using MDIcontainer

Something like...

Public Class form_master

    Private WithEvents sales_form As New form_sales

    Private Sub button_1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn_POS.Click
        If sales_form Is Nothing Then
            sales_form = New form_sales()
            sales_form.MdiParent = Me
            sales_form.Show()
        End If
        If sales_form.WindowState = FormWindowState.Minimized Then
            sales_form.WindowState = FormWindowState.Normal
        End If
        sales_form.Activate()
    End Sub

    Private Sub sales_form_FormClosed(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosedEventArgs) Handles sales_form.FormClosed
        sales_form = Nothing
    End Sub

End Class

To: Idle_Mind

how come i still can open duplicated form althought i used your code exactly?

The application doen't stop even I put the breakpoint at "If sales_form Is Nothing"
why?? (I was using visual studio .net 2005

Private Sub button_1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn_POS.Click

        If sales_form Is Nothing Then  (breakpoint)
            sales_form = New form_sales()
            sales_form.MdiParent = Me
            sales_form.Show()
        End If

        If sales_form.WindowState = FormWindowState.Minimized Then
            sales_form.WindowState = FormWindowState.Normal
        End If

        sales_form.Activate()

    End Sub

I found tat my application doesn't recognize my changed code...

For an instance..

Private Sub Button5_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button5.Click

    MsgBox("Hellow")

End Sub

The message box doesn't pop up when i click button5

Try deleting the "bin" subfolder in your project and running it again.

all right.. now the problem is..

sales_form doesn't pop up.... unless i changed the code

from "If sales_form Is Nothing Then" to "If sales_form Is Nothing = False Then"

----------------------------------------------------------------------------------------------

Public Class form_master

    Private WithEvents sales_form As New form_sales

    Private Sub sales_form_FormClosed(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosedEventArgs) Handles sales_form.FormClosed
        sales_form = Nothing
    End Sub

    Private Sub ToolStripButton1_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ToolStripButton1.Click

        If sales_form Is Nothing = False Then
            sales_form = New form_sales()
            sales_form.MdiParent = Me
            sales_form.Show()
        End If

        If sales_form.WindowState = FormWindowState.Minimized Then
            sales_form.WindowState = FormWindowState.Normal
        End If

        sales_form.Activate()
    End Sub

End Class

You're using VB.Net 2005 right?

Try:

        If IsNothing(sales_form) Then
            sales_form = New form_sales()
            sales_form.MdiParent = Me
            sales_form.Show()
        End If

Yup.. i'm using VS.net 2005 (VB)

i tried ur code oledi.. but the problem is existed