Understanding Adapter Pattern

Question

I am trying to understand Adapter pattern and its use in real world. After going through various articles on internet and www.dofactory.com, I created this sample code. I just want to know whether my understanding is correct. In the example below I have created MSDAO object in the Adaptor class. Later I changed it to OracleDAO.

class Client {   static void Main(string[] args)   {     ITarget objAdapter = new Adapter();     object dummyObject = objAdapter.GetData();   } }  Interface ITarget {   public void GetData(); }  //Decision to use MSDAO  class Adapter : ITarget {   public void GetData()   {     MSDAO objmsdao = new MSDAO();     objmsdao.GetData();   } }  //After a month, the decision to use OracaleDAO was taken, so the code change  class Adapter : ITarget {   public void GetData()   {     OracleDAO objoracledao = new OracleDAO();     objoracledao.GetData();   } } 

Answer 1

Generally the adapter pattern transforms one interface into another, but it can simply wrap the behavior to isolate your class from the underlying implementation. In your case, you are using an adapter, but you could just as easily have defined the DAO objects to simply implement the interface and programmed against the interface. The adapter pattern is usually used when you don't have control over the target class. My primary use of the adapter pattern would be to create wrappers for a framework class that doesn't implement an interface.

Say I want to mock out a framework class which doesn't implement an interface (and doesn't have virtual methods). With many mocking apis this is hard or impossible to do. What I will do, then, is define my own interface as a subset of the signature of the class I'm targeting. I implement a wrapper class that implements this interface and simply delegates the calls to the wrapped framework class. This wrapper class works as an adapter for the framework class. My classes use this adapter instead of the framework class, but get the framework class' behavior.

 public interface IFoo  {      void Bar();  }   public class FooWrapper : IFoo  {       private FrameworkFoo Foo { get; set; }        public FooWrapper( FrameworkFoo foo )       {            this.Foo = foo;       }        public void Bar()       {            this.Foo.Bar();       }  } 

Consider also the case where you have a couple of different classes that have basically the same functionality, but different signatures and you want to be able to use them interchangeably. If you can't transform these (or don't want to for other reasons), you may want to write an adapter class that defines a common interface and translates between that interface's methods and the methods available on the target classes.

Framework classes:

public class TargetA {     public void Start() { ... }     public void End() { ... } }  public class TargetB {     public void Begin() { ... }     public void Terminate() { ... } } 

An adapter for them

public interface ITargetAdapter {     void Open();     void Close(); }  public class AdapterA : ITargetAdapter {      private TargetA A { get; set; }       public AdapterA( TargetA a )      {            this.A = a;      }       public void Open() { this.A.Start(); }      public void Close() { this.A.End(); } }  public class AdapterB : ITargetAdapter {      private TargetB B { get; set; }       public AdapterB( TargetB a )      {            this.B = a;      }       public void Open() { this.B.Begin(); }      public void Close() { this.B.Terminate(); } } 

Then used as:

ITargetAdapter adapter = new AdapterA( new TargetA() ); adapter.Open(); adapter.Close();      

Answer 2

A canonical example inside the .NET framework exists in the System.Drawing.Bitmap class.

This Bitmap has a constructor that lets you load an image from a Stream:

public Bitmap(     Stream stream ) 

what you don't know, is that internally the .NET Bitmap class is a wrapper around the GDI+ Bitmap class, and its constructor that takes an IStream:

Bitmap(   [in]  IStream *stream,   [in]  BOOL useIcm ); 

So in the C# world, when i call:

new Bitmap(stream); 

it has to turn around and call:

IStream stm; IntPtr gpBitmap; GdipCreateBitmapFromStream(stm, out gpBitmap); 

The question is how to present a .NET Stream object to a method that expects a COM IStream interface.

Hence the internal GPStream class:

internal class GPStream : IStream {    GPStream(Stream stream) { ... } } 

You need to present an IStream interface to your Stream object:

IStream                                     Stream =======================================     ===================================== int Read(IntPtr buf, int len);          --> int Read(byte[] buffer, int offset, int count) int Write(IntPtr buf, int len);         --> void Write(byte[] buffer, int offset, int count); long Seek(long dlibMove, int dwOrigin); --> long Seek(long offset, SeekOrigin orgin) ...                                         ... 

So now you have an adapter:

And the code is something like:

IStream stm = new GPStream(stream); //adapter to convert Stream --> IStream IntPtr gpBitmap;  GdipCreateBitmapFromStream(stm, out gpBitmap);