Two dimensional array as buffer for screen - Practice Exercises C# Sharp

using System;
public class Array2D
{
    public static void Main()
    {
        char[,] position = new char[20, 70];

        Random generator = new Random();
        int i = 0;

        for (; i < 80; i++)
            position[generator.Next(0, 20), generator.Next(0, 70)] = 'X';

        for (i = 0; i < 20; i++)
        {
            for (int j = 0; j < 70; j++)
                Console.Write(position[i, j]);

            Console.WriteLine();
        }
    }
}