CreditCardMask.cs

using System;

public class CreditCardMask
{
    // Usually when you buy something, you're asked whether your credit card number,
    // phone number or answer to your most secret question is still correct. However,
    // since someone could look over your shoulder, you don't want that shown on your screen.
    // Instead, we mask it.
    // Your task is to write a function maskify, which changes all but the last four characters into '#'.

    // Examples
    // "4556364607935616" --> "############5616"
    // "64607935616" -->      "#######5616"
    // "1"           -->                "1"
    // ""            -->                 ""

    // "What was the name of your first pet?"
    // "Skippy" --> "##ippy"

    private const int UnMaskCount = 4;

    // return masked string
    public static string Maskify(string cc)
    {
        StringBuilder sb = new StringBuilder(cc);
        for (int i = 0; i < sb.Length - UnMaskCount; ++i)
            sb[i] = '#';

        return sb.ToString();
    }

}