Exception thrown when executing query

Question

I have vb.net app form, it contains id as combobox. after click on edit buton & chose an other id, i found error Like the picture below.so what to do?

Private Sub searchparfum()
    Dim dt As New DataTable
    Dim ds As New DataSet
    ds.Tables.Add(dt)
    Dim da As New OleDbDataAdapter("select num from parfum", MaConnection)
    da.Fill(dt)
    Dim r As DataRow
    cmb_parfum.AutoCompleteCustomSource.Clear()
    For Each r In dt.Rows
        cmb_parfum.AutoCompleteCustomSource.Add(r.Item(0).ToString)
    Next
    cmb_parfum.AutoCompleteMode = AutoCompleteMode.SuggestAppend
    cmb_parfum.AutoCompleteSource = AutoCompleteSource.CustomSource
End Sub

Private Sub cmb_parfum_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmb_parfum.SelectedIndexChanged
    qry = "select * from parfum where num ='" & (cmb_parfum.Text) & "' "
    cmd = New OleDbCommand(qry, MaConnection)
    cmd.ExecuteNonQuery()
    dr = cmd.ExecuteReader
    If dr.Read Then

        txt_num.Text = dr("num")
        lab_nom.Text = dr("nom")
        lab_prix.Text = dr("prix")
        stock_par.Text = dr("stock")
        txt_ventes.Text = dr("ventes")
        photo.Text = dr("photo")

    End If
End Sub

Answer 1

As the error message says, you don't have an open connection to the DB. Add an open and close and maybe wrap it in a Try.. Catch too like this:

    cmd = New OleDbCommand(qry, MaConnection)
    Try
        MaConnection.Open()
        cmd.ExecuteNonQuery()
        dr = cmd.ExecuteReader
        If dr.Read Then
            txt_num.Text = dr("num")
            lab_nom.Text = dr("nom")
            lab_prix.Text = dr("prix")
            stock_par.Text = dr("stock")
            txt_ventes.Text = dr("ventes")
            photo.Text = dr("photo")
        End If
    Catch ex As Exception
        ' Handle any exception
    Finally
        MaConnection.Close()
    End Try

Also look into using parameters in your SQL Commands too

Answer 2

Database object like commands and connection should be declared in the method where they are used so they can be properly disposed. Using...End Using blocks do this for you even if there is an error.

You would not use cmd.ExecuteNonQuery() for a Select statement. That is used only for Update , Insert or Delete commands.

You don't want the connection to remain open while you update the user interface. So, we load a data table which does not require the connection to remain open while we use the data. The reader does require an open connection.

Is photo really a string type?

Private ConStr As String = "Your connection string"

Private Sub cmb_parfum_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmb_parfum.SelectedIndexChanged
    Dim dt As New DataTable
    Dim qry = "select * from parfum where num = @Parfum;"
    Using cn As New OleDbConnection(ConStr),
            cmd As New OleDbCommand(qry, cn)
        cmd.Parameters.Add("@Parfum", OleDbType.VarWChar).Value = cmb_parfum.Text
        cn.Open()
        Using dr = cmd.ExecuteReader
            dt.Load(dr)
        End Using
    End Using 'connection is closed and disposed and command is disposed
    If dt.Rows.Count > 0 Then
        txt_num.Text = dt(0)("num").ToString
        lab_nom.Text = dt(0)("nom").ToString
        lab_prix.Text = dt(0)("prix").ToString
        stock_par.Text = dt(0)("stock").ToString
        txt_ventes.Text = dt(0)("ventes").ToString
        photo.Text = dt(0)("photo").ToString
    End If
End Sub