When you create an implementing class, you’re free to mark any or all of the methods from the interface as virtual
. Derived classes can then override or provide new implementations, just as they might with any other virtual instance method.
For example, a Document
class might implement the IStorable
interface and mark its Read( )
and Write( )
methods as virtual. In an earlier example, we created a base class Note
and a derived class Document
. While the Note
class implements Read( )
and Write( )
to save to a file, the Document
class might implement Read( )
and Write( )
to read from and write to a database.
Example 13-5 uses the Note
and Document
classes, but we’ve taken out the extra complexity we added in the last few examples, to focus on overriding an interface implementation. Note
implements the IStorable
-required Read( )
method as a virtual method, and Document
overrides that implementation.
Notice that Note
does not mark Write( )
as virtual. You’ll see the implications of this decision in the analysis that follows Example 13-5.
The complete listing is shown in Example 13-5.
Example 13-5. You can override an interface implementation in the same way that you would override any virtual method of a parent class
using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace Example_13_5_ _ _ _Overriding_Interface_Implementation { interface IStorable { void Read( ); void Write( ); } public class Note : IStorable { public Note(string s) { Console.WriteLine("Creating Note with: {0}", s); } // Note's version of Read( ) is virtual public virtual void Read( ) { Console.WriteLine("Note Read Method for IStorable"); } // Note's version of Write( ) is NOT virtual! public void Write( ) { Console.WriteLine("Note Write Method for IStorable"); } } public class Document : Note { public Document(string s) : base(s) { Console.WriteLine("Creating Document with: {0}", s); } // override the Read method public override void Read( ) { Console.WriteLine("Overriding the Read method for Document!"); } // implement my own Write method public new void Write( ) { Console.WriteLine("Implementing a new Write method for Document!"); } } class Tester { public void Run( ) { Note theNote = new Document("Test Document"); theNote.Read( ); theNote.Write( ); Console.WriteLine(" "); IStorable isStorable = theNote as IStorable; if (isStorable != null) { isStorable.Read( ); isStorable.Write( ); } Console.WriteLine(" "); // This time create a reference to the derived type Document theDoc = new Document("Second Test"); theDoc.Read( ); theDoc.Write( ); Console.WriteLine(" "); IStorable isStorable2 = theDoc as IStorable; if (isStorable != null) { isStorable2.Read( ); isStorable2.Write( ); } } static void Main( ) { Tester t = new Tester( ); t.Run( ); } } }
The output looks like this:
Creating Note with: Test Document Creating Document with: Test Document Overriding the Read method for Document! Note Write Method for IStorable Overriding the Read method for Document! Note Write Method for IStorable Creating Note with: Second Test Creating Document with: Second Test Overriding the Read method for Document! Implementing a new Write method for Document! Overriding the Read method for Document! Note Write Method for IStorable
In Example 13-5, the IStorable
interface is simplified for clarity’s sake:
interface IStorable { void Read( ); void Write( ); }
The Note
class implements the IStorable
interface:
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
:
public class Document : 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.
Overriding the Read method for Document! Note Write Method for IStorable
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:
Overriding the Read method for Document Note Write Method for IStorable
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");
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:
Overriding the Read method for Document! Note Write Method for IStorable