Professional C# 6 and .NET Core 1.0. Christian Nagel
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Название: Professional C# 6 and .NET Core 1.0
Автор: Christian Nagel
Издательство: John Wiley & Sons Limited
Жанр: Зарубежная образовательная литература
isbn: 9781119096634
isbn:
Constructors follow the same rules for overloading as other methods – that is, you can provide as many overloads to the constructor as you want, provided they are clearly different in signature:
However, if you supply any constructors that take parameters, the compiler does not automatically supply a default one. This is done only if you have not defined any constructors at all. In the following example, because a one-parameter constructor is defined, the compiler assumes that this is the only constructor you want to be available, so it does not implicitly supply any others:
If you now try instantiating a MyNumber object using a no-parameter constructor, you get a compilation error:
Note that it is possible to define constructors as private or protected, so that they are invisible to code in unrelated classes too:
This example hasn’t actually defined any public, or even any protected, constructors for MyNumber. This would actually make it impossible for MyNumber to be instantiated by outside code using the new operator (though you might write a public static property or method in MyNumber that can instantiate the class). This is useful in two situations:
• If your class serves only as a container for some static members or properties, and therefore should never be instantiated. With this scenario, you can declare the class with the modifier static. With this modifier the class can contain only static members and cannot be instantiated.
• If you want the class to only ever be instantiated by calling a static member function (this is the so-called factory pattern approach to object instantiation). An implementation of the Singleton pattern is shown in the following code snippet.
The Singleton class contains a private constructor, so you can instantiate it only within the class itself. To instantiate it, the static property Instance returns the field s_instance. If this field is not yet initialized (null), a new instance is created by calling the instance constructor. For the null check, the coalescing operator is used. If the left side of this operator is null, the right side of this operator is processed and the instance constructor invoked.
NOTE The coalescing operator is explained in detail in Chapter 8.
Calling Constructors from Other Constructors
You might sometimes find yourself in the situation where you have several constructors in a class, perhaps to accommodate some optional parameters for which the constructors have some code in common. For example, consider the following:
Both constructors initialize the same fields. It would clearly be neater to place all the code in one location. C# has a special syntax known as a constructor initializer to enable this:
In this context, the this keyword simply causes the constructor with the nearest matching parameters to be called. Note that any constructor initializer is executed before the body of the constructor. Suppose that the following code is run:
In this example, the two-parameter constructor executes before any code in the body of the one-parameter constructor (though in this particular case, because there is no code in the body of the one-parameter constructor, it makes no difference).
A C# constructor initializer may contain either one call to another constructor in the same class (using the syntax just presented) or one call to a constructor in the immediate base class (using the same syntax, but using the keyword base instead of this). It is not possible to put more than one call in the initializer.
Static Constructors
One feature of C# is that it is also possible to write a static no-parameter constructor for a class. Such a constructor is executed only once, unlike the constructors written so far, which are instance constructors that are executed whenever an object of that class is created:
One reason for writing a static constructor is if your class has some static fields or properties that need to be initialized from an external source before the class is first used.
The .NET runtime makes no guarantees about when a static constructor will be executed, so you should not place any code in it that relies on it being executed at a particular time (for example, when an assembly is loaded). Nor is it possible to predict in what order static constructors of different classes will execute. However, what is guaranteed is that the static constructor will run at most once, and that it will be invoked before your code makes any reference to the class. In C#, the static constructor is usually executed immediately before the first call to any member of the class.
Note that the static constructor does not have any access modifiers. It’s never called explicitly by any other C# code, but always by the .NET runtime when the class is loaded, so any access modifier such as public or private would be meaningless. For this same reason, the static constructor can never take any parameters, and there can be only one static constructor for a class. It should also be obvious that a static constructor can access only static members, not instance members, of the class.
It is possible to have a static constructor and a zero-parameter instance constructor defined in the same class. Although the parameter lists are identical, there is no conflict because the static constructor is executed when the class is loaded, but the instance constructor is executed whenever an instance is created. Therefore, there is no confusion about which constructor is executed or when.
If you have more than one class that has a static constructor, the static constructor that is executed first is undefined. Therefore, you should not put any code in a static constructor that depends on other static constructors having been or not having been executed. However, if any static fields have been given default values, these are allocated before the static constructor is called.
The next example illustrates the use of a static constructor. It is based on the idea of a program that has user preferences (which are presumably stored in some configuration file). To keep things simple, assume just one user preference – a quantity called BackColor that might represent the background color to be used in an application. Because we don’t want to get into the details of writing code to read data from an external source here, assume also that the preference is to have a background color of red on weekdays and green on weekends. All the program does is display the preference in a console window, but that is enough to see a static constructor at work.
The class UserPreferences is declared with the static modifier; thus it cannot be instantiated and can only contain static members. The static constructor initializes the BackColor property depending on the day of the week (code file StaticConstructorSample/UserPreferences.cs):
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