error - Object reference not set to an instance of an object

Question

I am trying to create a ticket system, this part of the system will display all of the acts performing on each specific day. I am also trying to show how many tickets/seats are available. I have a database with all seat locations and a boolean value to show if they are taken or not.

 Public ds As New DataSet 'used to store the basic elements of the database
Public con As New OleDb.OleDbConnection 'used to connect to the database
Public provider As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source ="
Public datafile As String = "Resources/database.accdb" 'database location and version
Public da As OleDb.OleDbDataAdapter
Public sqlstatement As String
Public connString As String = provider & datafile
 lbxActs.Items.Clear()
    Dim oDataRowView As DataRowView
    Dim sSelectedAssetType As String

 ds.Clear()
    con.ConnectionString = connString
    con.Open()
    sqlstatement = "SELECT ShowDate FROM AvailableDates"
    da = New OleDb.OleDbDataAdapter(sqlstatement, con)
    da.Fill(ds, "Dates")

    lbxDates.DataSource = ds.Tables("Dates")
    lbxDates.DisplayMember = "ShowDate"
    lbxDates.ValueMember = "ShowDate"
    con.Close()

    oDataRowView = CType(Me.lbxDates.SelectedItem, DataRowView)
    sSelectedAssetType = oDataRowView("ShowDate").ToString
    lbxActs.Items.AddRange(IO.File.ReadAllLines("Resources/" & sSelectedAssetType & ".txt"))



    con.Close()
    ds.Clear()
    con.ConnectionString = connString
    con.Open()                                          'Open connection to the database

    sqlstatement = "SELECT * FROM [Seats" & sSelectedAssetType & "] WHERE [Available] = True "


    da = New OleDb.OleDbDataAdapter(sqlstatement, con)
    da.Fill(ds, "seats")                           'Fill the data adapter
    con.Close()
    Dim recordCount, x As Short
    recordCount = 0
    x = 0
    recordCount = ds.Tables("seats").Rows.Count

    tbxLeftS.Text = recordCount

the first part of the program puts all the show dates into a list box, then depending on which one is clicked it shows a different list of acts from my text files. The second part of the program is supposed to use an SQL statement to find all the seat locations which are available. Then it counts the records and then displays the number in a text box.

Object reference not set to an instance of an object

This error appears 4 times and it highlights this line:

sSelectedAssetType = oDataRowView("ShowDate").ToString

Lets just talk about what that error means? Imagine you shipped a box to your friend... but you forgot to put anything inside the box. He gets it, expects to see his merchandise and its empty. It was a steak you were going to send. He cant eat the steak because the box is empty. You are trying to set a value to an object that doesnt exist yet. — logixologist, May 12 '16 at 13:22
you need to check if its null or not `If Not is dbnull(object)` — logixologist, May 12 '16 at 13:25
I have put the "ShowDate" listbox in the form load, the rest is when the selectedvalue changes — chandler, May 12 '16 at 13:26
Also to note: you probably want to look at the use of SQL Parameters. Its dangerous to write the code the way you are. — logixologist, May 12 '16 at 13:27

score 0 · Answer 1 · answered May 12 '16 at 13:31

0

To get rid of the error, its very likely that somehow oDataRowView is null. You need to check to make sure its not null before you try to convert it to a string. Do something like this:

IF Not isdbnull(oDataRowView)
 oDataRowView = CType(Me.lbxDates.SelectedItem, DataRowView)
End if

As to why you are getting a null value for oDataRowView, that will require you to debug further. Perhaps it could be an issue with PostBack (assuming this is a web page and not winforms.

answered May 12 '16 at 13:31

logixologist

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I have replaced the code with yours, but the same error still comes up? – chandler May 12 '16 at 13:36
Have you used your debugger to check the values. – logixologist May 12 '16 at 13:54

error - Object reference not set to an instance of an object

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