} // end if

}// end btnYesNo

I started by creating an instance of the DialogResult class. When I investigated the Show() method of the MessageBox class, I discovered that it returned an instance of the DialogResult class. The reply variable is intended to catch whatever result comes back after the message box communicates with the user.

The Show() method of the MessageBox class is heavily overloaded. It has 12 variations! I used one with four parameters because it looked as though this version would provide all the details I needed.

In this four−parameter version of the show command, the first parameter is the message you want to ask the user. The next parameter is the title of the box that appears with the message inside it. (Users almost always overlook this title, but being able to change it is still nice.) The third parameter is a special value that determines which buttons to show, and the fourth parameter is another special value that describes the icon to display. Both these last values are enumerations.

An enumeration, as you may recall from earlier in this chapter, is a predefined list of values. The color constants are an example of an enumeration. In fact, any property that displays a drop−down menu in the Form Designer is likely associated with an enumeration. The MessageBoxButtons enumeration contains a list of all the possible button combinations. (Look in the online documentation to see all the choices.) Likewise, the MessageBoxIcons enumeration contains a list of all the possible icons you can place in a message box. When you understand how they work, you can usually use the syntax completion feature of the IDE to figure out which buttons and icons you want, without having to consult the online help each time.

Retrieving Values from the MessageBox

If you have more than one button on a message box, it is important to determine which button the user pressed. The DialogResults object is the key to figuring out the user’s response. DialogResults has its own enumeration built in, describing all the various buttons that could have been pressed by the user. To figure out which button was pressed, simply compare the reply variable to the possible options in the DialogResults enumeration.

Coding the Lunar Lander

Building the Lunar Lander game simply requires putting together all the various elements in a new form. The program runs under a timer’s control. All user interaction happens through keyboard input, and the animations use images copied from an image list.

The Visual Design

I started by sketching out the visual design and the overall plan for the game. The visual interface is very simplistic. I wanted one ship, named picLander, and one landing platform, named picPlatform. Additionally, I added a series of labels to communicate various game variables to the user. Each of these labels is named to correspond to a specific variable. I’ll describe them in more detail in the ShowStats() method because I didn’t add them to the interface until I wrote that method.

In addition to the visual elements, I added a couple invisible controls. A timer control handles all the interval−based elements (which take up the bulk of the code), and I used an image list to support the variations of the lander spouting flames in different directions. The image list includes four

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images of the lander:

1.No flames

2.Flames on bottom

3.Flames on left

4.Flames on right

The Designer−Generated Code

Generally, I have not shown you the code generated by the Designer, but I show it to you here because I added a few elements. I’ll explain my modifications after the code listing.

using System;

using System.Drawing; using System.Collections;

using System.ComponentModel; using System.Windows.Forms; using System.Data;

namespace Lander

{

///<summary>

///Basic Arcade Game

///Demonstrates simple animation and keyboard controls

///and use of timer.

///Andy Harris, 1/16/02

///</summary>

public class theForm : System.Windows.Forms.Form

{

//my variables private double x, y; private double dx, dy;

private int fuel = 100; private int ships = 3; private int score = 0;

//will show new position of lander //difference in x and y

//how much fuel is left //number of ships player has //the player's current score

//created by designer

private System.Windows.Forms.Timer timer1;

private System.Windows.Forms.PictureBox picPlatform; private System.Windows.Forms.Panel pnlScore;

private System.Windows.Forms.Label lblDx; private System.Windows.Forms.Label lblDy; private System.Windows.Forms.Label lblShips; private System.Windows.Forms.Label lblFuel;

private System.Windows.Forms.PictureBox picLander; private System.Windows.Forms.ImageList myPics; private System.Windows.Forms.Label lblScore;

private System.ComponentModel.IContainer components;

public theForm()

{

//

// Required for Windows Form Designer support

//

InitializeComponent();

//I added this call, to a method that starts up a round initGame();

}

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///<summary>

///Clean up any resources being used.

