Object reference not set to an instance of an object...

**wrack** · Jun 4th, 2003, 03:58 AM

Hi,

I have following code to make a shopping cart and when I run it I get an error saying "Object reference not set to an instance of an object" and the error is in this line,

VB Code:

"Line 51:         For Each objDR In objDT.Rows"

Its the line in SUB AddToCart...

VB Code:

Private objDT As System.Data.DataTable
    Private objDR As System.Data.DataRow
 
    Private Sub Page_Load(ByVal s As Object, ByVal e As EventArgs)
        If Not IsPostBack Then
            makeCart()
        End If
    End Sub
 
    Function makeCart()
        objDT = New System.Data.DataTable("Cart")
        objDT.Columns.Add("ID", GetType(Integer))
        objDT.Columns("ID").AutoIncrement = True
        objDT.Columns("ID").AutoIncrementSeed = 1
 
        objDT.Columns.Add("Quantity", GetType(Integer))
        objDT.Columns.Add("Product", GetType(String))
        objDT.Columns.Add("Cost", GetType(Decimal))
 
        Session("Cart") = objDT
    End Function
 
    Sub AddToCart(ByVal s As Object, ByVal e As EventArgs)
        objDT = Session("Cart")
        Dim Product = ddlProducts.SelectedItem.Text
        Dim blnMatch As Boolean = False
 
        For Each objDR In objDT.Rows
            If objDR("Product") = Product Then
                objDR("Quantity") += txtQuantity.Text
                blnMatch = True
                Exit For
            End If
        Next
 
        If Not blnMatch Then
            objDR = objDT.NewRow
            objDR("Quantity") = txtQuantity.Text
            objDR("Product") = ddlProducts.SelectedItem.Text
            objDR("Cost") = Decimal.Parse(ddlProducts.SelectedItem.Value)
            objDT.Rows.Add(objDR)
        End If
        Session("Cart") = objDT
 
        dg.DataSource = objDT
        dg.DataBind()
 
        lblTotal.Text = "$" & GetItemTotal()
    End Sub
 
    Function GetItemTotal() As Decimal
        Dim intCounter As Integer
        Dim decRunningTotal As Decimal
 
        For intCounter = 0 To objDT.Rows.Count - 1
            objDR = objDT.Rows(intCounter)
            decRunningTotal += (objDR("Cost") * objDR("Quantity"))
        Next
 
        Return decRunningTotal
    End Function
 
    Sub Delete_Item(ByVal s As Object, ByVal e As DataGridCommandEventArgs)
        objDT = Session("Cart")
        objDT.Rows(e.Item.ItemIndex).Delete()
        Session("Cart") = objDT
 
        dg.DataSource = objDT
        dg.DataBind()
 
        lblTotal.Text = "$" & GetItemTotal()
    End Sub

Can anyone tell me how to create an object of this type...???

Cheers...

**Eras3r** · Jun 4th, 2003, 03:58 PM

Try this instead.....

Code:

Private objDT As New System.Data.DataTable
Private objDR As New System.Data.DataRow

if u dont use the above, then u will need these in your pageload...

Code:

Set objDT = New DataTable()
Set objDR = New DataRow()

**wrack** · Jun 4th, 2003, 05:56 PM

Originally posted by Eras3r
Try this instead.....

Code:

Private objDT As New System.Data.DataTable Private objDR As New System.Data.DataRow

if u dont use the above, then u will need these in your pageload...

Code:

Set objDT = New DataTable() Set objDR = New DataRow()

Thanks for the help but I am getting this error...

Overload resolution falied becasue no "New" is accessible.

I should have say that I am using VB.NET and framework 1.0.

Any idea what it can be...???

Cheers...

**Edneeis** · Jun 4th, 2003, 06:06 PM

You can't create a datatable in that way, it has required parameters in the constructor. I believe the same for the Datarow.

**wrack** · Jun 4th, 2003, 08:45 PM

Originally posted by Edneeis
You can't create a datatable in that way, it has required parameters in the constructor. I believe the same for the Datarow.

I am already defining DataTable as follows...

VB Code:

objDT = New System.Data.DataTable("Cart")

But Can't find proper way to create object for datarow...

Cheers...

**fungi** · Jun 5th, 2003, 08:46 AM

wrack, what you're doing looks legit to me.

Use the NewRow method to add rows to the table.

I don't think the error you are getting is related to objDR as the "For Each" should not error even if there are no rows.

I don't think you are referencing an instance of objDT at the time you get the error. Test it for Nothing to verify this theory.

How you're storing and retreiving it from a Session variable also looks good, but make sure you have session state persistance turned on...

**wrack** · Jun 5th, 2003, 10:33 AM

Originally posted by fungi
wrack, what you're doing looks legit to me.

