Verifying & Fix Decimal Rounding Issues

DbContext.cs

public class PomodoDbContext : DbContext, IPomodoDbContext
{
    static PomodoDbContext()
    {
        Database.SetInitializer<PomodoDbContext>(null);   
    }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        //this should just match whatever the DB column is
        modelBuilder.Entity<Invoice>()
            .Property(i => i.PretaxTotal).HasPrecision(18, 5);
        modelBuilder.Entity<Invoice>()
            .Property(i => i.TotalTax).HasPrecision(18, 5);
        modelBuilder.Entity<Invoice>()
            .Property(i => i.PosttaxTotal).HasPrecision(18, 5);

        modelBuilder.Entity<InvoiceItem>()
            .Property(i => i.Quantity).HasPrecision(18, 5);
        modelBuilder.Entity<InvoiceItem>()
            .Property(i => i.BasePrice).HasPrecision(18, 5);
        modelBuilder.Entity<InvoiceItem>()
            .Property(i => i.DiscountedPrice).HasPrecision(18, 5);
        modelBuilder.Entity<InvoiceItem>()
            .Property(i => i.Discount).HasPrecision(18, 5);
        modelBuilder.Entity<InvoiceItem>()
            .Property(i => i.TaxPaid).HasPrecision(18, 5);
        modelBuilder.Entity<InvoiceItem>()
            .Property(i => i.TotalPaid).HasPrecision(18, 5);
    }    
}

UnitTest.cs

[TestClass]
public class InvoiceDecimalTruncationTest
{
    [TestMethod]
    public void TestTruncation()
    {
        using(var db = new PomodoDbContext())
        {
            //first create entity in db.
            //be sure to use decimals that go beyond "2" precision
            var retailer = db.Retailers.First();
            var userId = db.AspNetUsers.First().Id;
            var productId = db.Products.First().Id;
            decimal taxAmt = 12.3456m;
            decimal pretaxTotal = 33.44556m;
            decimal discountedAmt = 29.9987m;
            decimal target = taxAmt + pretaxTotal;
            decimal quantity = 5.432m;
            var invoice = new Invoice
            {
                TotalTax = taxAmt,
                PretaxTotal = pretaxTotal,
                PosttaxTotal = target,
                RetailerId = retailer.Id,
                CreatedAt = DateTime.UtcNow,
                UpdatedAt = DateTime.UtcNow,
                InvoiceItems = new List<InvoiceItem>
                {
                    new InvoiceItem
                    {
                        CreatedAt = DateTime.UtcNow,
                        UpdatedAt = DateTime.UtcNow,
                        ProductId = productId,
                        Quantity = quantity,
                        BasePrice = pretaxTotal,
                        DiscountedPrice = discountedAmt,
                        TaxPaid = taxAmt,
                        TotalPaid = target
                    }
                }
            };

            Assert.AreEqual(taxAmt, invoice.TotalTax);
            Assert.AreEqual(pretaxTotal, invoice.PretaxTotal);
            Assert.AreEqual(target, invoice.PosttaxTotal);
            Assert.AreEqual(quantity, invoice.InvoiceItems.First().Quantity);
            Assert.AreEqual(pretaxTotal, invoice.InvoiceItems.First().BasePrice);
            Assert.AreEqual(discountedAmt, invoice.InvoiceItems.First().DiscountedPrice);
            Assert.AreEqual(taxAmt, invoice.InvoiceItems.First().TaxPaid);
            Assert.AreEqual(target, invoice.InvoiceItems.First().TotalPaid);

            db.Invoices.Add(invoice);
            db.SaveChanges();

            //need to detach and reload the item into memory to actually verify
            //if we don't detach, .First() will just reuse the above invoice object.
            var odb = ((IObjectContextAdapter)db).ObjectContext;
            odb.Detach(invoice.InvoiceItems.First());
            odb.Detach(invoice);

            var dbInvoice = db.Invoices
                .Include(i => i.InvoiceItems)
                .First(i => i.Id == invoice.Id);
            Assert.AreEqual(taxAmt, dbInvoice.TotalTax);
            Assert.AreEqual(pretaxTotal, dbInvoice.PretaxTotal);
            Assert.AreEqual(target, dbInvoice.PosttaxTotal);
            Assert.AreEqual(quantity, dbInvoice.InvoiceItems.First().Quantity);
            Assert.AreEqual(pretaxTotal, dbInvoice.InvoiceItems.First().BasePrice);
            Assert.AreEqual(discountedAmt, dbInvoice.InvoiceItems.First().DiscountedPrice);
            Assert.AreEqual(taxAmt, dbInvoice.InvoiceItems.First().TaxPaid);
            Assert.AreEqual(target, dbInvoice.InvoiceItems.First().TotalPaid);

            db.InvoiceItems.Remove(dbInvoice.InvoiceItems.First());
            db.Invoices.Remove(dbInvoice);
            db.SaveChanges();
        }
    }
}