///</summary>

protected override void Dispose( bool disposing )

{

if( disposing )

{

if (components != null)

{

components.Dispose();

}

}

base.Dispose( disposing );

}

#region Windows Form Designer generated code

///<summary>

///The main entry point for the application.

///</summary>

[STAThread]

static void Main()

{

Application.Run(new theForm());

}

The Designer−written code shows its usual lack of flair but is functional. Note that I hid the InitializeComponent() method call because you shouldn’t generally mess with it if you are using the Designer. I added several small but important things to the first chunk of code. I included a number of class−level variables and made a small modification to the constructor.

Class−Level Variables

Variables defined at the class level can be regarded as the DNA of an object. You can learn a lot about how the object works by understanding these variables, especially if the program is well written and documented. If these conditions are met, you can see an overview of the entire program’s structure by looking at the variables.

Several of the variables (x, y, dx, and dy) are used to position the lander on the screen. You might be surprised to see that all four of these variables are doubles, rather than the integers you’ve used throughout the chapter for positioning components. The Point and Location classes that form the basis of screen motion require integers, but when I was testing the game, I found that integers did not give me the fine−grained control that I wanted. In particular, the effects of gravity were too difficult to get right, within the limits of integers. I decided to do my calculations with doubles and then convert the values to integers when needed. (Don’t panic. I’ll explain this as it comes up in the code listing.)

The fuel, ships, and score variables are used in scorekeeping and to determine when the game is over. All are integers.

Trick Notice the comments after all the variables. Documenting all your important variables in this way is an excellent habit to form. You’ll find that the effort pays off when your program becomes complicated and you don’t remember what each of your cryptic variable names stands for.

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The Constructor

You might recall that a constructor is a special method that helps build an instance of a class. It always has the same name as the class, and is automatically called when the class is created. In this case, the constructor is a method named theForm(). The Designer automatically created this constructor and added a call to the InitializeComponent() method (which it also created automatically). However, I also wanted something to happen the first time the program started. I wanted to set up all the initial conditions for the game. The instructions for this are stored in the initGame() method, which you’ll investigate shortly.

Trap Sometimes you might be intimidated by the warnings not to change code created by the Designer. Although you certainly should be careful about doing this, you are the programmer. There is nothing wrong with adding your own code to a method generated by the Designer. In fact, it’s often necessary to do so. Still, you should avoid changing InitializeComponent unless you’re willing to finish writing the program without the assistance of the Designer. Changing the contents of that particular method can make it impossible for the Designer to read your code.

The timer1_Tick() Method

As you’ve seen throughout this chapter, much of the action in an arcade−style game happens in the tick method of the timer. The Lander game reinforces this observation.

private void timer1_Tick(object sender, System.EventArgs e) { //code that should happen on every timer tick

(10 times/sec)

//account for gravity dy+= .5;

//increment score for being alive score += 100;

//show ordinary (no flames) lander picLander.Image = myPics.Images[0];

//call helper methods to handle details moveShip();

checkLanding();

showStats();

} // end timer tick

This method does a great deal of work. All the main logic for the game flows through this method 10 times a second. However, many of the most critical elements are passed off to other methods.

First, I added .5 to dy to account for gravity. Each time the timer ticks, there will be a small force pulling the lander downwards. The exact amount for dy is a tricky thing to determine. This was the main reason I used doubles for the math. When dy had to be a whole number, gravity of 1 was just too powerful at 10 times per second, and 0 gave no gravity at all. One solution to the "heaviness" of a gravity of 1 is to lower the frame rate by changing the timer’s interval. When I tried this, the animation seemed too choppy. I decided, instead, to work with double values and then convert back to integers when needed. Sometimes you have to think creatively to get the results you want.

Then next thing I did was add a value to the score simply because the user survived another tick of the clock. It’s a long−standing tradition in arcade games never to add fewer than 100 points to the

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