public class Class1
{
    private int counter = 0;                    // this variable tracks where in the array we are.
    private int[] numberArray = new int[10];

    // this just fills out the number array with 0-9
    public void setup()
    {
        for (int i = 0; i < 10; i++)
        {
            numberArray[i] = i;
        }
    }

    // this is where the magic actually happens
    public void add()
    {
        // here is where you would get some user input etc
        int userInput = int.Parse(Console.ReadLine());

        // and this is where we use that input.
        numberArray[counter] = userInput;

        // and then we increase the counter variable so that it references the next member of the array
        counter++;
    }
}