public void CalculateTotalNoOfUnitsSoldTest()
        {
            //Arrange
            var invoiceRepo = new InvoiceRepository();
            var invoiceList = invoiceRepo.Retrieve();

            //Act
            var result = invoiceRepo.CalculateTotalNoOfUnitsSold(invoiceList);

            TestContext.WriteLine(result.ToString());
            //Assert
            Assert.IsNotNull(result);
        }