContentTemplate>         
                <asp:GridView ID="gvProgramDetails" runat="server" AutoGenerateColumns="False" 
                    CssClass="gridview" DataKeyNames="ProgramNumber" 
             AllowPaging="True" PageSize="2" AllowSorting="True" OnSorting="gvProgramDetails_Sorting">
                    <Columns>
                        <asp:CommandField ControlStyle-CssClass="button delete" ShowDeleteButton="True">
                            <ControlStyle CssClass="button delete" />
                        </asp:CommandField>
                        <asp:CommandField ControlStyle-CssClass="button save" ShowEditButton="True">
                            <ControlStyle CssClass="button save" />
                        </asp:CommandField>
                        <asp:BoundField DataField="ProgramNumber" HeaderText="ProgramNumber" 
                            InsertVisible="False" ReadOnly="True" SortExpression="ProgramNumber" />
                        <asp:BoundField DataField="ProgramName" HeaderText="ProgramName" 
                            SortExpression="ProgramName" />
                        <asp:BoundField DataField="ProgramStatus" HeaderText="ProgramStatus" 
                            SortExpression="ProgramStatus" />
                        <asp:BoundField DataField="RecordType" HeaderText="RecordType" 
                            SortExpression="RecordType" />
                        <asp:BoundField DataField="ProgramInformation" HeaderText="ProgramInformation" 
                            SortExpression="ProgramInformation" />
                    </Columns>
                </asp:GridView>
                </ContentTemplate>

default.aspx.vb

Imports System.Web.Services
Partial Class processes_ProgramTrack_Default
Public Property SortOrder() As String
        Get
            If (ViewState("SortOrder").ToString = "desc") Then
                ViewState("SortOrder") = "asc;"
            Else
                ViewState("SortOrder") = "desc"
            End If
            Return ViewState("SortOrder").ToString()
        End Get
        Set(ByVal value As String)
            ViewState("SortOrder") = value
        End Set
    End Property
    Public Sub bindGridView(Optional ByVal sortExp As String = "", Optional ByVal sortDir As String = "")
        Dim connstr As String = ConfigurationManager.ConnectionStrings("WEBConnectionString").ConnectionString
        Dim conn As New SqlConnection(connstr)
        If conn.State = ConnectionState.Closed Then
            conn.Open()
        End If
        Dim myDataView As New DataView()
        Dim mysqlCommand As New SqlCommand("SELECT tblPrgTrackPrograms.ProgramNumber, tblPrgTrackPrograms.ProgramName, tblPrgTrackPrograms.ProgramStatus, tblPrgTrackProgramDocumentation.RecordType, tblPrgTrackProgramDocumentation.ProgramInformation FROM tblPrgTrackPrograms INNER JOIN tblPrgTrackProgramDocumentation ON tblPrgTrackPrograms.ProgramNumber = tblPrgTrackProgramDocumentation.ProgramNumber")
        Dim myDataSet As New DataSet()
        Dim mySQLAdapter As New SqlDataAdapter(mysqlCommand)
        mySQLAdapter.SelectCommand.Connection = conn

        mySQLAdapter.Fill(myDataSet)
        myDataView = myDataSet.Tables(0).DefaultView
        If sortExp <> String.Empty Then

            myDataView.Sort = String.Format("{0}{1}", sortExp, sortDir)
        End If
        gvProgramDetails.DataSource = myDataView
        gvProgramDetails.DataBind()
    End Sub

    Protected Sub gvProgramDetails_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles gvProgramDetails.Load
        bindGridView()
    End Sub
    Protected Sub gvProgramDetails_PageIndexChanging(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewPageEventArgs) Handles gvProgramDetails.PageIndexChanging
        gvProgramDetails.PageIndex = e.NewPageIndex
        bindGridView()
    End Sub

    Protected Sub gvProgramDetails_RowDeleting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewDeleteEventArgs) Handles gvProgramDetails.RowDeleting

    End Sub

    Protected Sub gvProgramDetails_Sorting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewSortEventArgs) Handles gvProgramDetails.Sorting
        SortOrder = ViewState("SortOrder").ToString
        bindGridView(e.SortExpression, SortOrder)
    End Sub

End Class

I get the error message as a popup msgbox. What I think is happening, SortOrder isn’t getting assigned a value.

Answer 1

You need to make sure that you ViewState("SortOrder") has a value in it before you try to call ToString. Otherwise, it will throw that exception (as you’ve seen) because you cannot call the ToString method on null.

Essentially, you need to wrap your references to that ViewState variable in a “If (ViewState("SortOrder") IsNot Nothing Then” block, and possibly provide special handling in the “Else” section (if you need that).

Something like this for your property:

Public Property SortOrder() As String
    Get
        If (ViewState("SortOrder") IsNot Nothing Then
            If (ViewState("SortOrder").ToString = "desc") Then
                ViewState("SortOrder") = "asc;"
            Else
                ViewState("SortOrder") = "desc"
            End If
        End If
        Return ViewState("SortOrder").ToString()
    End Get
    Set(ByVal value As String)
        ViewState("SortOrder") = value
    End Set
End Property

And then the same thing for your sorting event:

Protected Sub gvProgramDetails_Sorting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewSortEventArgs) Handles gvProgramDetails.Sorting
    If (ViewState("SortOrder") IsNot Nothing Then
        SortOrder = ViewState("SortOrder").ToString
        bindGridView(e.SortExpression, SortOrder)
    End If
End Sub

Note: This is my best guess at your problem, given the information you’ve provided so far. If you provide the line that’s throwing the error, I’d be glad to take another look.