Interview task - SOLID Principle and TDD

I had a small technical C# task using SOLID principle and TDD. But I failed to prove my coding skill through the test.

Can anyone offer any small advice for me? I am really eager to learn what my faults are and how I can improve.

The interview question is here, and my answer is here.

I just reference here some pieces of my source code.

Program.cs

class Program
{
    static void Main(string[] args)
    {

        ////////////////////////////////////////////////////////////////////////////////
        // Story 1. As a donor
        Console.WriteLine("-------------------------------------- Story1\r\n");

        //1.1. Craete Tax Calculator
        decimal taxRate = 20;
        ITaxCalculator taxCalculator = new TaxCalculator(taxRate);

        //1.2. Display GiftAid
        Console.WriteLine("Please enter donation amount:");
        decimal Amount = decimal.Parse(Console.ReadLine());
        Console.WriteLine("=> Your Gift Aid is: {0} \r\n\r\n", taxCalculator.GetGiftAid(Amount));

        ////////////////////////////////////////////////////////////////////////////////
        // Story 2. As a site administrator
        Console.WriteLine("-------------------------------------- Story2\r\n");

        //2.1. Create using factory pattern
        taxCalculator = new TaxCalculatorFactory().GetObject(new TaxRepository());

        Console.WriteLine("Please enter donation amount:");
        Amount = decimal.Parse(Console.ReadLine());

        //2.2. Display GiftAid
        Console.WriteLine("=> Your Gift Aid is: {0} \r\n\r\n", taxCalculator.GetGiftAid(Amount));

        Console.WriteLine("Press any key to exit.");
        Console.ReadLine();
    }
}

Calculator Classes

```
interface ITaxCalculatorFactory
{
ITaxCalculator GetObject(decimal taxRate);
ITaxCalculator GetObject(ITaxRepository taxRepo);
}

public class TaxCalculatorFactory
{
public TaxCalculatorFactory()
{
}
public ITaxCalculator GetObject(decimal dAmount)
{
return

-
The interview defines 4 user stories but you have only really implemented 2 as far as I can see.

-
Story 3 says that the amount should be rounded to 2 decimal places. You have done that by applying the rounding to the result as part of a unit test which misses the point of the story (imho). The story says something about "should be displayed rounded to two decimal places" so it might need a tax calculator view which displays the results as per requirements.

-
You should avoid having two ways of creating your TaxCalculator objects. If you have a factory then you usually don't want the user to instantiate new objects by new. If I were to use your class I would not know which way to use and if there is a difference or not.

-
dAmount smells like hungarian notation of which I'm not a fan of and which is generally frowned upon.

-
I would prefer Create over GetObject as method name in the factory.

-
Remove the GetObject(decimal amount) factory method - you can pass a mock repository in unit tests which always returns a fixed amount.

StackExchange Code Review Q#41834, answer score: 16