How to close a form that open by a BackgroundWorker.? - vb.net

Question

How to close a form that open by a BackgroundWorker.? I need to close Form3 end of the Button1_Click. This is my code.
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
BackgroundWorker1.RunWorkerAsync()
con.Open()
Dim cmd As New SqlCommand(("SELECT * FROM sales"), con)
Dim da As New SqlDataAdapter
Dim ds As New Data.DataSet
da.SelectCommand = cmd
cmd.ExecuteNonQuery()
ds.Clear()
da.Fill(ds, "sales")
con.Close()
DataGridView1.DataSource = ds
DataGridView1.DataMember = "sales"
Form3.Close()
End Sub
Private Sub BackgroundWorker1_DoWork(sender As Object, e As DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Form3.ShowDialog()
End Sub

Answer 1

The problem with your code is that, by using the class name, you are using the default instance and default instances are thread-specific. That means that the Form3 instance that you're referring to in the DoWork event handler, which is executed on a secondary thread, is a different instance than the one you're referring to in the Click event handler, which is executed on the UI thread. In order to refer to the same instance, you would need to create an instance yourself and use that in both cases.
Private f3 As Form3
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
BackgroundWorker1.RunWorkerAsync()
'...
f3.Close()
End Sub
Private Sub BackgroundWorker1_DoWork(sender As Object, e As DoWorkEventArgs) Handles BackgroundWorker1.DoWork
f3 = New Form3
f3.ShowDialog()
End Sub
That code is problematic though, because you're then calling the Close method on a thread other than the one the form was created on. To avoid that, you need to marshal a method call to the thread that the form was created on.
f3.Invoke(Sub() f3.Close())
That addresses your question but the real issue here is that you are trying to display a modal dialogue to prevent access to the calling form while it's doing some work but you're failing in that aim. Because you're calling ShowDialog on a secondary thread, it won't actually behave the way a modal dialogue should. That's because modality is thread-specific too.
You're actually doing things the wrong way around. You should not be displaying the form on a secondary thread and doing the work on the UI thread. Rather, you should be displaying the form on the UI thread and doing the work on a secondary thread.
Click here for a demo that does as I describe in that last paragraph.