public void MessageAndCause()
        {
            QRBillGenerationException e = Assert.Throws <QRBillGenerationException>(
                () => ThrowNestedNullRefException()
                );

            Assert.Equal("QRS", e.Message);
            Assert.Same(typeof(NullReferenceException), e.InnerException.GetType());
        }
        public void MessageOnly()
        {
            QRBillGenerationException e = Assert.Throws <QRBillGenerationException>(
                () => ThrowQRBillException()
                );

            Assert.Equal("ABC", e.Message);
            Assert.Null(e.InnerException);
        }