[RESOLVED] Please solution Object reference not set to an instance of an object.

Dear All Master
I use openfolderdialog for connection to jet.oledb. Please solution.

Code:

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Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim FolderBrowserDialog1 As New FolderBrowserDialog()
        FolderBrowserDialog1.SelectedPath = Directory.GetCurrentDirectory
        If FolderBrowserDialog1.ShowDialog() = DialogResult.OK Then
            TextBox1.Text = FolderBrowserDialog1.SelectedPath
        End If
    End Sub
    Dim Path As String = TextBox1.Text 'Object reference not set to an instance of an object.
    Dim cn = "provider=Microsoft.Jet.OLEDB.4.0; data source=" & Path & "; Extended Properties=dBase IV"
    Private Sub PopulateDataGridView()
        Try
            dt = New DataTable
            Dim query = "select ITM FROM ITEM."
            Using adapter As New OleDbDataAdapter(query, cn.ToString)
                adapter.Fill(dt)
            End Using
            Me.DataGridView1.DataSource = dt
        Catch myerror As OleDbException
            MessageBox.Show("Error: " & myerror.Message)
        Finally
        End Try
    End Sub

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Thanks
roy88

Code:

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Dim Path As String
Dim cn As String

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim FolderBrowserDialog1 As New FolderBrowserDialog()
    FolderBrowserDialog1.SelectedPath = Directory.GetCurrentDirectory
    If FolderBrowserDialog1.ShowDialog() = DialogResult.OK Then
        TextBox1.Text = FolderBrowserDialog1.SelectedPath
        Path = TextBox1.Text
        cn = "provider=Microsoft.Jet.OLEDB.4.0; data source=" & Path & "; Extended Properties=dBase IV"
    End If
End Sub

Private Sub PopulateDataGridView()
    Try
        dt = New DataTable
        Dim query = "select ITM FROM ITEM."
        Using adapter As New OleDbDataAdapter(query, cn.ToString)
            adapter.Fill(dt)
        End Using
        Me.DataGridView1.DataSource = dt
    Catch myerror As OleDbException
        MessageBox.Show("Error: " & myerror.Message)
    Finally
    End Try
End Sub

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Code:

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Using adapter As New OleDbDataAdapter(query, cn)

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Quote:

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Originally Posted by .paul.

Code:

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Using adapter As New OleDbDataAdapter(query, cn)

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dear mr. .paul.
Thanks for the reply from you, but there's a problem.

Code:

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Using adapter As New OleDbDataAdapter(query, cn)
                adapter.Fill(dt) 'The ConnectionString property has not been initialized.
  End Using

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Code:

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Dim con As New OleDbConnection(cn)
Using adapter As New OleDbDataAdapter(query, con)
    adapter.Fill(dt)
End Using

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Quote:

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Originally Posted by .paul.

Code:

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Dim con As New OleDbConnection(cn)
Using adapter As New OleDbDataAdapter(query, con)
    adapter.Fill(dt)
End Using

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dear mr. .paul.
Thanks for the reply from you, but still problem.

Code:

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Dim con As New OleDbConnection(cn)
            Using adapter As New OleDbDataAdapter(query, con)
                adapter.Fill(dt) 'The ConnectionString property has not been initialized.
            End Using

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Have you tried stepping through the code and checking thing? Make sure cn is the value it should be. When something isn't right, debugging and stepping through the code should be the first thing you should be doing to diagnose the problem. Have you checked to make sure that the connection string in cn has been constructed correctly? You're fairly new here, but this isn't how it works here. This isn't therapy for you to work out your problems. We should be a last resort, when you can't figure it out. But you first must make an effort. And at the moment, it doesn't look like you are. And saying things like "still problems" isn't much help either. I'll give you credit though, you're posting code and pointing at the line, that's more effort than we usually get from new posters, so I'm willing to work with that, but I'd like to see more of the code... For instance, I don't see how PopulateDataGridView is called -- it doesn't seem to be from the button click -- ... and since the code in question is there, how is that getting called?

\-tg

Funny enough, one of my very first codebank entries does just what you're wanting to do: https://www.vbforums.com/showthread....openfiledialog

Dear All master,

Below is the full code from me.

