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The keyword this refers to the current instance of an object. The this reference is a hidden parameter in every nonstatic method of a class (we ll discuss static methods shortly). There are three ways in which the this reference is typically used. The first way is to qualify instance members that have the same name as parameters, as in the following:
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public void SomeMethod (int length) { this.length = length; }
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In this example, SomeMethod( ) takes a parameter (length) with the same name as a member variable of the class. The this reference is used to resolve the ambiguity. Whereas this.length refers to the member variable, length refers to the parameter. You can, for example, use the this reference to make assigning to a field more explicit:
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public void SetBox(int length, { this.length = length; this.width = newWidth; height = newHeight;
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int newWidth, int newHeight) // use of "this" required // use of "this" optional // use of "this" not needed
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If the name of the parameter is the same as the name of the member variable, you must use the this reference to distinguish between the two, but if the names are different (such as newWidth and newHeight), the use of the this reference is optional.
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The argument in favor of naming the argument to a method that is the same as the name of the member is that the relationship between the two is made explicit. The counterargument is that using the same name for both the parameter and the member variable can cause confusion as to which one you are referring to at any given moment.
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The second use of the this reference is to pass the current object as a parameter to another method, as in the following code:
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class SomeClass { public void FirstMethod(OtherClass otherObject) { otherObject.SecondMethod(this); } // ... }
This code snippet establishes two classes, SomeClass and OtherClass (the definition of OtherClass isn t shown here). SomeClass has a method named FirstMethod( ), and OtherClass has a method named SecondMethod( ). Inside FirstMethod( ), we d like to invoke SecondMethod( ), passing in the current object (an instance of SomeClass) for further processing. To do so, you pass in the this reference, which refers to the current instance of SomeClass. The third use of this is with indexers, which we cover in 14.
Static and Instance Members
The fields, properties, and methods of a class can be either instance members or static members. Instance members are associated with instances of a type, whereas static members are associated with the class itself, and not with any particular instance. All methods are instance methods unless you explicitly mark them with the keyword static. The vast majority of methods will be instance methods. The semantics of an instance method are that you are taking an action on a specific object. From time to time, however, it is convenient to be able to invoke a method without having an instance of the class, and for that, you will use a static method.
You access a static member through the name of the class in which it is declared. For example, suppose you have a class named Button and have instantiated objects of that class named btnUpdate and btnDelete. Suppose that the Button class has an instance method Draw( ) and a static method GetButtonCount( ). The job of Draw( ) is to draw the current button, and the job of GetButtonCount( ) is to return the number of buttons currently visible on the form. Since GetButtonCount( ) applies to more than just the one button, it wouldn t make sense to call it on a specific instance of Button; therefore, it s static. You access an instance method through an instance of the class that is, through an object:
btnUpdate.SomeMethod( );
You access a static method through the class name, not through an instance:
Button.GetButtonCount( );
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