Cannot get web app to work with MS Access databases

[ChemTech]

I have two web pages that perform statistical calculations using data from MS Access tables. In VS 2019 it works fine. I have not been able to make it work after I deploy the pages to the web. I have rewritten the code using numerous approaches, but cannot get it to work. My latest is very simple, display the tables on the web page using GridView then select values from the Gridview but I am getting a null reference that I cannot get rid of. I had edited the webconfig and on and on.

[techgnome]

Well, a null reference error means that some object, somewhere in a line of code you're using is NULL or Nothing. What exactly that could be is anyone's guess at this point. It could be an object, it could be a database field, it could be the sun, it could be the North Star. Thing is, we don't know, since you didn't supply the problem code. So I don't know what help we can be. All I can suggest at this point is to remove the eels from your hovercraft and try again.

-tg

[ChemTech]

Well, a null reference error means that some object, somewhere in a line of code you're using is NULL or Nothing. What exactly that could be is anyone's guess at this point. It could be an object, it could be a database field, it could be the sun, it could be the North Star. Thing is, we don't know, since you didn't supply the problem code. So I don't know what help we can be. All I can suggest at this point is to remove the eels from your hovercraft and try again.

-tg

Thanks for the initial start, the coding is as follows:

 Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
        Dim sLines1() As String, L1 As Integer
        sLines1 = Split(TextBox1.Text, vbCrLf)
        Dim sLines1check As String = sLines1(UBound(sLines1))
        Dim location1 As Integer = sLines1.Length - 1
        If sLines1check = "" Then
            Dim sLinesa(location1 - 1) As String
            Array.Copy(sLines1, 0, sLinesa, 0, location1)
            Dim Xinput11 = GetXinput1sLinesa(sLinesa)
            Dim xyCount1 As Integer = sLinesa.Length
            Dim Xinput22(xyCount1 - 1) As Single
            For L1 = 0 To UBound(Xinput11)
                Xinput22(L1) = Xinput11(L1)
            Next
            Session("Xinput1") = Xinput11
            Session("Xinput2") = Xinput22
            Session("xyCount") = xyCount1
        Else
            Dim Xinput11() As String = sLines1
            For L1 = 0 To UBound(sLines1)
                Xinput11(L1) = sLines1(L1)
            Next
            Dim xyCount1 As Integer = sLines1.Length
            Dim check As String = Xinput11(UBound(sLines1))
            Dim Xinput22(xyCount1 - 1) As Single
            For L1 = 0 To UBound(Xinput11)
                Xinput22(L1) = Xinput11(L1)
            Next
            Session("Xinput1") = Xinput11
            Session("Xinput2") = Xinput22
            Session("xyCount") = xyCount1
        End If
        Dim Xinput1() As String = Session("Xinput1")
        Dim Yinput1() As String = Session("Yinput1")
        Dim Xinput2() As Single = Session("Xinput2")
        Dim xyCount As Integer = Session("xyCount")
        'Dim numofdata As Integer = xyCount
        Dim df1 As Integer = xyCount - 2
        Dim SqlDataSource1 As New DataSet
        Dim SqlDataSource2 As New DataSet

#Disable Warning IDE0059 ' Unnecessary assignment of a value
        Dim conflevel As New Single
#Enable Warning IDE0059 ' Unnecessary assignment of a value

        conflevel = GridView2.SelectedRow.Cells(1).Text

        'Dim studentt As String
        Dim studentt As New Single '= GridView1.SelectedRow.Cells(6).Text
        If conflevel = "t095" Then
            studentt = GridView1.SelectedRow.Cells(6).Text
            'studentt = GridView1.SelectedRow.Cells(conflevel).Text
        End If

#Disable Warning BC42104 ' Variable is used before it has been assigned a value
        TextBox3.Text = studentt
#Enable Warning BC42104 ' Variable is used before it has been assigned a value
        'Dim MyCount As Single
        'MyCount = 0
        Dim avgx, addn, Dev, AddDev, AvgDev, Stdev1, Stdev2, AddDev1 As Single
        'Dim Nx As Integer

        'Nx = "0"
        Dim I As Integer
        For L1 = 0 To UBound(Xinput1)
#Disable Warning IDE0054 ' Use compound assignment
            addn = addn + Xinput1(L1)
#Enable Warning IDE0054 ' Use compound assignment
        Next L1
        avgx = addn / xyCount
        TextBox4.Text = Format(avgx, "0.00000")

