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VALUE VS. REFERENCE TYPES
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this.y = y; } private int x; private int y; }
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A struct is typically used for a lightweight class and often contains data members, but no methods, as in this example. We use the same new operator to create an instance of a struct:
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Point p = new Point(10, 20);
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Small structs can be more efficient than classes because they avoid the extra level of indirection associated with a reference, and they don t have to be garbage collected. However, when you pass a struct to a method, a copy of the struct is passed. In contrast, when you pass a class instance, a reference is passed. Therefore, passing large structs as parameters can negatively impact performance. A struct cannot inherit from another struct or class, nor can it serve as a base for inheritance. However, a struct can implement interfaces. 2.2.2 Boxing and unboxing To preserve the everything is an object philosophy, .NET provides a corresponding reference type for every value type. This is known as the value type s boxed type. For example, if you store a value type in a reference type, the value type is automatically boxed:
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int i = 123; object o = i;
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In this example, we define i as an integer with the value 123. Then, we create an object reference, o, and assign i to it. This causes an implicit boxing operation. Boxing causes the runtime to create a new object, containing a copy of i s value, on the heap. A reference to this object is stored in o. The original value type, i, is unaffected by the operation. In contrast, unboxing must be explicitly requested with a cast:
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int i = (int)o; // unbox o to i
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In this case, we re unboxing o into an integer i. For this to succeed, o must contain a reference to an integer type and we must make our intentions clear by casting o to an integer. Any attempt to unbox to an incompatible type, such as from a string to an integer, will generate an InvalidCastException error at run time. Boxing and unboxing provide an elegant way to allow programs to use small, efficient value types without the overhead of full-blown heap-allocated objects, while, at the same time, allowing such values to take on the form of an object reference whenever necessary.
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UNDERSTANDING TYPES AND ASSEMBLIES
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Note that automatic boxing means that you can use an array of objects as a generic collection:
object[] arr = {"cat", 1, 2.3, 'C'};
This example creates an array containing string, integer, double, and character elements. However, you ll need to cast the individual array elements to the correct type when unboxing.
EXPLORING SYSTEM.OBJECT
System.Object is the ultimate superclass from which all .NET reference types implicitly derive. It provides several useful members that you should become familiar with:
Equals An overloaded method that comes in both static and virtual instance versions, and tests whether two object instances are equal. The default implementation tests for reference equality, not value equality. Therefore Equals returns true if the object passed as an argument is the same instance as the current object. It may make sense to override this method. For example, the built-in string type overrides Equals to return true if the two strings contain the same characters. Finalize A protected virtual instance method that is automatically executed when an object is destroyed. You can override this method in derived classes to free resources and perform cleanup before the object is garbage collected. In C#, this method is not directly overridden. Instead C++-style destructor syntax is used, as we ll see in a moment. GetHashCode A public virtual instance method that produces a hash code for the object. GetHashCode returns an integer value that can be used as a hash code to store the object in a hash table. If you override Equals, then you should also override GetHashCode, since two objects, which are equal, should return the same hash code. GetType A public instance method that returns the type of the object, thereby facilitating access to the type s metadata. Under .NET, applications and components are self-describing and that description is stored with the component in the form of metadata. This contrasts with alternative schemes in which such data was typically stored as IDL, or in TypeLibs, or in the registry. GetType returns a Type object that programs can use to retrieve details of the type s members and even create an instance of the type and invoke those members. This process of type inspection and dynamic invocation is known as reflection, and we examine it in more detail later in this chapter. GetType is not virtual because its implementation is the same for all types. MemberwiseClone A protected instance method that returns a shallow copy of the object.
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