Yes, the classic arcade game has returned and is appearing in C# Corner complete with sound and authentic aliens and source code. It's not quite as good as the real game, but with a few adjustments it can get there. I actually got many of the sounds and images from a web site that is bent on celebrating this traditional ground breaking game: http://spaceinvaders.retrogames.com. This made writing the game a little bit easier since they supplied many of the files in which you can capture the images and sounds. 

 Figure 1 - The Space Invader Game

Note: This article is actually an update of the original space invaders article I posted in 2001. It adds a few minor features such as displaying remaining lives and spiraling bombs.

Playing the Game

This version is played using the left and right arrow keys to move the man and the space bar to fire. If you hit an alien, you get 10, 20, or 30 points, depending on which one you hit. If a bomb hits you, you die and you only have 3 lives. The shields will protect you from a bomb, so you can hide behind them while you wait for the bombs to pass. Occasionally, a saucer flies across the screen. If you hit it, you get a suprise score (up to 150 points). 

Design of the Game

The design for this game is shown below:

Figure 2 - Space Invader UML Design Reverse engineered using WithClass 2000

The Design basically has a base class GameObject which knows how to draw the game pieces bitmap.  Subclassed under this object is all the component classes of the game: Man, Invader, Bullet, Bomb and Shield. A row of Invader classes is controlled through the InvaderRow class. The form contains an array of  InvaderRows, an array of Shields, and a Man.

The Timer Event Handler

The entire game is played off of the timer component. After each tick of the timer, the game checks to see if their are bomb collisions, bullet collisions, and whether or not to move the invaders. It also handles the key presses from the user here so as not to change the timing of the game. If a key is pressed, it is remembered by the key press event, but nothing more happens in the key press event handler.  Below is the code executed with each entry into the timer event handler:

Painting the Game Components

The drawing of all the components is triggered by the Invalidate called in the timer event handler. Below is the paint event handler that paints all of the game elements:

private void Form1_Paint(object sender, System.Windows.Forms.PaintEventArgs e)
{
// get the graphics surface
Graphics g = e.Graphics;
// Draw the entire graphics surface black
g.FillRectangle(Brushes.Black, 0, 0, ClientRectangle.Width, ClientRectangle.Height);
// Draw the man
TheMan.Draw(g);
// Draw the score
TheScore.Draw(g);
// Draw the the number of lives remaining
_livesIndicator.Draw(g);
// Draw the Bullet shot
if (ActiveBullet)
{
TheBullet.Draw(g);
}
// Draw the Saucer
if (SaucerStart)
{
CurrentSaucer.Draw(g);
}
// Draw the Space Invaders
for (int i = 0; i < kNumberOfRows; i++)
{
TheInvaders = InvaderRows[i];
TheInvaders.Draw(g);
}
// Draw the Shields
for (int i = 0; i < kNumberOfShields; i++)
{
Shields[i].Draw(g);
}
}

Listing 2 - The Paint Event Handler in the Form

Spinning Bombs with Graphic Paths

Short of using a bitmap, graphic paths give you a nice way to draw complex shapes with lines and curves.  Graphic paths can also be operated on with matrix transformations so you can easily rotate, flip, scale, and translate a graphics path on the screen.  Listing 3 shows how a bomb is drawn on a screen using the Draw method in the Bomb class. Before the bomb is drawn, however, two graphics paths are created. One path is created for the zig zag bomb and a one graphics path is created for the bombs reflection on the Y axis.  When we draw the bomb in the Draw method we determine which path to draw by an inversion flag.  The inversion flag is toggled each time in the draw routine, so each path is drawn alternately. As the path is flipped back and forth along the y-axis, it looks like the bomb is spinning down towards the ship.

Conclusion:

Although an old arcade game, Space Invaders is still fun to play. GDI+ provides enough graphic power to recreate the game in its entirety. Using bitmaps, graphic paths, simple shapes, and a timer, those invaders are soon moving across the screen and dropping bombs on your unsuspecting ship. Using the old windows multimedia library you can also play wave files and bring back the original sounds and explosions. Space Invaders was designed with OOP in mind, so I hope this article gives you a good basis for thinking about how to design games with objects.  In the meantime, don't let those invaders get the best of you!