如何检查字符串.net中某个位置的字符

Question

In my project the customer have a card and a 7 letter particular security code is in it. I want to ask 3 letters from that security code by position.

eg. card security code is  **57GHY58**

I want to ask what is the Character at 2,4 and 7 position in your security code ?

answer is  **7H8**

How to generate that question with random postion and how to check it ?

Answer 1

private static int[] GetThreeRandomNumbers()
{
    List<int> list = new List<int>();
    Random r = new Random();
    while (list.Count < 3)
    {
        int num = r.Next(1, 7);
        if (!list.Contains(num))
        {
            list.Add(num);
        }
    }
    list.Sort();
    return list.ToArray();
}

Answer 2

You have a string with indices 0-6. You need to pick 3 indices from that range randomly. The Random class will help you with this, have a look at its method Random.Next(int, int) , it will return you a random number from the specified range. Then the only other thing you have to do is skip the indices you have already used.