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I created a login project in visual studio using VB.NET and MS Access as database. When I run the project for login it gives me the following error below:

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

What I have tried:

Imports System.Data.OleDb
Public Class Login
    Dim conn As New OleDb.OleDbConnection
    Dim dbProvider As String = "PROVIDER=Microsoft.Jet.OLEDB.4.0;"
    Dim dbSource As String = "Data Source = C:\Users\Sam\Documents\User Accounts.mdb"
    Dim adapter As OleDbDataAdapter
    Dim ds As DataSet
    Private Sub LinkLabel1_LinkClicked(sender As Object, e As LinkLabelLinkClickedEventArgs) Handles LinkLabel1.LinkClicked
        Register.ShowDialog()
    End Sub

    Private Sub LinkLabel2_LinkClicked(sender As Object, e As LinkLabelLinkClickedEventArgs) Handles LinkLabel2.LinkClicked
        Forgot_Pssowrd.ShowDialog()
    End Sub

    Private Sub Loginbtn_Click(sender As Object, e As EventArgs) Handles Loginbtn.Click
        ds = New DataSet
        adapter = New OleDbDataAdapter("insert into [tbl_Users] ([Username],[Password])VALUES" &
                                       "('" & txtUser.Text & "','" & txtPassWrd.Text & "')", conn)
        adapter.Fill(ds, "tbl_Users")

        If ds.Tables("tbl_Users").Rows.Count > 0 Then
            MsgBox("Login Sucess")
            txtUser.Clear()
            txtPassWrd.Clear()

        Else
            MsgBox("Invalid credentials")
        End If

    End Sub

    Private Sub Login_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        conn.ConnectionString = dbProvider & dbSource
        txtUser.Select()
    End Sub
End Class
Posted
Updated 21-Jun-22 0:03am

That's the least of your problems ... so little code, so many problems.

Let's start with the one you have noticed, because it's one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

But then we get to the serious ones: Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:
SQL
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SQL
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'
Which SQL sees as three separate commands:
SQL
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
SQL
DROP TABLE MyTable;
A perfectly valid "delete the table" command
SQL
--'
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

And in a login system? That's just asking for trouble as I don't even have to have to register with you to do it!

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

And the other serious problem: Never store passwords in clear text - it is a major security risk. There is some information on how to do it here: Password Storage: How to do it.[^]

And remember: if you have any European Union users then GDPR applies and that means you need to handle passwords as sensitive data and store them in a safe and secure manner. Text is neither of those and the fines can be .... um ... outstanding. In December 2018 a German company received a relatively low fine of €20,000 for just that.
   
Look at the code:
VB
  1  Dim conn As New OleDb.OleDbConnection
  2  Dim dbProvider As String = "PROVIDER=Microsoft.Jet.OLEDB.4.0;"
  3  Dim dbSource As String = "Data Source = C:\Users\Sam\Documents\User Accounts.mdb"
  4  Dim adapter As OleDbDataAdapter
  5  Dim ds As DataSet
  6  Private Sub LinkLabel1_LinkClicked(sender As Object, e As LinkLabelLinkClickedEventArgs) Handles LinkLabel1.LinkClicked
  7      Register.ShowDialog()
  8  End Sub
  9  
 10  Private Sub LinkLabel2_LinkClicked(sender As Object, e As LinkLabelLinkClickedEventArgs) Handles LinkLabel2.LinkClicked
 11      Forgot_Pssowrd.ShowDialog()
 12  End Sub
 13  
 14  Private Sub Loginbtn_Click(sender As Object, e As EventArgs) Handles Loginbtn.Click
 15      ds = New DataSet
 16      adapter = New OleDbDataAdapter("insert into [tbl_Users] ([Username],[Password])VALUES" &
 17                                     "('" & txtUser.Text & "','" & txtPassWrd.Text & "')", conn)

At line 1 you create a new OleDb.OleDbConnection, but it has not been initialised to point to any actual object; i.e. it is a NULL reference. At line 17 you try to use that connection reference to insert some data into your database. But since it is still not initialised you get the ""Object reference not set to an instance of an object"" error. So that is an easy one to fix.

The more important corrections are as explained by OriginalGriff in Solution 1.
   

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