how to generate qr code in asp.net using c# Inheritance and Polymorphism in C#

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Inheritance and Polymorphism
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We ll get into the nuances of the question in the preceding paragraph in a minute, but let s assume for the time being that our answer to the question is yes (which, on face value, seems reasonable). Example 4-3 shows how we use inheritance in C#.
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class FireChief : Firefighter { public void TellFirefighterToExtinguishFire (Firefighter colleague) { colleague.ExtinguishFire(); } }
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Notice that we use the colon in the class declaration to indicate that FireChief is a Firefighter. We then say that Firefighter is a base class of FireChief. Looking at the relationship from the other direction, we can also say that FireChief is a derived class of Firefighter. We ve added the extra function that allows the chief to tell a firefighter to extinguish a fire which encapsulates that extra responsibility. What we haven t had to do is to duplicate all the functionality of the firefighter; that comes along anyway. We can now use the fire chief just as we would a firefighter, as shown in Example 4-4.
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Firetruck truckOne = new Firetruck(); FireChief bigChiefHarry = new FireChief { Name = "Harry" }; truckOne.Driver = bigChiefHarry; bigChiefHarry.Drive(truckOne, new Point(100,300));
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Firefighter joe = new Firefighter { Name = "Joe" }; bigChiefHarry.TellFirefighterToExtinguishFire(joe);
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Because bigChiefHarry is an object of type FireChief, and a FireChief is a Fire fighter, we can assign him to be the driver of a truck and tell him to drive it somewhere. But because he is a FireChief, we can also ask him to tell Joe to put out the fire when he gets there. Wherever we talk about a FireChief, we can treat the object as a Firefighter. This use of one type as though it were one of its bases is an example of polymorphism. Equally, we could phrase that the other way around: we can successfully substitute an instance of a more-derived class where we expect a base class. This is known as the Liskov Substitution Principle (LSP) after computer scientist Barbara Liskov, who articulated the idea in a paper she delivered in 1987.
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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
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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);
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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:
'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; }
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:
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.
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