Question

Suppose a user minimize my visual basic application to the taskbar notification icon. Now I want when user open a new instance, the old one should restore.

Answer 1

You can often do this fairly simply using DDE in a degenerate way:

Form1.frm

Option Explicit
'This is Form1.  To use as DDE source at design time we set:
'   Form1.LinkMode = 1 (Source, i.e. vbLinkSource).
'   Form1.LinkTopic = "Form1" (default).
'
'Note we use (hidden) Label1 on this Form as a DDE destination.

Private PrevState As Integer

Private Sub Form_LinkExecute(CmdStr As String, Cancel As Integer)
    'Got a "command" so restore Form1 and accept the command.
    WindowState = PrevState
    Caption = "I am awake!"
    Cancel = False
End Sub

Private Sub Form_Load()
    PrevState = WindowState
End Sub

Private Sub Form_Resize()
    If WindowState <> vbMinimized Then PrevState = WindowState
End Sub

Module1.bas

Option Explicit

Private Sub Main()
    Load Form1
    'After Form1 is loaded (hidden), try DDE link to possible prior copy.
    With Form1.Label1
        .LinkTopic = App.EXEName & "|Form1"
        On Error Resume Next
        .LinkMode = vbLinkManual
        If Err.Number = 0 Then
            On Error GoTo 0
            'Link succeeded.  Wake up prior copy via pushback to
            'the DDE source, then unload Form1 and terminate.
            .LinkExecute "Wake up!"
            Unload Form1
        Else
            On Error GoTo 0
            'Link failed, so we're 1st.  Show Form1.
            Form1.Show vbModal
        End If
    End With
End Sub