Function PrevInstance() As Boolean
If Ubound(Diagnostics.Process.GetProcessesByName( _
  Diagnostics.Process.GetCurrentProcess).ProcessName)) > 0 _
Then
     Return True 
Else 
     Return False  
End If
End Function

Public Shared Sub Main()
        'Für jede Applikation eine neue Id nehmen!!!
        Const idMutex$ = "{0670FCFC-CCC2-4597-96F6-6AFD78C6BF8D}"
        Dim mut As Threading.Mutex
        Dim createdNew As Boolean
 
        mut = New Threading.Mutex(True, idMutex, createdNew)
        If createdNew Then
            mut.ReleaseMutex()
        Else
            Debug.WriteLine("Es läuft bereits eine Instanz des Programms")
            ActivatePrevInst() 
            Return
        End If
 
        Dim fmMain As New Form1()
        Application.Run(fmMain)
 
        mut.Close()
 
End Sub

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) _
      Handles MyBase.Load
 
        Api.SetProp(Me.Handle, ClsStart.IdWinProp, 1)
 
    End Sub
    Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
        'Wenn die Form die selbstdefinierte Nachricht erhält,
        'wird sie aktiviert.
        If m.Msg = Api.WM_PRIVATE_ACTIVATE Then
            Me.Activate()
        End If
 
        MyBase.WndProc(m)
    End Sub

    'Für jede Applikation bzw. jedes Fenster einer Anwendung eine neue ID 
    ' nehmen!!!
    Public Const IdWinProp$ = "{CF9E5190-770E-4e62-AAAB-7C0020E52129}"
    Friend Shared Sub ActivatePrevInst()
        'Sendet die selbstdefinierte Nachricht an das Fenster der schon 
        ' laufenden
        'Instanz
        Dim hwnd As IntPtr = GetPrevInstWnd()
 
        If hwnd.Equals(IntPtr.Zero) Then
            Return
        End If
 
        Api.SendMessage(hwnd, Api.WM_PRIVATE_ACTIVATE, 0, 0)
    End Sub
 
    Private Shared Function GetPrevInstWnd() As IntPtr
        'Durchsucht alle Fenster bis es die Form gefunden hat,
        'die die selbstdefinierte Eigenschaft ClsStart.IdWinProp aht
        'und gibt das Fensterhandle zurück
        Dim hWnd As IntPtr
        Dim ret%
 
        hWnd = Api.GetDesktopWindow
 
        hWnd = Api.GetWindow(hWnd, Api.GW_CHILD)
        Do Until hWnd.Equals(IntPtr.Zero)
            ret = Api.GetProp(hWnd, IdWinProp)
            If ret Then
                Return hWnd
            End If
 
            hWnd = Api.GetWindow(hWnd, Api.GW_HWNDNEXT)
        Loop
 
        Return IntPtr.Zero
    End Function

Public Class Api
    Public Declare Function SetProp Lib "user32" Alias "SetPropA" ( _
        ByVal hwnd As IntPtr, _
        ByVal lpString As String, _
        ByVal hData As Int32) As Int32
 
    Public Declare Function GetProp Lib "user32" Alias "GetPropA" ( _
        ByVal hwnd As IntPtr, _
        ByVal lpString As String) As Int32
 
    Public Declare Function GetDesktopWindow Lib "user32" () As IntPtr
 
    Public Declare Function GetWindow Lib "user32" ( _
        ByVal hwnd As IntPtr, _
        ByVal wCmd As Int32) As IntPtr
 
    Public Declare Function SendMessage Lib "user32" Alias "SendMessageA" ( _
        ByVal hwnd As IntPtr, _
        ByVal wMsg As Int32, _
        ByVal wParam As Int32, _
        ByVal lParam As Int32) As Int32
 
    Public Const GW_CHILD As Int32 = 5
    Public Const GW_HWNDNEXT As Int32 = 2
 
    Public Const WM_APP As Int32 = &H8000
    Public Const WM_PRIVATE_ACTIVATE = WM_APP + 1
End Class