public class Note : IStorable in Visual C#

Encode Code 128C in Visual C# public class Note : IStorable

The designer of Note has opted to make the Read(...) method virtual but not to make the Write(...) method virtual:

public virtual void Read( ) public void Write( )

In a real-world application, you would almost certainly mark both methods as virtual, but we ve differentiated them to demonstrate that the developer is free to pick and choose which methods are made virtual.

The new class, Document, derives from Note:

It is not necessary for Document to override Read( ), but it is free to do so and has done so here:

public override void Read( )

To illustrate the implications of marking an implementing method as virtual, the Run( ) method calls the Read( ) and Write( ) methods in four ways: Through the Note class reference to a Document object Through an interface reference created from the Note class reference to the Document object Through a Document object Through an interface reference created from the Document object Virtual implementations of interface methods are polymorphic, just like the virtual methods of classes.

When you call the nonpolymorphic Write( ) method on the IStorable interface cast from the derived Document, you actually get the Note s Write method, because Write( ) is implemented in the base class and is nonvirtual. To see polymorphism at work with interfaces, you ll create a reference to the Note class and initialize it with a new instance of the derived Document class:

Note theDocument = new Document("Test Document");

Invoke the Read and Write methods:

theDocument.Read( ); theDocument.Write( );

The output reveals that the (virtual) Read( ) method is called polymorphically that is, the Document class overrides the Note class s Read( ), and the nonvirtual Write( ) method of the Note class is invoked because the Write( ) method was not made virtual.

The overridden method of Read( ) is called because you ve created a new Document object:

Note theDocument = new Document("Test Document");

The nonvirtual Write method of Note is called because you ve assigned theDocument to a reference to a Note:

Note theDocument = new Document("Test Document");

To illustrate calling the methods through an interface that is created from the Note class reference to the Document object, create an interface reference named isDocument. Use the as operator to cast the Note (theDocument) to the IStorable reference:

IStorable isDocument = theDocument as IStorable;

Then invoke the Read( ) and Write( ) methods for theDocument through that interface:

if (isDocument != null) { isDocument.Read( ); isDocument.Write( ); }

The output is the same: once again, the virtual Read( ) method is polymorphic, and the nonvirtual Write( ) method is not:

Next, create a second Document object, this time assigning its address to a reference to a Document, rather than a reference to a Note. This will be used to illustrate the final cases (a call through a Document object and a call through an interface created from the Document object):

Document theDoc = new Document("Second Test");

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Call the methods on the Document object:

theDoc.Read( ); theDoc.Write( );

Again, the virtual Read( ) method is polymorphic and the nonvirtual Write( ) method is not, but this time you get the Write( ) method for Document because you are calling the method on a Document object:

Overriding the Read method for Document! Implementing a new Write method for Document!

Finally, cast the Document object to an IStorable reference and call Read( ) and Write( ):

IStorable isDocument2 = theDoc as IStorable; if (isDocument != null) { isDocument2.Read( ); isDocument2.Write( ); }

The Read( ) method is called polymorphically, but the Write( ) method for Note is called because Note implements IStorable, and Write( ) is not polymorphic: