539: Declaring a Static Property class Employee in Visual C#

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Listing 539: Declaring a Static Property class Employee
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{ //
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public static int NextId
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return _NextId;
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private set
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{ _NextId = value;
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Static Members
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public static int _NextId = 42;
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// }
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It is almost always better to use a static property rather than a public static field because public static fields are callable from anywhere whereas a static property offers at least some level of encapsulation
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Static Classes Some classes do not contain any instance fields Consider, for example, a Math class that has functions corresponding to the mathematical operations Max() and Min(), as shown in Listing 540
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Listing 540: Declaring a Static Class
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// Static class introduced in C# 20
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public static class SimpleMath
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{ // params allows the number of parameters to vary static int Max(params int[] numbers) { // Check that there is a least one item in numbers if(numbersLength == 0) { throw new ArgumentException( "numbers cannot be empty"); }
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int result;
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result = numbers[0];
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foreach (int number in numbers)
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if(number > result)
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{ result = number; } }
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return result;
} // params allows the number of parameters to vary static int Min(params int[] numbers) { // Check that there is a least one item in numbers if(numbersLength == 0)
5: Classes
throw new ArgumentException(
"numbers cannot be empty"); }
int result;
result = numbers[0];
foreach (int number in numbers)
if(number < result)
{ result = number; } }
return result;
This class does not have any instance fields (or methods), and therefore, creation of such a class would be pointless Because of this, the class is decorated with the static keyword The static keyword on a class provides two facilities First, it prevents a programmer from writing code that instantiates the SimpleMath class Second, it prevents the declaration of any instance fields or methods within the class Since the class cannot be instantiated, instance members would be pointless One more distinguishing characteristic of the static class is that the C# compiler automatically marks it as abstract and sealed within the CIL This designates the class as inextensible; in other words, no class can be derived from it or instantiate it
Extension Methods
Consider the SystemIODirectoryInfo class which is used to manipulate filesystem directories The class supports functionality to list the files and subdirectories (DirectoryInfoGetFiles()) as well as the capability to move the directory (DirectoryInfoMove()) One feature it doesn t support directly is copy If you needed such a method you would have to implement it, as shown earlier in Listing 537 The DirectoryInfoExtensionCopy() method is a standard static method declaration However, notice that calling this Copy() method is different from calling the DirectoryInfoMove() method This is unfortunate Ideally, we
Extension Methods
want to add a method to DirectoryInfo so that, given an instance, we could call Copy() as an instance method directoryCopy() C# 30 simulates the creation of an instance method on a different class via extension methods To do this we simply change the signature of our static method so that the first parameter, the data type we are extending, is prefixed with the this keyword (see Listing 541)
Listing 541: Static Copy Method for DirectoryInfo public static class DirectoryInfoExtension
public static void CopyTo(
this DirectoryInfo sourceDirectory, string target, SearchOption option, string searchPattern) { // } }
// DirectoryInfo directory = new DirectoryInfo("\\Source"); directoryCopyTo("\\Target", SearchOptionAllDirectories, "*"); //
Via this simple addition to C# 30, it is now possible to add instance methods to any class, even classes that are not within the same assembly The resultant CIL code, however, is identical to what the compiler creates when calling the extension method as a normal static method Extension method requirements are as follows
The first parameter corresponds to the type on which the method
extends or operates
To designate the extension method, prefix the extended type with the this modifier To access the method as an extension method, import the extending
type s namespace via a using directive (or place the extending class in the same namespace as the calling code) If the extension method signature matches a signature on the extended type already (that is, if CopyTo() already existed on DirectoryInfo), the extension method will never be called except as a normal static method