        For I = 0 To UBound(Xinput1)
            If xyCount <> 0 Then
                Dev = Math.Abs(Xinput1(I) - avgx)
#Disable Warning IDE0054 ' Use compound assignment
                AddDev = AddDev + Dev
#Enable Warning IDE0054 ' Use compound assignment
            End If
        Next I

        If AddDev <> 0 Then
            AvgDev = AddDev / xyCount
            ' TextBox31.Text = AvgDev
        End If

        For I = 0 To UBound(Xinput1)
            If xyCount <> 0 Then
                Stdev1 = (Xinput1(I) - avgx) ^ 2
#Disable Warning IDE0054 ' Use compound assignment
                AddDev1 = AddDev1 + Stdev1
#Enable Warning IDE0054 ' Use compound assignment
            End If
        Next I

        If avgx <> 0 And xyCount > 1 Then
            Stdev2 = Math.Sqrt(AddDev1 / (xyCount - 1))
            TextBox5.Text = Format(Stdev2, "0.00000")
            TextBox6.Text = Format((studentt * Stdev2) / Math.Sqrt(xyCount), "0.00000")
        End If
        Dim conlim As Single = TextBox6.Text
        TextBox7.Text = xyCount
        TextBox8.Text = Format(TextBox4.Text - TextBox6.Text, "0.00000")
        TextBox9.Text = Format(TextBox4.Text + conlim, "0.00000")
    End Sub

    'Private Function GetConnectionString() As String
    'Return "Provider = Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\bmham\source\repos\WebApplication1\WebApplication1\App_Data\Studentst1.mdb"
    'End Function

    'Private Function GetConnectionString() As String
    'Return "Provider = Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\Studentst1.mdb"
    'End Function

    'Private Function GetConnectionString() As String
    'Return "Provider = Microsoft.ACE.OLEDB.12.0;Data Source=GridView1"
    'End Function

    'Protected Sub SqlDataSource2_Selecting(sender As Object, e As SqlDataSourceSelectingEventArgs)

    'End Sub

End Class

[wes4dbt]

Still, you didn't tell us which line of code is causing the Null reference. That's the place we need to start.

[ChemTech]

Still, you didn't tell us which line of code is causing the Null reference. That's the place we need to start.

In VS2019 the code runs fine with no errors. Its when I run the page on the web server. I get back the same null reference error with the same line of code at line 134. I change the coding around and it always says at line 134. It is after I publish the page to the web server and run the page. If it was in VS I could solve the problem usually. Here is a copy of the error page I get:


Server Error in '/WebApplication' Application.
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

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

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:


[NullReferenceException: Object reference not set to an instance of an object.]
WebApplication1.WebForm22.Button6_Click(Object sender, EventArgs e) in C:\Users\bmham\source\repos\WebApplication1\WebApplication1\Chap5-stat2.aspx.vb:134
System.Web.UI.WebControls.Button.OnClick(EventArgs e) +9780226
System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +204
System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +12
System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +15
System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +35
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1639

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.7.3701.0

[ChemTech]

Perhaps I should pose this question in another way. I have coded the web page to get data from a MS Access database file. I was having trouble getting the code after deploying to the web to get the data from the database tables. I found that the sql was not working. I also found that when I used GridView the web page was getting the data from the tables and displaying on the web page. So, I turned to trying to select the data from the GridViews and that is the code above. But now I get this null reference.

[jmcilhinney]

I get back the same null reference error with the same line of code at line 134.

If only there was a way that we could find out what line in the code you posted is that line 134. If there was someone who already knew that then they could tell us. Alas, I can't think of any such person.

Regardless, NullReferenceExceptions are actually quite easy to diagnose. There are generally not may distinct references used on any one line of code so you can fairly easily test each one to determine which is Nothing. You can add code to do that and log the results, so you can still get that information outside the debugger. Once you know which reference is the issue, you can look backwards through the code to see where it should have been assigned a value. You can then add further code to check whether or not that's happening. If it's not then you can try to work out how and why that might be happening. We can help with that, if we are provided with all the relevant information. The most likely explanation is that your Production database contains different data to your Debug/Test database and you are incorrectly assuming that something exists when it might not. It's impossible to be more specific without you providing specific information.