public void XmlRepoTests_Can_CRUD_Baum()
        {
            var baum = new Tannenbaum()
            {
                Height = 17
            };
            var newHeight = 452;

            {//INSERT
                var repo = new XmlRepository();
                repo.Add(baum);
                repo.Save();
            }


            {//check Read
                var repo = new XmlRepository();

                Assert.IsTrue(repo.GetAll <Tannenbaum>().Any(x => x.Height == baum.Height));
                var loaded = repo.GetById <Tannenbaum>(baum.Id);
                Assert.IsNotNull(loaded);
                Assert.AreEqual(baum.Height, loaded.Height);

                //UPDATE
                loaded.Height = 452;
                repo.Save();
            }

            {//check UPDATE + DELETE
                var repo   = new XmlRepository();
                var loaded = repo.GetById <Tannenbaum>(baum.Id);
                Assert.AreEqual(newHeight, loaded.Height);

                repo.Delete(loaded);
                repo.Save();
            }

            {//check DELETE
                var repo   = new XmlRepository();
                var loaded = repo.GetById <Tannenbaum>(baum.Id);
                Assert.IsNull(loaded);
            }
        }