Use the NewRow method to add rows to the table.

I don't think the error you are getting is related to objDR as the "For Each" should not error even if there are no rows.

I don't think you are referencing an instance of objDT at the time you get the error. Test it for Nothing to verify this theory.

How you're storing and retreiving it from a Session variable also looks good, but make sure you have session state persistance turned on...

Can you please tell me how exactly you are intended to check this...I am confused...

Cheers..

**fungi** · Jun 5th, 2003, 12:47 PM

Here's a quick prog I whipped up that uses most of your code. Let me know how it runs..

CODE BEHIND

Code:

Public Class WebForm1
    Inherits System.Web.UI.Page
    Protected WithEvents txtQty As System.Web.UI.WebControls.TextBox
    Protected WithEvents txtProduct As System.Web.UI.WebControls.TextBox
    Protected WithEvents txtCost As System.Web.UI.WebControls.TextBox
    Protected WithEvents cmdAdd As System.Web.UI.WebControls.Button
    Private objDR As System.Data.DataRow
    Private objDT As System.Data.DataTable


#Region " Web Form Designer Generated Code "

    'This call is required by the Web Form Designer.
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()

    End Sub

    Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
        'CODEGEN: This method call is required by the Web Form Designer
        'Do not modify it using the code editor.
        InitializeComponent()
    End Sub

#End Region

    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        If Not IsPostBack Then
            makeCart()
        Else
            objDT = Session("Cart")
        End If

    End Sub

    Function showcart()

        If objDT Is Nothing Then
            Response.Write("objDT is Nothing")
            Exit Function
        End If

        Dim szLine As String
        Response.Write(objDT.Rows.Count.ToString & " rows<BR>")
        For Each objDR In objDT.Rows
            szLine = "[Id] " & objDR("id") & "  "
            szLine += "[Qty] " & objDR("quantity") & "  "
            szLine += "[Prod] " & objDR("product") & "  "
            szLine += "[Qty] " & objDR("cost") & "<BR>"
            Response.Write(szLine)
        Next

    End Function

    Function makeCart()
        objDT = New System.Data.DataTable("Cart")
        objDT.Columns.Add("ID", GetType(Integer))
        objDT.Columns("ID").AutoIncrement = True
        objDT.Columns("ID").AutoIncrementSeed = 1

        objDT.Columns.Add("Quantity", GetType(Integer))
        objDT.Columns.Add("Product", GetType(String))
        objDT.Columns.Add("Cost", GetType(Decimal))

        Session("Cart") = objDT
    End Function

    Private Sub cmdAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdAdd.Click

        objDR = objDT.NewRow()
        objDR("Quantity") = CInt(txtQty.Text)
        objDR("Product") = txtProduct.Text
        objDR("Cost") = CDec(txtCost.Text)

        objDT.Rows.Add(objDR)

        Session("Cart") = objDT

        showcart()


    End Sub

End Class

ASPX

Code:

<%@ Page Language="vb" AutoEventWireup="false" Codebehind="WebForm1.aspx.vb" Inherits="test4.WebForm1"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
	<HEAD>
		<title>WebForm1</title>
		<meta name="GENERATOR" content="Microsoft Visual Studio.NET 7.0">
		<meta name="CODE_LANGUAGE" content="Visual Basic 7.0">
		<meta name="vs_defaultClientScript" content="JavaScript">
		<meta name="vs_targetSchema" content="http://schemas.microsoft.com/intellisense/ie5">
	</HEAD>
	<body>
		<form id="Form1" method="post" runat="server">
			<P>
				<asp:TextBox id="txtQty" runat="server" Width="65px">1</asp:TextBox>&nbsp;&nbsp;
				<asp:TextBox id="txtProduct" runat="server" Width="68px">Prod1</asp:TextBox>&nbsp;&nbsp;
				<asp:TextBox id="txtCost" runat="server" Width="69px">1.00</asp:TextBox>&nbsp;&nbsp;
				<asp:Button id="cmdAdd" runat="server" Text="Add"></asp:Button></P>
			<P>&nbsp;</P>
		</form>
	</body>
</HTML>

**wrack** · Jun 5th, 2003, 08:06 PM

I am getting this error...

**fungi** · Jun 6th, 2003, 07:51 AM

I'm afraid I haven't got a clue what your error is

Are you having success running other ASP.NET apps?

**wrack** · Jun 6th, 2003, 09:29 AM

Originally posted by fungi
I'm afraid I haven't got a clue what your error is

Are you having success running other ASP.NET apps?

I figured out that I can't run any ASP.NET app...As a matter of fact I can't even uninstall VS.NET or install it again...

Give me sometime and I will check it out...

Cheers...

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