Code:

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Public Class Form1
    Dim dt As New DataTable
    Dim Path As String
    Dim cn As String
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        PopulateDataGridView()
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim FolderBrowserDialog1 As New FolderBrowserDialog()
        FolderBrowserDialog1.SelectedPath = Directory.GetCurrentDirectory
        If FolderBrowserDialog1.ShowDialog() = DialogResult.OK Then
            TextBox1.Text = FolderBrowserDialog1.SelectedPath
            Path = TextBox1.Text
            cn = "provider=Microsoft.Jet.OLEDB.4.0; data source=" & Path & "; Extended Properties=dBase IV"
        End If
    End Sub
    Private Sub PopulateDataGridView()
        Try
            dt = New DataTable
            Dim query = "select ITM FROM ITEM."
            Dim con As New OleDbConnection(cn)
            Using adapter As New OleDbDataAdapter(query, con)
                adapter.Fill(dt) 'The ConnectionString property has not been initialized.
            End Using
            Me.DataGridView1.DataSource = dt
        Catch myerror As OleDbException
            MessageBox.Show("Error: " & myerror.Message)
        Finally
        End Try
    End Sub
End Class

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for which problem I mark the color red in the code.
Thanks
roy88

Code:

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Public Class Form1
    Dim dt As New DataTable
    Dim Path As String
    Dim cn As String
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        PopulateDataGridView()
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim FolderBrowserDialog1 As New FolderBrowserDialog()
        FolderBrowserDialog1.SelectedPath = Directory.GetCurrentDirectory
        If FolderBrowserDialog1.ShowDialog() = DialogResult.OK Then
            TextBox1.Text = FolderBrowserDialog1.SelectedPath           
        End If
    End Sub
    Private Sub PopulateDataGridView()
        Try
            dt = New DataTable
            Dim query = "select ITM FROM ITEM."
            Path = TextBox1.Text
            cn = "provider=Microsoft.Jet.OLEDB.4.0; data source=" & Path & "; Extended Properties=dBase IV"
            Dim con As New OleDbConnection(cn)
            Using adapter As New OleDbDataAdapter(query, con)
                adapter.Fill(dt) 'The ConnectionString property has not been initialized.
            End Using
            Me.DataGridView1.DataSource = dt
        Catch myerror As OleDbException
            MessageBox.Show("Error: " & myerror.Message)
        Finally
        End Try
    End Sub
End Class

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Quote:

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Originally Posted by .paul.

Code:

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Public Class Form1
    Dim dt As New DataTable
    Dim Path As String
    Dim cn As String
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        PopulateDataGridView()
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim FolderBrowserDialog1 As New FolderBrowserDialog()
        FolderBrowserDialog1.SelectedPath = Directory.GetCurrentDirectory
        If FolderBrowserDialog1.ShowDialog() = DialogResult.OK Then
            TextBox1.Text = FolderBrowserDialog1.SelectedPath           
        End If
    End Sub
    Private Sub PopulateDataGridView()
        Try
            dt = New DataTable
            Dim query = "select ITM FROM ITEM."
            Path = TextBox1.Text 'error invalid argument
            cn = "provider=Microsoft.Jet.OLEDB.4.0; data source=" & Path & "; Extended Properties=dBase IV"
            Dim con As New OleDbConnection(cn)
            Using adapter As New OleDbDataAdapter(query, con)
                adapter.Fill(dt)
            End Using
            Me.DataGridView1.DataSource = dt
        Catch myerror As OleDbException
            MessageBox.Show("Error: " & myerror.Message)
        Finally
        End Try
    End Sub
End Class

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Dear mr..paul.
Thanks for your reply. But I run the code from you there is an error "invalid argument".
I mark the color red in the code
Thanks
roy88

You need to learn how to use the debugger. If you had done any debugging then the issue would have been obvious. You are using 'cn' in the 'PopulateDataGridView' method and you're calling that from the 'Load' event handler of the form. You're not setting 'cn' until you click 'Button1' though, which obviously happens later. Simple logic should tell you what the problem is but a couple of breakpoints - one where you set 'cn' and one where you use 'cn' - would have demonstrated it clearly. You should not be posting a question here without having debugged your code first. If you don't know how to use the debugger, stop what you're doing and learn. It is essential knowledge for every developer, regardless of skill level.

