Public Class FileLocker
 
    Dim FileStream As System.IO.FileStream
 
    Sub New()
    End Sub
 
    Public Function Release() As Boolean
        Release = False
        Try
            FileStream.Close()
            Return True
        Catch ex As Exception
            Return False
        End Try
    End Function
 
    Public Function Lock(ByVal Filename As String) As Boolean
        Lock = False
        Try
            Try
                FileStream.Close()
            Catch ex As Exception
            End Try
            FileStream = New System.IO.FileStream(Filename, IO.FileMode.Open, _
              IO.FileAccess.ReadWrite, IO.FileShare.Read)
            Return True
        Catch ex As Exception
            Return False
        End Try
    End Function
 
End Class

Public Class Form1
 
    Dim filelocker3 As New FileLocker
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Label1.Text = filelocker3.Lock("C:\test.txt").ToString
    End Sub
 
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button2.Click
        Label2.Text = filelocker3.Release.ToString
    End Sub
 
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button3.Click
        Dim Filestream1 As New System.IO.FileStream("C:\test.txt", _
        System.IO.FileMode.Open, IO.FileAccess.Read)
    End Sub
 
End Class

Danke, an ALLE !!