534: Declaring a Static Field in Visual C#.NET

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Listing 534: Declaring a Static Field
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class Employee { public Employee(string firstName, string lastName) { FirstName = firstName; LastName = lastName; Id = NextId; NextId++; } // public public public public public // } static int Id string string string int NextId; { get; set; } FirstName { get; set; } LastName { get; set; } Salary { get; set; }
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In this example, the NextId field declaration includes the static modifier and therefore is called a static field Unlike Id, a single storage location for NextId is shared across all instances of Employee Inside the Employee constructor, you assign the new Employee object s Id the value of NextId
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Static Members
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immediately before incrementing it When another Employee class is created, NextId will be incremented and the new Employee object s Id field will hold a different value Just as instance fields (nonstatic fields) can be initialized at declaration time, so can static fields, as demonstrated in Listing 535
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Listing 535: Assigning a Static Field at Declaration
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class Employee { // public static int NextId = 42; // }
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Unlike with instance fields, if no initialization for a static field is provided, the static field will automatically be assigned its default value (0, null, false, and so on), and it will be possible to access the static field even if it has never been explicitly assigned Nonstatic fields, or instance fields, have a new value for each object to which they belong In contrast, static fields don t belong to the instance, but rather to the class itself As a result, you access a static field from outside a class via the class name Consider the new Program class shown in Listing 536 (using the Employee class from Listing 534)
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Listing 536: Accessing a Static Field
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using System; class Program { static void Main() { EmployeeNextId = 1000000; Employee employee1 = new Employee( "Inigo", "Montoya"); Employee employee2 = new Employee( "Princess", "Buttercup"); ConsoleWriteLine( "{0} {1} ({2})", employee1FirstName, employee1LastName, employee1Id);
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ConsoleWriteLine( "{0} {1} ({2})", employee2FirstName, employee2LastName, employee2Id); ConsoleWriteLine("NextId = {0}", EmployeeNextId); } // }
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Output 59 shows the results of Listing 536
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OUTPUT 59:
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Inigo Montoya (1000000) Princess Buttercup (1000001) NextId = 1000002
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To set and retrieve the initial value of the NextId static field, you use the class name, Employee, not a variable name The only time you can eliminate the class name is from within code that appears within the class itself In other words, the Employee() constructor did not need to use EmployeeNextId because the code appeared within the context of the Employee class itself, and therefore, the context was already understood from the scope In fact, the context is the scope Even though you refer to static fields slightly differently than instance fields, it is not possible to define a static and an instance field with the same name in the same class The possibility of mistakenly referring to the wrong field is high, and therefore, the C# designers decided to prevent such code Therefore, overlap in names will introduce conflict within the declaration space
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BEGINNER TOPIC Data Can Be Associated with Both a Class and an Object Both classes and objects can have associated data, just as can the molds and the widgets created from them
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Static Members
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For example, a mold could have data corresponding to the number of widgets it created, the serial number of the next widget, the current color of the plastic injected into the mold, and the number of widgets it produces per hour Similarly, a widget has its own serial number, its own color, and perhaps the date and time when the widget was created Although the color of the widget corresponds to the color of the plastic within the mold at the time the widget was created, it obviously does not contain data corresponding to the color of the plastic currently in the mold, or the serial number of the next widget to be produced In designing objects, programmers should take care to declare both fields and methods appropriately as static or instance-based In general, you should declare methods that don t access any instance data as static methods, and methods that access instance data (where the instance is not passed in as a parameter) as instance methods Static fields store data corresponding to the class, such as defaults for new instances or the number of instances that have been created Instance fields store data associated with the object
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Static Methods Just like static fields, you access static methods directly off the class name (ConsoleReadLine(), for example) Furthermore, it is not necessary to have an instance in order to access the method Listing 537 provides another example of both declaring and calling a static method
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