Public Class Form1
 
    Dim pb As New PictureBox With
        {.Parent = Me, .Top = 10, .Left = 10, .Width = 300, .Height = 300,
        .SizeMode = PictureBoxSizeMode.Zoom, .BackColor = Color.White,
        .BorderStyle = BorderStyle.Fixed3D}
 
    Dim WithEvents btnCreateBitmap As New Button With
        {.Parent = Me, .Top = 320, .Left = 10, .Width = 300, .Text = "Create"}
    Dim WithEvents btnKillBitmap As New Button With
        {.Parent = Me, .Top = 360, .Left = 10, .Width = 300, .Text = "Kill"}
    Dim WithEvents btnSaveBitmap As New Button With
        {.Parent = Me, .Top = 400, .Left = 10, .Width = 300, .Text = "Save"}
 
    Dim Gleisplan As Bitmap
    Dim rndm As New Random(Now.Millisecond)
 
 
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
 
        Me.MinimumSize = New Size(350, 500)
    End Sub
 
 
    Private Sub CreateBitmap()
 
        DeleteBitmap() 'Freigabe sicher stellen
 
        Dim Gpx, GPy, x1, y1, x2, y2 As Integer
        Gpx = 24361
        GPy = 5966
 
        Gleisplan = New Bitmap(Gpx, GPy,
             Imaging.PixelFormat.Format32bppArgb)
 
        pb.Image = Gleisplan
 
        Using g As Graphics = Graphics.FromImage(Gleisplan),
                pn As New System.Drawing.Pen(Color.Blue, 8)
            g.Clear(Color.Yellow)
            For i As Integer = 0 To 10
                x2 = x1
                y2 = y1
                x1 = CInt(rndm.NextDouble() * Gpx)
                y1 = CInt(rndm.NextDouble() * GPy)
                g.DrawLine(pn, x1, y1, x2, y2)
            Next i
        End Using
 
    End Sub
 
 
    Private Sub SaveBitmap()
        If Gleisplan Is Nothing Then Exit Sub
 
        Dim filepath As String = "G:\downloads\gleisplan.png"
        IO.File.Delete(filepath)
        Gleisplan.Save(filepath, Drawing.Imaging.ImageFormat.Png)
    End Sub
 
 
    Private Sub DeleteBitmap()
 
        'Referenz in der Picturebox aufheben
        pb.Image = Nothing
 
        If Gleisplan IsNot Nothing Then
            'Windows-Resourcen (Handles etc.) der Bitmap entfernen
            Gleisplan.Dispose
            'Letzte Referenz auf die Bitmap aufheben
            '(Löschen der Net-Klasse anfordern)
            Gleisplan = Nothing
        End If
    End Sub
 
    Private Sub btnKillBitmap_Click(sender As Object, e As EventArgs) Handles _
      btnKillBitmap.Click
        DeleteBitmap()
    End Sub
 
    Private Sub btnCreateBitmap_Click(sender As Object, e As EventArgs) Handles _
      btnCreateBitmap.Click
        CreateBitmap()
    End Sub
 
    Private Sub btnSaveBitmap_Click(sender As Object, e As EventArgs) Handles _
      btnSaveBitmap.Click
        SaveBitmap()
    End Sub
End Class