Quote:

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Originally Posted by jmcilhinney

You need to learn how to use the debugger. If you had done any debugging then the issue would have been obvious. You are using 'cn' in the 'PopulateDataGridView' method and you're calling that from the 'Load' event handler of the form. You're not setting 'cn' until you click 'Button1' though, which obviously happens later. Simple logic should tell you what the problem is but a couple of breakpoints - one where you set 'cn' and one where you use 'cn' - would have demonstrated it clearly. You should not be posting a question here without having debugged your code first. If you don't know how to use the debugger, stop what you're doing and learn. It is essential knowledge for every developer, regardless of skill level.

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The problem in the code below

Code:

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Path = TextBox1.Text 'error invalid argument

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That is NOT the line causing the error. As jmcilhinney told you, you’re running your code when TextBox1.Text = “”
Therefore cn is invalid…

You should do as jmcilhinney told you to. You’re really not helping yourself…

Quote:

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Originally Posted by roy88

The problem in the code below

Code:

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Path = TextBox1.Text 'error invalid argument

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You just ignored everything I posted previously. The issue is exactly the same, i.e. you're trying to use data that you haven't actually got yet. Instead of blindly copying and pasting code that others write, how about you put some thought and effort into this for yourself? The fact that I told you asked about effectively the exact same problem that I already told you how to resolve shows that you're not doing either.

Quote:

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Originally Posted by .paul.

That is NOT the line causing the error. As jmcilhinney told you, you’re running your code when TextBox1.Text = “”
Therefore cn is invalid…

You should do as jmcilhinney told you to. You’re really not helping yourself…

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Dear Mr. .paul.
what you say is true, so what is the solution to avoid empty textbox1.text If I keep using button1 for FolderBrowserDialog.Below is the full code from me without using button1 succeeds.

Code:

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Public Class Form1
    Dim dt As New DataTable
    Dim Path As String
    Dim cn As String
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If File.Exists(Path) = False Then
            MessageBox.Show("Do you want to change path of the database?", "System Message", MessageBoxButtons.OK)
            Dim FolderBrowserDialog1 As New FolderBrowserDialog()
            FolderBrowserDialog1.SelectedPath = Directory.GetCurrentDirectory
            If FolderBrowserDialog1.ShowDialog() = DialogResult.OK Then
                TextBox1.Text = FolderBrowserDialog1.SelectedPath
            End If
        End If
        PopulateDataGridView()
    End Sub
 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    End Sub
    Private Sub PopulateDataGridView()
        Try
            dt = New DataTable
            Path = TextBox1.Text
            cn = "provider=Microsoft.Jet.OLEDB.4.0; data source=" & Path & "; Extended Properties=dBase IV"

            Dim query = "select ITM FROM ITEM."
            Dim con As New OleDbConnection(cn)

            Using adapter As New OleDbDataAdapter(query, con)
                adapter.Fill(dt)
            End Using
            Me.DataGridView1.DataSource = dt
        Catch myerror As OleDbException
            MessageBox.Show("Error: " & myerror.Message)
        Finally
        End Try
    End Sub

End Class

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thanks
roy88

You're still not really putting anything into this. You should be working out the logic well before writing any code but you have no idea what the logic is here. Think about it! If the user needs to supply information in order for the application to connect to the database, how can the application possibly connect to the database before the user has even seen a form in order to supply that information? It makes no sense so why are you writing code to do it? If the grid requires you to connect to the database and connecting the database requires input from the user then you can't possibly populate the grid before the user has provided that input, so don't write code to do it and then be amazed that it doesn't work. Work out the logic first, then write code to implement that logic. If you do that then your code won't try to do illogical things.

Quote:

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Originally Posted by roy88

Private Sub PopulateDataGridView()
Try
dt = New DataTable
Path = TextBox1.Text
cn = "provider=Microsoft.Jet.OLEDB.4.0; data source=" & Path & "; Extended Properties=dBase IV"

Stop
'inspect cn here. Be sure you have the right connection string
'examples of connection strings are available at connectionstrings.com

Dim query = "select ITM FROM ITEM."
Dim con As New OleDbConnection(cn)

Using adapter As New OleDbDataAdapter(query, con)
adapter.Fill(dt)
End Using
Me.DataGridView1.DataSource = dt
Catch myerror As OleDbException
MessageBox.Show("Error: " & myerror.Message)
Finally
End Try
End Sub

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See the notes in the code snippet

Quote:

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Originally Posted by .paul.

See the notes in the code snippet

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dear Mr. .paul.

Thank you for your reply, sorry I'm late replying.
thanks
roy88