vb.net generate barcode image Part II Designing Types in C#

Painting PDF 417 in C# Part II Designing Types

Part II Designing Types
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In this code, the new keyword tells the compiler to emit metadata, making it clear to the CLR that BetterPhone s EstablishConnection method is intended to be treated as a new function that is introduced by the BetterPhone type . The CLR will know that there is no relationship between Phone s and BetterPhone s methods . When the same application code (in the Main method) executes, the output is as follows:
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BetterPhone.Dial BetterPhone.EstablishConnection Phone.Dial Phone.EstablishConnection
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This output shows that Main s call to Dial calls the new Dial method defined by BetterPhone.Dial, which in turn calls the virtual EstablishConnection method that is also defined by BetterPhone . When BetterPhone s EstablishConnection method returns, Phone s Dial method is called . Phone s Dial method calls EstablishConnection, but because BetterPhone s EstablishConnection is marked with new, BetterPhone s EstablishConnection method isn t considered an override of Phone s virtual EstablishConnection method . As a result, Phone s Dial method calls Phone s EstablishConnection method this is the expected behavior . Note If the compiler treated methods as overrides by default (as a native C++ compiler does),
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the developer of BetterPhone couldn t use the method names Dial and EstablishConnection. This would most likely cause a ripple effect of changes throughout the entire source code base, breaking source and binary compatibility . This type of pervasive change is undesirable, especially in any moderate-to-large project . However, if changing the method name causes only moderate updates in the source code, you should change the name of the methods so the two different meanings of Dial and EstablishConnection don t confuse other developers .
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Alternatively, CompanyB could have gotten the new version of CompanyA s Phone type and decided that Phone s semantics of Dial and EstablishConnection are exactly what it s been looking for . In this case, CompanyB would modify its BetterPhone type by removing its Dial method entirely . In addition, because CompanyB now wants to tell the compiler that BetterPhone s EstablishConnection method is related to Phone s EstablishConnection method, the new keyword must be removed . Simply removing the new keyword isn t enough, though, because now the compiler can t tell exactly what the intention is of BetterPhone s EstablishConnection method . To express his intent exactly, the CompanyB developer must also change BetterPhone s EstablishConnection method from virtual to override. The following code shows the new version of BetterPhone:
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6 Type and Member Basics
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namespace CompanyB { public class BetterPhone : CompanyA.Phone { // Delete the Dial method (inherit Dial from base). // Remove 'new' and change 'virtual' to 'override' to // mark this method as having a relationship to the base // type's EstablishConnection method. protected override void EstablishConnection() { Console.WriteLine("BetterPhone.EstablishConnection"); // Do work to establish the connection. } } }
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Now when the same application code (in the Main method) executes, the output is as follows:
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Phone.Dial BetterPhone.EstablishConnection
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This output shows that Main s call to Dial calls the Dial method defined by Phone and inherited by BetterPhone. Then when Phone s Dial method calls the virtual EstablishConnection method, BetterPhone s EstablishConnection method is called because it overrides the virtual EstablishConnection method defined by Phone .
7
Constants and Fields
In this chapter: Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
In this chapter, I ll show you how to add data members to a type . Specifically, we ll look at constants and fields .
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