Façade defines

Definition

Provide a unified interface to a set of interfaces in a subsystem. Façade defines a higher-level interface that makes the subsystem easier to use.

Frequency of use:

High

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UML class diagram

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Participants

    The classes and objects participating in this pattern are:

• Facade   (MortgageApplication)
  • knows which subsystem classes are responsible for a request.
  • delegates client requests to appropriate subsystem objects.
• Subsystem classes   (Bank, Credit, Loan)
  • implement subsystem functionality.
  • handle work assigned by the Facade object.
  • have no knowledge of the facade and keep no reference to it.

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Structural code in C#

This structural code demonstrates the Facade pattern which provides a simplified and uniform interface to a large subsystem of classes.

                 





using System;










namespace DoFactory.GangOfFour.Facade.Structural




{




  /// <summary>




  /// MainApp startup class for Structural




  /// Facade Design Pattern.




  /// </summary>




  class MainApp




  {




    /// <summary>




    /// Entry point into console application.




    /// </summary>




    public static void Main()




    {




      Facade facade = new Facade();










      facade.MethodA();




      facade.MethodB();










      // Wait for user




      Console.ReadKey();




    }




  }










  /// <summary>




  /// The 'Subsystem ClassA' class




  /// </summary>




  class SubSystemOne




  {




    public void MethodOne()




    {




      Console.WriteLine(" SubSystemOne Method");




    }




  }










  /// <summary>




  /// The 'Subsystem ClassB' class




  /// </summary>




  class SubSystemTwo




  {




    public void MethodTwo()




    {




      Console.WriteLine(" SubSystemTwo Method");




    }




  }










  /// <summary>




  /// The 'Subsystem ClassC' class




  /// </summary>




  class SubSystemThree




  {




    public void MethodThree()




    {




      Console.WriteLine(" SubSystemThree Method");




    }




  }










  /// <summary>




  /// The 'Subsystem ClassD' class




  /// </summary>




  class SubSystemFour




  {




    public void MethodFour()




    {




      Console.WriteLine(" SubSystemFour Method");




    }




  }










  /// <summary>




  /// The 'Facade' class




  /// </summary>




  class Facade




  {




    private SubSystemOne _one;




    private SubSystemTwo _two;




    private SubSystemThree _three;




    private SubSystemFour _four;










    public Facade()




    {




      _one = new SubSystemOne();




      _two = new SubSystemTwo();




      _three = new SubSystemThree();




      _four = new SubSystemFour();




    }










    public void MethodA()




    {




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




      _one.MethodOne();




      _two.MethodTwo();




      _four.MethodFour();




    }










    public void MethodB()




    {




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




      _two.MethodTwo();




      _three.MethodThree();




    }




  }




}


 
        
            

Output

MethodA() ----
SubSystemOne Method
SubSystemTwo Method
SubSystemFour Method

MethodB() ----
SubSystemTwo Method
SubSystemThree Method

---

Real-world code in C#

This real-world code demonstrates the Facade pattern as a MortgageApplication object which provides a simplified interface to a large subsystem of classes measuring the creditworthyness of an applicant.

                 





using System;










namespace DoFactory.GangOfFour.Facade.RealWorld




{




  /// <summary>




  /// MainApp startup class for Real-World 




  /// Facade Design Pattern.




  /// </summary>




  class MainApp




  {




    /// <summary>




    /// Entry point into console application.




    /// </summary>




    static void Main()




    {




      // Facade




      Mortgage mortgage = new Mortgage();










      // Evaluate mortgage eligibility for customer




      Customer customer = new Customer("Ann McKinsey");




      bool eligible = mortgage.IsEligible(customer, 125000);










      Console.WriteLine("\n" + customer.Name +




          " has been " + (eligible ? "Approved" : "Rejected"));










      // Wait for user




      Console.ReadKey();




    }




  }










  /// <summary>




  /// The 'Subsystem ClassA' class




  /// </summary>




  class Bank




  {




    public bool HasSufficientSavings(Customer c, int amount)




    {




      Console.WriteLine("Check bank for " + c.Name);




      return true;




    }




  }










  /// <summary>




  /// The 'Subsystem ClassB' class




  /// </summary>




  class Credit




  {




    public bool HasGoodCredit(Customer c)




    {




      Console.WriteLine("Check credit for " + c.Name);




      return true;




    }




  }










  /// <summary>




  /// The 'Subsystem ClassC' class




  /// </summary>




  class Loan




  {




    public bool HasNoBadLoans(Customer c)




    {




      Console.WriteLine("Check loans for " + c.Name);




      return true;




    }




  }










  /// <summary>




  /// Customer class




  /// </summary>




  class Customer




  {




    private string _name;










    // Constructor




    public Customer(string name)




    {




      this._name = name;




    }










    // Gets the name




    public string Name




    {




      get { return _name; }




    }




  }










  /// <summary>




  /// The 'Facade' class




  /// </summary>




  class Mortgage




  {




    private Bank _bank = new Bank();




    private Loan _loan = new Loan();




    private Credit _credit = new Credit();










    public bool IsEligible(Customer cust, int amount)




    {




      Console.WriteLine("{0} applies for {1:C} loan\n",




        cust.Name, amount);










      bool eligible = true;










      // Check creditworthyness of applicant




      if (!_bank.HasSufficientSavings(cust, amount))




      {




        eligible = false;




      }




      else if (!_loan.HasNoBadLoans(cust))




      {




        eligible = false;




      }




      else if (!_credit.HasGoodCredit(cust))




      {




        eligible = false;




      }










      return eligible;




    }




  }




}


 
 
       
            

Output

Ann McKinsey applies for $125,000.00 loan

Check bank for Ann McKinsey
Check loans for Ann McKinsey
Check credit for Ann McKinsey

Ann McKinsey has been Approved