Sub abrunden(ByVal was As Object, _
  ByVal x As Integer, ByVal y As Integer, _
  ByVal width As Integer, ByVal height As Integer, _
  ByVal radius As Integer)
 
 
  Dim gp As System.Drawing.Drawing2D.GraphicsPath = _
    New System.Drawing.Drawing2D.GraphicsPath()
 
  gp.AddLine(x + radius, y, x + width - radius, y)
  gp.AddArc(x + width - radius, y, radius, radius, 270, 90)
  gp.AddLine(x + width, y + radius, x + width, y + height - radius)
  gp.AddArc(x + width - radius, y + height - radius, radius, radius, 0, 90)
  gp.AddLine(x + width - radius, y + height, x + radius, y + height)
  gp.AddArc(x, y + height - radius, radius, radius, 90, 90)
  gp.AddLine(x, y + height - radius, x, y + radius)
  gp.AddArc(x, y, radius, radius, 180, 90)
  gp.CloseFigure()
 
  was.region = New System.Drawing.Region(gp)
  gp.Dispose()
End Sub

Private Sub Form1_Load(sender As Object, e As System.EventArgs) _
  Handles Me.Load
  ' Ecken abrufen
  abrunden(Me, 0, 0, Me.Width, Me.Height, 20)
End Sub

Private Sub Form1_Resize(sender As Object, e As System.EventArgs) Handles _
  Me.Resize
 ' Ecken abrufen
  abrunden(Me, 0, 0, Me.Width, Me.Height, 20)
End Sub