public async Task User1_transfer_to_both_users_at_same_time_method_1()
        {
            var contextCreator = new TestingDbContextCreator("User1_transfer_to_both_users_at_same_time_method_1");
            var context        = contextCreator.CreateSimpleBankDbContextForTesting();
            // Arrange the data for the test
            decimal user1Balance = 10000; decimal user2Balance = 100; decimal user3Balance = 1000;

            await this.CreateUserTask("user1", "12345", user1Balance, context);

            await this.CreateUserTask("user2", "12345", user2Balance, context);

            await this.CreateUserTask("user3", "12345", user3Balance, context);

            var context1 = contextCreator.GetFreshDbContext();
            var context2 = contextCreator.GetFreshDbContext();

            var user1InContext1 = context1.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "user1");
            var user2InContext1 = context1.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "user2");
            var user1InContext2 = context2.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "user1");
            var user3InContext2 = context2.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "user3");

            // Act
            // initialize the transfer in 2 different dbContext
            var transferInContext1 = new UserService(context1).Transfer(
                user1InContext1.ID, user2InContext1.ID,
                900,
                user1InContext1.Timestamp, user2InContext1.Timestamp);

            var transferInContext2 = new UserService(context2).Transfer(
                user1InContext2.ID, user3InContext2.ID,
                1000,
                user1InContext2.Timestamp, user3InContext2.Timestamp);

            // exec 2 transfer in parallel
            var allTasks = await Task.WhenAll(transferInContext1, transferInContext2);

            // Assert
            // assert there is existing ErrorList in allTasks
            Assert.True(allTasks.Any(x => x.ErrorList.Count > 0));
            // create new dbContext to query latest data in database
            using (var assertContext = contextCreator.GetFreshDbContext())
            {
                var updatedUser1 = assertContext.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "user1");
                var updatedUser2 = assertContext.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "user2");
                var updatedUser3 = assertContext.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "user3");

                // assert the balance of user1 is 9100 (if transfer1 is success) OR 9000 (if transfer2 is success)
                Assert.True(updatedUser1.Balance == 9100 || updatedUser1.Balance == 9000);
                // assert the balance of user2 is 1000 AND balance of user3 is 1000 if the transferInContext1 is success
                // OR assert the balance of user2 is 100 AND balance of user3 is 2000 if the transferInContext2 is success
                Assert.True((updatedUser2.Balance == 1000 && updatedUser3.Balance == 1000) ||
                            (updatedUser2.Balance == 100 && updatedUser3.Balance == 2000));
            }
        }
        public async Task User1_transfer_to_both_users_at_same_time_method_2()
        {
            var contextCreator = new TestingDbContextCreator("User1_transfer_to_both_users_at_same_time_method_2");
            var context        = contextCreator.CreateSimpleBankDbContextForTesting();
            // Arrange the data for the test
            decimal userABalance = 10000; decimal userBBalance = 100; decimal userCBalance = 1000;

            await this.CreateUserTask("userA", "6789", userABalance, context);

            await this.CreateUserTask("userB", "6789", userBBalance, context);

            await this.CreateUserTask("userC", "6789", userCBalance, context);

            var context1 = contextCreator.GetFreshDbContext();
            var context2 = contextCreator.GetFreshDbContext();

            // create context1 data
            var userAInContext1 = context1.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "userA");
            var userBInContext1 = context1.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "userB");
            // create context2 data
            var userAInContext2 = context2.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "userA");
            var userCInContext2 = context2.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "userC");

            // Act
            // exec 2 transfer in parallel
            ConcurrentDictionary <string, Task <BankUserUpdateModel> > resultInParallel = new ConcurrentDictionary <string, Task <BankUserUpdateModel> >();

            Parallel.Invoke(
                () =>
            {
                var transfer = new UserService(context1).Transfer(
                    userAInContext1.ID, userBInContext1.ID,
                    900,
                    userAInContext1.Timestamp, userBInContext1.Timestamp);
                resultInParallel.TryAdd("transferInContext1", transfer);
            },
                () =>
            {
                var transfer = new UserService(context2).Transfer(
                    userAInContext2.ID, userCInContext2.ID,
                    1000,
                    userAInContext2.Timestamp, userCInContext2.Timestamp);
                resultInParallel.TryAdd("transferInContext2", transfer);
            });

            // Assert
            // assert there is existing ErrorList in resultInParallel
            Assert.True(resultInParallel.Any(x => x.Value.Result.ErrorList.Count > 0));
            // create new dbContext to query latest data in database
            using (var assertContext = contextCreator.GetFreshDbContext())
            {
                var updatedUserA = assertContext.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "userA");
                var updatedUserB = assertContext.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "userB");
                var updatedUserC = assertContext.BankUsers.AsNoTracking().SingleOrDefault(x => x.AccountNumber == "userC");

                // assert the balance of userA is 9100 (if transfer1 is success) OR 9000 (if transfer2 is success)
                Assert.True(updatedUserA.Balance == 9100 || updatedUserA.Balance == 9000);
                // assert the balance of userB is 1000 AND balance of userC is 1000 if the transferInContext1 is success
                // OR assert the balance of userB is 100 AND balance of userC is 2000 if the transferInContext2 is success
                Assert.True((updatedUserB.Balance == 1000 && updatedUserC.Balance == 1000) ||
                            (updatedUserB.Balance == 100 && updatedUserC.Balance == 2000));
            }
        }