5: Classes in C#.NET

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5: Classes
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Note that specializing a type via inheritance (which I will cover in 6) is preferable to using an extension method Extension methods do not provide a clean versioning mechanism since the addition of a matching signature to the extended type will take precedence over the extension method without warning of the change The subtlety of this is more pronounced for extended classes whose source code you don t control Another minor point is that, although development IDEs support IntelliSense for extension methods, it is not obvious that a method is an extension method by simply reading through the calling code In general, use extension methods sparingly
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In addition to properties and the access modifiers we looked at earlier in the chapter, there are several other specialized ways of encapsulating the data within a class For instance, there are two more field modifiers The first is the const modifier, which you already encountered when declaring local variables The second is the capability of fields to be defined as read-only
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Just as with const values, a const field contains a compile-time-determined value that cannot be changed at runtime Values such as pi make good candidates for constant field declarations Listing 542 shows an example of declaring a const field
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Listing 542: Declaring a Constant Field class ConvertUnits
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public const float CentimetersPerInch = 254F; public const int CupsPerGallon = 16;
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// }
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Constant fields are static automatically, since no new field instance is required for each object instance Declaring a constant field as static explicitly will cause a compile error It is important that the types of values used in public constant expressions are permanent in time Values such as pi, Avogadro s number, and
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the circumference of the Earth are good examples However, values that could potentially change over time are not Build numbers, population counts, and exchange rates would be poor choices for constants
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ADVANCED TOPIC Public Constants Should Be Permanent Values public constants should be permanent because changing their value will not necessarily take effect in the assemblies that use it If an assembly references constants from a different assembly, the value of the constant is compiled directly into the referencing assembly Therefore, if the value in the referenced assembly is changed but the referencing assembly is not recompiled, then the referencing assembly will still use the original value, not the new value Values that could potentially change in the future should be specified as readonly instead
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readonly Unlike const, the readonly modifier is available only for fields (not for
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local variables) and it declares that the field value is modifiable only from inside the constructor or directly during declaration Listing 543 demonstrates how to declare a readonly field
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Listing 543: Declaring a Field As readonly class Employee
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public Employee(int id)
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{ Id = id; } //
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public readonly int Id; public void SetId(int newId)
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{ // ERROR: read-only fields cannot be set // outside the constructor // Id = newId; } // }
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5: Classes
Unlike constant fields, readonly fields can vary from one instance to the next In fact, a readonly field s value can change from its value during declaration to a new value within the constructor Furthermore, readonly fields occur as either instance or static fields Another key distinction is that you can assign the value of a readonly field at execution time rather than just at compile time Using readonly with an array does not freeze the contents of the array It freezes the number of elements in the array because it is not possible to reassign the readonly field to a new instance However, the elements of the array are still writeable
Nested Classes
In addition to defining methods and fields within a class, it is also possible to define a class within a class Such classes are nested classes You use a nested class when the class makes little sense outside the context of its containing class Consider a class that handles the command-line options of a program Such a class is generally unique to each program and there is no reason to make a CommandLine class accessible from outside the class that contains Main() Listing 544 demonstrates such a nested class