int x = _, y = _; // replace _ with the start x and y. I used the player's coordinate in my own code.
int replaceId = _, id = _; // replaceId is the only block Id it will replace, and it will be replaced with `id`.
List<Point> points = new List<Point>(); // Point is from System.Drawing & lets you remember x's/y's.

Block[, ,] tempMap; // replace with whatever method you use to read the map.
// It needs to contain all the blocks in the room. Or, at least the ones you are using.
// I use a three-dimensional array of Block objects. Blocks are found by typing Map[x,y,layer].

do // using a do-while instead of a while because the list starts at 0 and it wouldn't execute.
{
    // if you're on the replace id, replace it and remove it from the list.
    if (tempMap[x, y, 0].Id == replaceId)
    {
        Build(id, x, y);
        tempMap[x, y, 0] = new Block(id, x, y);;

        for (int ind = 0; ind < points.Count; ind++)
        {
            if (points[ind].X == x && points[ind].Y == y)
            {
                points.RemoveAt(ind);
            }
        }
    }
    else // If you're not on the replace id, you're on the border. Select a random point and remove it from the list.
    {
        var RanPoint = points[MyRandom.Next(points.Count)]; // Do not use `new Random` here. Do it outside the loop.
        x = RanPoint.X;
        y = RanPoint.Y;

        for (int ind = 0; ind < points.Count; ind++)
        {
            if (points[ind].X == x && points[ind].Y == y)
            {
                points.RemoveAt(ind);
            }
        }
    }

    // Move to the right.
    x++;

    if (tempMap[x, y, 0].Id == replaceId) // If it's on the replace id, replace it and add it to the list.
    {
        b.Push.Build(id, x, y);
        tempMap[x, y, 0] = new Block(id, x, y);;
        points.Add(new Point(x, y));
    }

    // Moooove to the left.
    x -= 2;

    if (tempMap[x, y, 0].Id == replaceId)
    {
        b.Push.Build(id, x, y);
        tempMap[x, y, 0] = new Block(id, x, y);;
        points.Add(new Point(x, y));
    }

    // 2 hops this time (go one space above the original)
    x++;
    y++;

    if (tempMap[x, y, 0].Id == replaceId)
    {
        b.Push.Build(id, x, y);
        tempMap[x, y, 0] = new Block(id, x, y);;
        points.Add(new Point(x, y));
    }

    // go one space below the original
    y -= 2;

    if (tempMap[x, y, 0].Id == replaceId)
    {
        b.Push.Build(id, x, y);
        tempMap[x, y, 0] = new Block(id, x, y);;
        points.Add(new Point(x, y));
    }

    y++;

    // You just added a few points to the list. Return, select a random one, delete it, and repeat until there are none left.
    // Often times it will select a point that is already surrounded with dots, making the process much quicker.
} while (points.Count > 0);