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EXPLORING SYSTEM.OBJECT
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ReferenceEquals A public static method that tests whether two object instances are the same instance. If you ve overridden the Equals method to test for value equality, then you can use ReferenceEquals instead to test for reference equality. ToString A public virtual instance method that returns a string representation of the object. By default, ToString returns the fully qualified name of the object s type. In practice, ToString is typically overridden to provide a more meaningful string representation of the object s data members. 2.3.1 Overriding System.Object methods Let s see how we might leverage System.Object in a class of our own. Suppose we had a class representing a pixel in a 256 x 256 pixel graphics coordinate system. Listing 2.2 illustrates how we might implement this class while overriding Equals, GetHashCode, and ToString, to our advantage.
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Listing 2.2 Overriding System.Object methods
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public class Pixel { public Pixel(byte x, byte y) { this.x = x; this.y = y; } private byte x; private byte y; public override string ToString() { // return "(x,y)"... return "(" + x + "," + y + ")"; } public override bool Equals(object o) { try { Pixel p = (Pixel)o; return p.x == x && p.y == y; } catch (Exception) { return false; } } public override int GetHashCode() { // shift x one byte to the left... // and add y... return (x<<8) + y; } }
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UNDERSTANDING TYPES AND ASSEMBLIES
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The implementation of ToString simply returns the x and y coordinates in parentheses, separated by a comma. Methods such as Console.WriteLine automatically call the ToString method on their arguments. Therefore, in this example, we can create and display a pixel, p, as follows:
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Pixel p = new Pixel(200, 150); Console.WriteLine(p); // displays "(200,150)"
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Equals returns true if the argument is a pixel with identical coordinates, while GetHashCode combines the coordinates into a single integer value suitable for use
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as a unique key. This ensures that pixels, which are in fact equal, will hash to the same value.
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UNDERSTANDING FINALIZATION
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As we saw above, reference types are created on the managed heap. Management of the heap is performed automatically by the runtime and involves garbage-collecting unused objects when necessary to reclaim heap space. System.Object includes a protected, virtual, instance method called Finalize that can be overridden by all classes in order to free resources before an object is reclaimed by the garbage collector. In C#, you do not directly override Finalize as you would other System.Object methods such as Equals or ToString. Instead C# uses the familiar C++ destructor notation, ~ClassName(), to identify the finalization method.
Coding a finalizer Listing 2.3 provides a simple example of a base class and a derived class, each containing a finalizer that displays a message when the object is finalized.
Listing 2.3 Finalization of a derived class
// file : finalize.cs // compile : csc finalize.cs using System; public class BaseClass { public BaseClass() { Console.WriteLine("creating BaseClass"); } ~BaseClass() { Console.WriteLine("finalizing BaseClass"); } } public class DerivedClass : BaseClass { public DerivedClass() { Console.WriteLine("creating DerivedClass"); } ~DerivedClass() {
UNDERSTANDING FINALIZATION
Console.WriteLine("finalizing DerivedClass"); } } public class Go { public static void Main() { DerivedClass dc = new DerivedClass(); } }
The Go.Main() method simply creates an instance of DerivedClass and exits. The output generated, when you compile and run this program, is shown in figure 2.2.
Finalization of a derived class
Note that the base class constructor is automatically called before that of the derived class. The finalizers are called in reverse order when the derived object is garbage collected. This ensures that the base class object is not destroyed while a derived object still exists, and thus ensures that the finalization chain is correctly followed. Typically, since finalization takes time, you should avoid coding a finalizer unless your program absolutely requires it, such as when it needs to close files, or network connections. 2.4.2 Finalization and the Dispose method If it is necessary to explicitly free resources when an object is destroyed, the .NET documentation recommends writing a Dispose method and calling it explicitly, as we do in listing 2.4. The System.IDisposable interface is provided for this purpose.
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