void Fill(int x, int y, int fillId, int replaceId = 0)
{
    List<Point> points = new List<Point>(); // Point is from System.Drawing & lets you remember x's/y's.
    Block[, ,] map;                         // replace with whatever method you use to read the map.
                                            //   It needs to contain all the blocks in the room.
                                            //   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 (map[x, y, 0].Id == replaceId)
        {
            Build(fillId, x, y);

            map[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);
            }
        }

        // Where there is replaceId, draw and add a point. Update the map and it becomes exponentially faster.
        UpdatePoint(x + 1, y, fillId, replaceId, map, points);
        UpdatePoint(x - 1, y, fillId, replaceId, map, points);
        UpdatePoint(x, y + 1, fillId, replaceId, map, points);
        UpdatePoint(x, y - 1, fillId, replaceId, map, points);

        // You just added a few points to the list, generally 2-3.
        // Return, select a random one, delete it, and repeat until there are none left.
        // What you end up with in the List<Point> points is all the points that have not been checked, aka the ones on the front line.
    } while (points.Count > 0); // Once the list is empty, it is done.
} // end Fill function

void UpdatePoint(int x, int y, int fillId, int replaceId, out Block[, ,] map, out List<Point> points)
{
    if (map[x, y, 0].Id == replaceId)
    {
        Build(fillId, x, y);

        map[x, y, 0] = new Block(id, x, y);

        points.Add(new Point(x, y));
    }
}

void Build(int id, int x, int y)
{
    _
    // Insert whatever build method you use.
}