Conversion Using the as Operator in Visual C#

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Conversion Using the as Operator
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The advantage of the is operator is that it enables verification that a data item is of a particular type The as operator goes one step further It attempts a conversion to a particular data type just like a cast does In contrast to a cast, however, the as operator assigns null to the target if the conversion is unsuccessful This is significant because it avoids the exception that could result from casting Listing 623 demonstrates using the as operator
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Listing 623 Data Conversion Using the as Operator
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object Print(IDocument document) { if(thing != null) { // Print document } else { } } static void Main() { object data; // Print( data as Document ); }
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By using the as operator, you are able to avoid additional try-catch handling code if the conversion is invalid because the as operator provides a way to attempt a cast without throwing an exception if the cast fails One advantage of the is operator over the as operator is that the latter cannot successfully determine the underlying type The latter potentially casts up or down an inheritance chain, as well as across to types supporting the cast operator Therefore, unlike the as operator, the is operator can determine the underlying type
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Summary
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This chapter discussed how to specialize a class by deriving from it and adding additional methods and properties This included a discussion of the private and protected access modifiers that control the level of encapsulation This chapter also investigated the details of overriding the base class implementation, and alternatively hiding it using the new modifier To control overriding, C# provides the virtual modifier, which identifies to the deriving class developer which members she intends for derivation For preventing any derivation altogether you learned about the sealed modifier on the class Similarly, the sealed modifier on a member prevents further overriding from subclasses This chapter ended with a brief discussion of how all types derive from object 9 discusses this derivation further, with a look at how object includes three virtual methods with specific rules and guidelines that govern overloading Before you get there, however, you need to consider another programming paradigm that builds on object-oriented programming: interfaces This is the subject of 7
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7 Interfaces
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Polymorphism is available not only via inheritance (as discussed in the previous chapter), but also via interfaces Unlike abstract classes, interfaces cannot include any implementation Like abstract classes, however, interfaces define a set of members that classes can rely on in order to support a particular feature By implementing an interface, a class defines its capabilities The interface implementation relationship is a "can do" relationship: The class can do what the interface requires The interface defines the contract between the classes that implement the interface and the classes that use the interface Classes that implement interfaces define methods with the same signatures as the implemented interfaces This chapter discusses defining, implementing, and using interfaces
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Introducing Interfaces
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Beginner Topic: Why Interfaces
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Implemented interfaces are like appliances with wall plugs The wall plug is the interface that appliances support in order to receive AC power An appliance can use that power in countless ways, but in order to plug into a wall socket, an appliance must supply a compatible wall plug What the appliance does with the power corresponds to how an interface implementation varies from class to class The specification that defines a wall plug is the contract that must be supported in order for an appliance to plug into the wall plug Similarly, an interface defines a contract that a class must support in order to gain the capability that the interface provides Consider the following example: An innumerable number of file compression formats are available (zip, 7-zip, cab, lha, tar, targz, tarbz2 , bh , rar, arj, arc, ace, zoo, gz , bzip2, xxe, mime, uue, and yenc, just to name a few) If you created classes for each compression format, you could end up with different method signatures for each compression implementation and no ability for a standard calling convention across them Although the method signature could be defined in an abstract member of a base class, deriving from a common base type uses up a class's one and only inheritance, with an unlikely chance of sharing code across the various compression implementations, thereby making the potential of a base class implementation useless Instead of sharing a common base class, each compression class needs to implement a common interface Interfaces define the contract that a class supports in order to interact with the other classes that expect the interface Although there are many potential compression algorithms, if all of them could implement the IFileCompression interface and its Compress() and Uncompress() methods, then the code for calling the algorithm on any particular compression class would simply involve a cast to the IFileCompression interface and a call into the members, regardless of which class implemented the methods The result is polymorphism because each compression class has the same method signature but individual implementations of that signature
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The naming convention for interfaces is to use Pascal case, with an I prefix The IFileCompression interface shown in Listing 71 is an example of such a name and interface definition
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