.NET Design Patterns

Bridge


 Definition
 UML diagram
 Participants
 Structural code in C#
 Real-world code in C#
 .NET Optimized code in C#



Definition

Decouple an abstraction from its implementation so that the two can vary independently.

Frequency of use:
Medium




UML class diagram






Participants


    The classes and objects participating in this pattern are:

  • Abstraction   (BusinessObject)
    • defines the abstraction's interface.
    • maintains a reference to an object of type Implementor.
  • RefinedAbstraction   (CustomersBusinessObject)
    • extends the interface defined by Abstraction.
  • Implementor   (DataObject)
    • defines the interface for implementation classes. This interface doesn't have to correspond exactly to Abstraction's interface; in fact the two interfaces can be quite different. Typically the Implementation interface provides only primitive operations, and Abstraction defines higher-level operations based on these primitives.
  • ConcreteImplementor   (CustomersDataObject)
    • implements the Implementor interface and defines its concrete implementation.



Structural code in C#


This structural code demonstrates the Bridge pattern which separates (decouples) the interface from its implementation. The implementation can evolve without changing clients which use the abstraction of the object.

                 

using System;

 

namespace DoFactory.GangOfFour.Bridge.Structural

{

  /// <summary>

  /// MainApp startup class for Structural

  /// Bridge Design Pattern.

  /// </summary>

  class MainApp

  {

    /// <summary>

    /// Entry point into console application.

    /// </summary>

    static void Main()

    {

      Abstraction ab = new RefinedAbstraction();

 

      // Set implementation and call

      ab.Implementor = new ConcreteImplementorA();

      ab.Operation();

 

      // Change implemention and call

      ab.Implementor = new ConcreteImplementorB();

      ab.Operation();

 

      // Wait for user

      Console.ReadKey();

    }

  }

 

  /// <summary>

  /// The 'Abstraction' class

  /// </summary>

  class Abstraction

  {

    protected Implementor implementor;

 

    // Property

    public Implementor Implementor

    {

      set { implementor = value; }

    }

 

    public virtual void Operation()

    {

      implementor.Operation();

    }

  }

 

  /// <summary>

  /// The 'Implementor' abstract class

  /// </summary>

  abstract class Implementor

  {

    public abstract void Operation();

  }

 

  /// <summary>

  /// The 'RefinedAbstraction' class

  /// </summary>

  class RefinedAbstraction : Abstraction

  {

    public override void Operation()

    {

      implementor.Operation();

    }

  }

 

  /// <summary>

  /// The 'ConcreteImplementorA' class

  /// </summary>

  class ConcreteImplementorA : Implementor

  {

    public override void Operation()

    {

      Console.WriteLine("ConcreteImplementorA Operation");

    }

  }

 

  /// <summary>

  /// The 'ConcreteImplementorB' class

  /// </summary>

  class ConcreteImplementorB : Implementor

  {

    public override void Operation()

    {

      Console.WriteLine("ConcreteImplementorB Operation");

    }

  }

}


Output
ConcreteImplementorA Operation
ConcreteImplementorB Operation




Real-world code in C#


This real-world code demonstrates the Bridge pattern in which a BusinessObject abstraction is decoupled from the implementation in DataObject. The DataObject implementations can evolve dynamically without changing any clients.

                 

using System;

using System.Collections.Generic;

 

namespace DoFactory.GangOfFour.Bridge.RealWorld

{

  /// <summary>

  /// MainApp startup class for Real-World

  /// Bridge Design Pattern.

  /// </summary>

  class MainApp

  {

    /// <summary>

    /// Entry point into console application.

    /// </summary>

    static void Main()

    {

      // Create RefinedAbstraction

      Customers customers = new Customers("Chicago");

 

      // Set ConcreteImplementor

      customers.Data = new CustomersData();

 

      // Exercise the bridge

      customers.Show();

      customers.Next();

      customers.Show();

      customers.Next();

      customers.Show();

      customers.Add("Henry Velasquez");

 

      customers.ShowAll();

 

      // Wait for user

      Console.ReadKey();

    }

  }

 

  /// <summary>

  /// The 'Abstraction' class

  /// </summary>

  class CustomersBase

  {

    private DataObject _dataObject;

    protected string group;

 

    public CustomersBase(string group)

    {

      this.group = group;

    }

 

    // Property

    public DataObject Data

    {

      set { _dataObject = value; }

      get { return _dataObject; }

    }

 

    public virtual void Next()

    {

      _dataObject.NextRecord();

    }

 

    public virtual void Prior()

    {

      _dataObject.PriorRecord();

    }

 

    public virtual void Add(string customer)

    {

      _dataObject.AddRecord(customer);

    }

 

    public virtual void Delete(string customer)

    {

      _dataObject.DeleteRecord(customer);

    }

 

    public virtual void Show()

    {

      _dataObject.ShowRecord();

    }

 

    public virtual void ShowAll()

    {

      Console.WriteLine("Customer Group: " + group);

      _dataObject.ShowAllRecords();

    }

  }

 

  /// <summary>

  /// The 'RefinedAbstraction' class

  /// </summary>

  class Customers : CustomersBase

  {

    // Constructor

    public Customers(string group)

      : base(group)

    {

    }

 

    public override void ShowAll()

    {

      // Add separator lines

      Console.WriteLine();

      Console.WriteLine("------------------------");

      base.ShowAll();

      Console.WriteLine("------------------------");

    }

  }

 

  /// <summary>

  /// The 'Implementor' abstract class

  /// </summary>

  abstract class DataObject

  {

    public abstract void NextRecord();

    public abstract void PriorRecord();

    public abstract void AddRecord(string name);

    public abstract void DeleteRecord(string name);

    public abstract void ShowRecord();

    public abstract void ShowAllRecords();

  }

 

  /// <summary>

  /// The 'ConcreteImplementor' class

  /// </summary>

  class CustomersData : DataObject

  {

    private List<string> _customers = new List<string>();

    private int _current = 0;

 

    public CustomersData()

    {

      // Loaded from a database

      _customers.Add("Jim Jones");

      _customers.Add("Samual Jackson");

      _customers.Add("Allen Good");

      _customers.Add("Ann Stills");

      _customers.Add("Lisa Giolani");

    }

 

    public override void NextRecord()

    {

      if (_current <= _customers.Count - 1)

      {

        _current++;

      }

    }

 

    public override void PriorRecord()

    {

      if (_current > 0)

      {

        _current--;

      }

    }

 

    public override void AddRecord(string customer)

    {

      _customers.Add(customer);

    }

 

    public override void DeleteRecord(string customer)

    {

      _customers.Remove(customer);

    }

 

    public override void ShowRecord()

    {

      Console.WriteLine(_customers[_current]);

    }

 

    public override void ShowAllRecords()

    {

      foreach (string customer in _customers)

      {

        Console.WriteLine(" " + customer);

      }

    }

  }

}


Output
Jim Jones
Samual Jackson
Allen Good

------------------------
Customer Group: Chicago
Jim Jones
Samual Jackson
Allen Good
Ann Stills
Lisa Giolani
Henry Velasquez
------------------------




.NET Optimized code in C#


The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, object initializers, automatic properties, etc. You can find an example on our Singleton pattern page.

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