自定义绘制方法不能正常工作

我想尝试一下《俄罗斯方块》的克隆，但我遇到了一个问题。我有一个Tetromino类，在它的Draw()方法，以使事情更容易时绘制每一块屏幕。然而，它不能正常工作，它似乎画不出任何东西，除非我删除graphics.Clear(Color.Black)线，这当然会导致混乱的结果(因为绘制表面并没有清除游戏循环的每次迭代)。我传递图形对象作为参数在我的方法。我错过什么了吗?此外，我使用自定义Timer类来提高准确性，但我认为这应该无关紧要。

Game.cs:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace Tetris
{
    public partial class Game : Form
    {
        private Timer timer;
        private Graphics graphics;
        private List<Tetromino> tetrominos;
        private static readonly Random random = new Random();
        private const int SCREEN_WIDTH = 300;
        private const int SCREEN_HEIGHT = 600;
        public Game()
        {
            InitializeComponent();
            timer = new Timer();
            this.Text = "Tetris";
            this.DoubleBuffered = true;
            this.ClientSize = new Size(SCREEN_WIDTH, SCREEN_HEIGHT);
            this.FormBorderStyle = FormBorderStyle.FixedSingle;
            this.StartPosition = FormStartPosition.CenterScreen;
            this.MaximizeBox = false;
            graphics = this.CreateGraphics();
            tetrominos = new List<Tetromino>();
        }
        public void GameLoop()
        {
            // This is here just for testing.
            int tetromino = random.Next(0, 6);
            tetrominos.Add(new Tetromino((Tetromino.Tetrominos)tetromino));
            while (this.Created)
            {
                timer.Reset();
                GameUpdate();
                GameDraw();
                Application.DoEvents();
                while (timer.GetTicks() < 33) ;
            }
        }
        private void GameUpdate()
        {
            for (int i = 0; i < tetrominos.Count; i++)
                tetrominos[i].Offset(0, 5);
        }
        private void GameDraw()
        {
            graphics.Clear(Color.Black);
            for (int i = 0; i < tetrominos.Count; i++)
                tetrominos[i].Draw(graphics);
        }
    }
}

Tetromino.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Drawing;
namespace Tetris
{
    class Tetromino
    {
        private List<Rectangle> _tetromino;
        private Brush brush;
        public Tetromino(Tetrominos tetromino)
        {
            _tetromino = new List<Rectangle>();
            switch (tetromino)
            {
                //---[][][][]---
                case Tetrominos.I:
                    brush = Brushes.Cyan;
                    _tetromino.Add(new Rectangle(90, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X + 30, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[1].X + 30, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[2].X + 30, -60, 30, 30));
                    break;
                //----[][]----
                //----[][]----
                case Tetrominos.O:
                    brush = Brushes.Yellow;
                    _tetromino.Add(new Rectangle(120, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X + 30, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X + 30, -30, 30, 30));
                    break;
                //---- [] -----
                //---[][][]----
                case Tetrominos.T:
                    brush = Brushes.Purple;
                    _tetromino.Add(new Rectangle(120, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X - 30, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[1].X + 30, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[2].X + 30, -30, 30, 30));
                    break;
                //---- [][]----
                //---[][] -----
                case Tetrominos.S:
                    brush = Brushes.Green;
                    _tetromino.Add(new Rectangle(120, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X + 30, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X - 30, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X, -30, 30, 30));
                    break;
                //---[][] -----
                //---- [][]----
                case Tetrominos.Z:
                    brush = Brushes.Red;
                    _tetromino.Add(new Rectangle(90, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X + 30, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X + 30, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[2].X + 30, -30, 30, 30));
                    break;
                //---[]  ------
                //---[][][]----
                case Tetrominos.J:
                    brush = Brushes.Blue;
                    _tetromino.Add(new Rectangle(90, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X + 30, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[2].X + 30, -30, 30, 30));
                    break;
                //-----  []----
                //---[][][]----
                case Tetrominos.L:
                    brush = Brushes.Orange;
                    _tetromino.Add(new Rectangle(150, -60, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[0].X - 60, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[1].X + 30, -30, 30, 30));
                    _tetromino.Add(new Rectangle(_tetromino[2].X + 30, -30, 30, 30));
                    break;
            }
        }
        public void Offset(int x, int y)
        {
            for (int i = 0; i < _tetromino.Count; i++)
                _tetromino[i] = new Rectangle(_tetromino[i].X + x, _tetromino[i].Y + y, 30, 30);
        }
        public void Draw(Graphics graphics)
        {
            for (int i = 0; i < _tetromino.Count; i++)
                graphics.FillRectangle(brush, _tetromino[i]);
        }
        public void Rotate()
        {
            // TO-DO: Implement
        }
        public enum Tetrominos { I, O, T, S, Z, J, L }
    }
}

Timer.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;
namespace Tetris
{
    class Timer
    {
        [DllImport("kernel32.dll")]
        private static extern long GetTickCount();
        private long startTick;
        public Timer() { Reset(); }
        public void Reset() { startTick = 0; }
        public long GetTicks() => GetTickCount() - startTick;
    }
}