Private Function impersonateValidUser(ByVal userName As String, ByVal domain As _
  String, ByVal password As String) As Boolean
 
        Dim tempWindowsIdentity As WindowsIdentity
        Dim token As IntPtr = IntPtr.Zero
        Dim tokenDuplicate As IntPtr = IntPtr.Zero
        impersonateValidUser = False
 
        If RevertToSelf() Then
            If LogonUserA(userName, domain, password, _
              LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, token) <> _
              0 Then
                If DuplicateToken(token, 2, tokenDuplicate) <> 0 Then
                    tempWindowsIdentity = New WindowsIdentity(tokenDuplicate)
                    impersonationContext = tempWindowsIdentity.Impersonate()
                    If Not impersonationContext Is Nothing Then
                        impersonateValidUser = True
                    End If
                End If
            End If
        End If
        If Not tokenDuplicate.Equals(IntPtr.Zero) Then
            CloseHandle(tokenDuplicate)
        End If
        If Not token.Equals(IntPtr.Zero) Then
            CloseHandle(token)
        End If
    End Function
 
    ' Aufruf der Funktion z.B.
    impersonateValidUser("username", "domäne", "passwort")