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It is quite possible to derive one class from another in a way that means we can t treat the derived class as its base type. The derived class could change the meaning or behavior of a function with the same signature as its base, or throw errors in situations where the base promised that everything would be fine say, the base accepted parameters in the range 1 10, where the derived class accepts parameters in the range 2 5. This violates the LSP, which is a very poor design practice, but it is very easy to slip into, especially if the classes evolve independently over time.

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What happens if our client doesn t know that Harry is a fire chief, though What if we refer to the object via a reference typed to Firefighter instead

FireChief bigChiefHarry = new FireChief { Name = "Harry" }; // Another reference to Harry, but as a firefighter Firefighter stillHarry = bigChiefHarry; Firefighter joe = new Firefighter { Name = "Joe" }; stillHarry.TellFirefighterToExtinguishFire(joe);

You know that stillHarry is referencing an object that is a FireChief, with that extra method on it. But the compiler produces a long, semicomprehensible error full of useful suggestions if you try to compile and execute this code:

Note I refer to the device s coordinates as global because they are shared between all painters working

'Firefighter' does not contain a definition for 'TellFirefighterToExtinguishFire' and no extension method 'TellFirefighterToExtinguishFire' accepting a first argument of type 'Firefighter' could be found (are you missing a using directive or an assembly reference )

The compiler is being rather tactful. It is assuming that you must ve forgotten to include some external reference that s got a suitable extension method definition to fix your problem. (We ll be looking at that technique in a later chapter, by the way.) Unfortunately, the real reason for our bug is hidden in the error s opening salvo: we re trying to talk to a FireChief method through a variable that is strongly typed to be a Firefighter, and you can t call on any members of the derived class through a reference typed to a base. So, if we can t use a derived member from a reference to a base type, is there any way we can refine these classes so that Harry never puts out a fire, but always passes responsibility to his Number One when he s asked to do so, regardless of whether we happen to know that he s a FireChief After all, he knows that he s the boss! To get started, we ll have to make a few changes to the model to accommodate this idea of the chief s Number One. In other words, we need to create an association between the FireChief and his NumberOne. Remember that we typically implement this as a read/write property, which we can add to the FireChief:

public Firefighter NumberOne { get; set; }

on the device (and widgets, if the device happens to be a screen). Each painter is then transformed to a point relevant to its purpose. Other commonly used notations are physical device coordinates and logical local coordinates.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" > <head><title> Untitled Page </title></head> <body> <form name="aspnetForm" method="post" action="Default3.aspx" id="aspnetForm"> <div> <input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwULLTEwMDUyNjYzMjhkZERQ3hQz51DsahcItBSVyAcTtqP7" /> </div>

And let s change the main function so that it does what we want (see Example 4-5).

// A reference to Joe, Harry's number one Firefighter joe = new Firefighter { Name = "Joe" }; // Firefighter harry is really a firechief, with joe as his NumberOne Firefighter harry = new FireChief { Name = "Harry", NumberOne = joe }; // Harry is just a firefighter, so he can extinguish fires // but we want him to get joe to do the work harry.ExtinguishFire();

But if we compile that, here s the output we get:

The coordinate system of a painter can be translated as well (an example of such a translation is shown in Figure 7-18). In the figure, the gray box is what is drawn in relation to the original coordinate system. The coordinate system is transformed through the following call: painter.translate( 30, 30 ); The result is that the rectangle is drawn where the black rectangle is the coordinate system has been shifted to the right and downward.

Harry is putting out the fire!

That s not what we want at all. What we want is a different implementation for that ExtinguishFire method if we re actually a FireChief, rather than an ordinary Firefighter.

So the implementation for the ExtinguishFire method that we want on the FireChief looks like this:

public void ExtinguishFire() { // Get our number one to put out the fire instead TellFirefighterToExtinguishFire(NumberOne); }

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