AutoCAD 2010 :: How To Set The Drawing Limits

In VB i had:

Dim newLimits(0 To 3) As Double
newLimits(0) = x1 - (x2 - x1)
newLimits(1) = y1 - (y2 - y1)
newLimits(2) = x2 + (x2 - x1)
newLimits(3) = y2 + (y2 - y1)
AcadDoc.Limits = newLimits

Now i thougt this works in this way:

 /// <summary>        ///     Testfunktion für Limits        /// </summary>        private static void Test()        {            var acDoc = Application.DocumentManager.MdiActiveDocument;            var acDb = acDoc.Database;  acDb.Limmin = new Point2d(10, 50);            acDb.Limmax = new Point2d(10, 50);        }

But this is not working

I get the following Error: eLockViolation

I think this is from the Transaction and OpenMode.ForWrite - but where do i get such a transaction and the database open for writing.