The connection was not closed the connection current state is open

Question

Hye... I currently doing my final year project using vb.net and i got this error. Im tryin' to fix the error but still not succeed. I use ms access for database in my project. I try to put con.Open() before the 'dt' statement and con.Close() after the 'cboProduct.Select' but the result is the same. Really appreciate your helps. Thanks :)

'GLOBAL DECLARATIONS
Dim conString As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\Acer User\Documents\MAKLUMAT IVENTORI.accdb"
Dim con As OleDbConnection = New OleDbConnection(conString)
Dim adapter As New OleDbDataAdapter
Dim cmd As New OleDbCommand
Dim dt As New DataTable
Dim ds As New DataSet

Private Sub RefreshData()
    dt = New DataTable
    ds = New DataSet
    ds.Tables.Add(dt)

    adapter = New OleDbDataAdapter("Select * FROM product WHERE lab_kod='66'", con)
    adapter.Fill(dt)
    DataGridView1.DataSource = dt.DefaultView

    labelID.Text = getAutoID()
    lblLabCode.Text = "66"
    cboProduct.Select()

    Dim v_SQL As String = "SELECT * FROM kategori_product"
    cmd = New OleDbCommand(v_SQL, con)
    con.Open()

    Dim v_dataReader As OleDbDataReader = cmd.ExecuteReader()
    Dim v_dataTable As New DataTable
    v_dataTable.Columns.Add("product_kod", Type.GetType("System.String"))

    If v_dataReader.HasRows Then
        While v_dataReader.Read
            v_dataTable.Rows.Add(v_dataReader.GetString(0))
        End While
        cboProduct.DataSource = v_dataTable
    End If

    cboProduct.DisplayMember = "product_kod"
    cboProduct.ValueMember = "product_kod"
    v_dataReader.Close()
    con.Close()
End Sub

Answer 1

1. Don't store short-lived objects, such as database connections, as global variables.
2. In fact, don't use global variables, ever, at all.
   _{(though I note that assuming this is a Class and not a Module those are actually class fields, not globals, but you're still misusing them)}
3. Your OleDbDataAdapter.Fill call requires the connection to be open, but you call .Fill(dt) before calling con.Open()
4. Use Using (using() in C#) blocks to ensure your database connection is always closed, even in the event of failure.