print barcode with vb.net 13: Interfaces in Visual C#.NET

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13: Interfaces
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Now, suppose you are the author of a Document class, which specifies that Document objects can be stored in a database. You decide to have Document implement the IStorable interface. It isn t required that you do so, but by implementing the IStorable interface, you signal to potential clients that the Document class can be used just like any other IStorable object. This will, for example, allow your clients to add your Document objects to an array of IStorable references:
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IStorable[] myStorableArray = new IStorable[3];
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As we discussed earlier, the array doesn t specifically need to know that it holds a Document object, just that it holds objects that implement IStorable. To implement the IStorable interface, use the same syntax as though the new Document class were inheriting from IStorable a colon (:) followed by the interface name:
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public class Document : IStorable
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You can read this as define a public class named Document that implements the IStorable interface. The compiler distinguishes whether the colon indicates inheritance or implementation of an interface by checking to see whether IStorable is defined, and whether it is an interface or base class. If you derive from a base class and you also implement one or more interfaces, you use a single colon and separate the base class and the interfaces by commas. The base class must be listed first; the interfaces may be listed in any order.
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public MyBigClass : TheBaseClass, IPrintable, IStorable, IClaudius, IAndThou
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In this declaration, the new class MyBigClass derives from TheBaseClass and implements four interfaces. Suppose that the definition of IStorable requires a void Read( ) method, and a void Write( ) method that takes an object. In that case, your definition of the Document class that implements the IStorable interface might look like this:
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public class Document : IStorable { public void Read( ) {...} public void Write(object obj) {...} // ... }
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It is now your responsibility, as the author of the Document class, to provide a meaningful implementation of the IStorable methods. Having designated Document as implementing IStorable, you must implement all the IStorable methods, or you will generate an error when you compile. Example 13-1 illustrates defining and implementing the IStorable interface. Have a look at it first, and we ll take it apart afterward.
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Implementing an Interface |
using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_13_1_ _ _ _Implementing_Interface { interface IStorable { void Read( ); void Write( object obj ); int Status { get; set; } }
public class Document : IStorable { public Document( string s ) { Console.WriteLine( "Creating document with: {0}", s ); } #region IStorable public void Read( ) { Console.WriteLine( "Executing Document's Read Method for IStorable" ); } public void Write( object o ) { Console.WriteLine( "Executing Document's Write Method for IStorable" ); } // property required by IStorable public int Status { get; set;} #endregion } class Tester { public void Run( ) { Document doc = new Document( "Test Document" ); doc.Status = -1; doc.Read( );
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13: Interfaces
Console.WriteLine( "Document Status: {0}", doc.Status ); } static void Main( ) { Tester t = new Tester( ); t.Run( ); } } }
The output looks like this:
Creating document with: Test Document Executing Document's Read Method for IStorable Document Status: -1
De ning the Interface
In Example 13-1, the first few lines define an interface, IStorable, which has two methods (Read( ) and Write( )) and a property (Status) of type int:
interface IStorable { void Read( ); void Write(object obj); int Status { get; set; } }
Notice that the IStorable method declarations for Read( ) and Write( ) do not include access modifiers (public, protected, internal, private). In fact, providing an access modifier generates a compile error. Interface methods are implicitly public because an interface is a contract meant to be used by other classes. In addition, you must declare these methods to be public, and not static, when you implement the interface. In the interface declaration, the methods are otherwise defined just like methods in a class: you indicate the return type (void), followed by the identifier (Write), followed by the parameter list (object obj), and, of course, you end all statements with a semicolon. The methods in the interface declaration have no body, however. An interface can also require that the implementing class provide a property (see 8 for a discussion of properties). Notice that the declaration of the Status property does not provide an implementation for get( ) and set( ), but simply designates that there must be a get( ) and a set( ):
int Status { get; set; }
You can t define member variables in an interface, but defining properties like this has the same practical effect.
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