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Although bugs are probably the most obvious type of error, we won t actually be dealing with them directly in this chapter. We will, however, look at how our error-handling techniques can make it easier (or harder!) to find the bugs that are often the cause of the other, better defined issues. Let s get started with an example we can use to look at error-handling techniques. We re going to branch out into the world of robotics for this one, and build a turtle-controlling application. The real-world turtle is a rectangular piece of board on which are mounted two motors that can drive two wheels. The wheels are located in the middle of the left and right edges of the board, and there are nondriven castor wheels at the front and back to give it a bit of stability. We can drive the two motors independently: we can move forward, move backward, or stop. And by moving the wheels in different directions, or moving one wheel at time, we can steer it about a bit like a tank. Let s create a class to model our turtle (see Example 6-1).

class Turtle { // The width of the platform public double PlatformWidth { get; set; } // The height of the platform public double PlatformHeight { get; set; } // The speed at which the motors drive the wheels, // in meters per second. For ease, we assume that takes account // of the distance traveled by the tires in contact // with the ground, and any slipping public double MotorSpeed { get; set; } // The state of the left motor public MotorState LeftMotorState { get; set; }

// The state of the right motor public MotorState RightMotorState { get; set; } // The current position of the turtle public Point CurrentPosition { get; private set; } // The current orientation of the turtle public double CurrentOrientation { get; private set; }

}

A property is a member that represents an item of data in a class instance or class. Using a property appears very much like writing to, or reading from, a field. The syntax is the same. For example, the following code shows the use of a class called MyClass that has both a public field and a public property. From their usage, you cannot tell them apart. MyClass mc = new MyClass(); mc.MyField = 5; mc.MyProperty = 10; // Assigning to a field // Assigning to a property

// The current state of a motor enum MotorState { Stopped, Running, Reversed }

In addition to the motor control, we can define the size of the platform and the speed at which the motors rotate the wheels. We also have a couple of properties that tell us where the turtle is right now, relative to its point of origin, and the direction in which it is currently pointing. To make our turtle simulator actually do something, we can add a method which makes time pass. This looks at the state of the different motors and applies an appropriate algorithm to calculate the new position of the turtle. Example 6-2 shows our first, somewhat naive, go at it.

// Run the turtle for the specified duration public void RunFor(double duration) { if (LeftMotorState == MotorState.Stopped && RightMotorState == MotorState.Stopped) { // If we are at a full stop, nothing will happen return; } // The motors are both // then we just drive if ((LeftMotorState == RightMotorState == (LeftMotorState == running in the same direction MotorState.Running && MotorState.Running) || MotorState.Reversed &&

RightMotorState == MotorState.Reversed)) Drive(duration); return; The motors are running in opposite directions, so we don't move, we just rotate about the center of the rig ((LeftMotorState == MotorState.Running && RightMotorState == MotorState.Reversed) || (LeftMotorState == MotorState.Reversed && RightMotorState == MotorState.Running)) Rotate(duration); return;

WriteLine("{0} {1}", mc.MyField, mc.MyProperty); // Read field and property A property, like a field, has the following characteristics: It is a named class member. It has a type. It can be assigned to and read from. Unlike a field, however, a property is a function member. It does not allocate memory for data storage! It executes code. A property is a named set of two matching methods called accessors. The set accessor is used for assigning a value to the property. The get accessor is used for retrieving a value from the property. Figure 6-7 shows representations of a property. The code on the left shows the syntax of declaring a property named MyValue, of type int. The figure on the right shows how I will graphically represent properties in this text. The accessors are shown sticking out the back, because, as you shall soon see, they are not directly callable.

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