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Source Code

The CarEvents project is located under the Chapter 11 subdirectory.

A “Prim-and-Proper” Event

Truth be told, there is one final enhancement we could make to the current iteration of the Car type

that mirrors Microsoft’s recommended event pattern. As you begin to explore the events sent by a

given type in the base class libraries, you will find that the first parameter of the underlying delegate

is a System.Object, while the second parameter is a type deriving from System.EventArgs.

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The System.Object argument represents a reference to the object that sent the event (such as

the Car), while the second parameter represents information regarding the event at hand. The

System.EventArgs base class represents an event that is not sending any custom information:

public class EventArgs

{

public static readonly System.EventArgs Empty;

public EventArgs();

}

For simple events, you can pass an instance of EventArgs directly. However, when you wish to

pass along custom data, you should build a suitable class deriving from EventArgs. For our example,

assume we have a class named CarEventArgs, which maintains a string representing the message

sent to the receiver:

public class CarEventArgs : EventArgs

{

public readonly string msg;

public CarEventArgs(string message)

{

msg = message;

}

}

With this, we would now update the CarEventHandler delegate as follows (the events would be

unchanged):

public class Car

{

public delegate void CarEventHandler(object sender, CarEventArgs e);

...

}

Here, when firing our events from within the Accelerate() method, we would now need to

supply a reference to the current Car (via the this keyword) and an instance of our CarEventArgs

type. For example, consider the following update:

public void Accelerate(int delta)

{