Sandsim/ConsoleApp1/Program.cs

using System;
using System.Threading;

const int width = 50;
const int height = 10;

const int Air = 0;
const int Sand = 1;

int[] grid = new int[width * height];
int spawnX = width / 2;
int spawnY = 0;
grid[spawnY * width + spawnX] = Sand;

while (true)
{
    // INPUT:
    while (Console.KeyAvailable)
    {
        var key = Console.ReadKey(intercept: true).Key;
        if (key == ConsoleKey.Spacebar)
        {
            int spawnIndex = spawnY * width + spawnX;
            if (grid[spawnIndex] == Air)
                grid[spawnIndex] = Sand;
        }
        if (key == ConsoleKey.RightArrow) spawnX += 1;
        if (key == ConsoleKey.LeftArrow) spawnX -= 1;

        spawnX = Math.Clamp(spawnX, 0, width - 1);

    }

    // UPDATE:
    Console.Clear();
    for (int y = height - 2; y >= 0; y--)
    {
        int row = y * width;
        int rowBelow = (y + 1) * width;

        bool leftToRight = (y & 1) == 0;
        int xStart = leftToRight ? 0 : width - 1;
        int xEnd   = leftToRight ? width : -1;
        int xStep  = leftToRight ? 1 : -1;

        for (int x = xStart; x != xEnd; x += xStep) 
        {
            int idx = row + x;
            if (grid[idx] != Sand) continue;

            int downIdx = rowBelow + x;
            if (grid[downIdx] == Air)
            {
                grid[downIdx] = Sand;
                grid[idx] = Air;
                continue;
            }

            bool canLeft  = x > 0;
            bool canRight = x + 1 < width;

            int downLeftIdx  = rowBelow + (x - 1);
            int downRightIdx = rowBelow + (x + 1);

            bool emptyLeft  = canLeft  && grid[downLeftIdx]  == Air;
            bool emptyRight = canRight && grid[downRightIdx] == Air;

            if (!emptyLeft && !emptyRight) continue;

            bool goLeft = emptyLeft && (!emptyRight || ((x + y) & 1) == 0);
            int target = goLeft ? downLeftIdx : downRightIdx;

            grid[target] = Sand;
            grid[idx] = Air;
        }
    }


    // RENDER:
    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x++)
        {
            int v = grid[y * width + x];
            char c = v == Sand ? '#' : '.';
            Console.Write(c);
        }
        Console.WriteLine();
    }

    Thread.Sleep(25);
}