VB2010 & MSACCESS | Object reference not set to an instance of an object - vb.net

Question

i'm making a login form and every time i try to login(whether with a correct or not user) it gives me the same error.
"Object reference not set to an instance of an object"
Here's the code:
Public Class login
Dim conn As New OleDb.OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0; Data Source=C:\Users\Fausto\Documents\GitHub\CRE\cre.accdb;")
Dim sql As String = "SELECT USERID FROM USERS WHERE USERID=#p_userid AND PASSWORD=#p_passw;"
Dim cmd As New OleDb.OleDbCommand(sql, conn)
Dim da As New OleDb.OleDbDataAdapter
Private Sub login_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
End Sub
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
'I figured how to make comments in VB! Yeey
'Temporarely redirecting to menuForm
With cmd
.Parameters.AddWithValue("p_userid", username.Text)
.Parameters.AddWithValue("p_passw", password.Text)
End With
Try
conn.Open()
Dim usr As String = cmd.ExecuteScalar.ToString
If Not (IsDBNull(usr)) AndAlso usr = username.Text Then
Me.DialogResult = Windows.Forms.DialogResult.OK
Else
Me.DialogResult = Windows.Forms.DialogResult.Cancel
End If
Catch ex As Exception
MsgBox("Could not connect to DB hahahaha" & Environment.NewLine & "Error message: " & ex.Message)
Me.DialogResult = Windows.Forms.DialogResult.Cancel
Finally
conn.Close()
Me.Close()
End Try
End Sub
Private Sub closeBtn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles closeBtn.Click
End
End Sub
Private Sub PictureBox1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles PictureBox1.Click
credits.Show()
End Sub
End Class
I'm using Visual Basic Express and Microsoft Access 2010,
Thanks :D

Answer 1

I saw several issues, but I think the one addressed in the question is here:
Dim conn As New OleDb.OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0; Data Source=C:\Users\Fausto\Documents\GitHub\CRE\cre.accdb;")
Dim sql As String = "SELECT USERID FROM USERS WHERE USERID=#p_userid AND PASSWORD=#p_passw;"
Dim cmd As New OleDb.OleDbCommand(sql, conn)
The problem is that those variables are class-level, initialized prior to the class constructor, and the order in which the variables are initialized is not guaranteed. What is happening is that, at the point when the New OleDb.OleDbCommand(sql, conn) code executes, the conn variable is not guaranteed to have initialized.
This omission will fly under the radar, all the way until you try to execute a command, here:
Dim usr As String = cmd.ExecuteScalar.ToString
Aside from needing parentheses for the function call, it's not unil this point that the OleDbCommand object finally tries to use the connection supplied earlier and discovers that there's Nothing there.
While I have your attention, I also need to point out that you're using the wrong parameter notation for OleDb (you need to use ? placeholders instead of #NameParameters). Also, this is a horrible way to handle authentication. NEVER store a password in plain text like that.