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Replacing Methods in Derived Classes
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So the implementation for the ExtinguishFire method that we want on the FireChief looks like this:
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public void ExtinguishFire() { // Get our number one to put out the fire instead TellFirefighterToExtinguishFire(NumberOne); }
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What happens if we just add that function to our FireChief and compile and run Well, it compiles, but when we run it, it still says:
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Harry is putting out the fire!
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It seems to have completely ignored our new function! Let s go back and have a look at that compiler output again. You ll see that although it built and ran, there s a warning (you may have to rebuild to get it to appear again; Choose Rebuild Solution from the Build menu):
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'FireChief.ExtinguishFire()' hides inherited member 'Firefighter.ExtinguishFire()'. Use the new keyword if hiding was intended.
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It is a good idea to leave all your compiler warnings on and work until you are both error and warning free. That way, when something crops up unexpectedly like this, you can spot it easily, rather than burying it in a pile of stuff you re habitually ignoring.
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It is telling us that, rather than replacing the implementation on the base class, our method (with matching signature) is hiding it; and that if this is what we really meant to do, we should add the keyword new to the method.
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Hiding Base Members with new
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OK, let s do that:
public new void ExtinguishFire() { // Get our number one to put out the fire instead TellFirefighterToExtinguishFire(NumberOne); }
We typically add the new modifier between the accessibility modifier and the return value.
Compile and run again. You ll notice that we ve gotten rid of the warning, but the output hasn t changed:
Harry is putting out the fire!
What s going on This method-hiding approach is actually letting a single object provide different implementations for the ExtinguishFire method. The implementation we get is based on the type of the variable we use, rather than the type of object to which the variable refers. You can see that happening if we use the code in Example 4-6 in our client.
// 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 FireChief harry = new FireChief { Name = "Harry", NumberOne = joe }; Firefighter harryAsAFirefighter = harry; // Harry is just a firefighter, so he can extinguish fires // but as a firechief he gets joe to do the work harry.ExtinguishFire(); // While as a firefighter he does it himself harryAsAFirefighter.ExtinguishFire();
The output we get now looks like this:
Joe is putting out the fire! Harry is putting out the fire!
When we talk to our Harry object through a FireChief reference, he gets Joe to put out the fire. If we talk to the object through a Firefighter reference, he does it himself. Same object, but two completely different implementations. Why might we want to do that Let s say we had multiple fire chiefs on a job, but it is our policy that a chief acting as another chief s Number One is not allowed to delegate the job again. Our code models exactly this behavior, as shown in Example 4-7.
Of course, whether that s desirable behavior is another matter entirely we ve ended up with such radically different approaches to putting out a fire that it might be better to separate them back out into functions with different names. When you go through a refactoring process such as this, it is a good idea to check that you re still happy with the semantic implications of your code. Ideally, you want to end up with a neat design, but a superficially neat design that makes no sense is not helpful.
// A reference to Joe, Harry's number one Firefighter joe = new Firefighter { Name = "Joe" }; // FireChief harry has joe as his NumberOne FireChief harry = new FireChief { Name = "Harry", NumberOne = joe }; FireChief tom = new FireChief { Name = "Tom", NumberOne = harry }; // Harry is just a firefighter, so he can extinguish fires // but as a firechief he gets joe to do the work harry.ExtinguishFire(); // But if Tom is asked to extinguish a fire, he asks Harry to do it // Our policy dictates that Harry has to do it himself, not delegate to // Joe this time. tom.ExtinguishFire();
Harry delegates to Joe when he is asked to do it himself, because we are calling through a reference to a FireChief. Tom is also a FireChief, and we are calling through a reference to him as a FireChief, so he delegates to Harry; but when Harry is asked to do it in his role as a Firefighter (remember, the NumberOne property is a reference to a Firefighter), he does it himself, because we are now calling the method through that reference typed to Firefighter. So our output looks like this:
Joe is putting out the fire! Harry is putting out the fire!
That s all very well, but we don t actually want that restriction the fire chief should be allowed to pass the work off to his subordinate as often as he likes, regardless of who he asked to do it.
There s one big caveat regarding everything we ve just shown about method hiding: I can t think of the last time I used this feature in a real application, but I see the warning from time to time and it usually alerts me to a mistake in my code. We ve wanted to illustrate how method hiding works, but we discourage you from using it. The main reason to avoid method hiding with new is that it tends to surprise your clients, and that, as we ve established, is not a good thing. (Would you really expect behavior to change because the type of the variable, not the underlying object, changes ) While method hiding is absolutely necessary for some corner cases, we usually treat this warning as an error, and think very carefully about what we re doing if it comes up. 9 times out of 10, we ve got an inadvertent clash of names.
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