Definition

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Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

UML class diagram

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Participants

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The classes and/or objects participating in this pattern are:

• Strategy - declares an interface common to all supported algorithms. Context uses this interface to call the algorithm defined by a ConcreteStrategy.
• ConcreteStrategy - implements the algorithm using the Strategy interface

Context
is configured with a ConcreteStrategy object

maintains a reference to a Strategy object

may define an interface that lets Strategy access its data.

Sample code in C#

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This allows clients to dynamically change sorting strategies including Quicksort, Shellsort, and Mergesort.

  /// <summary>
  /// MainApp startup class for Real-World
  /// Strategy Design Pattern.
  /// </summary>
  class MainApp
  {
    /// <summary>
    /// Entry point into console application.
    /// </summary>
    static void Main()
    {
      // Two contexts following different strategies
      SortedList studentRecords = new SortedList();
 
      studentRecords.Add("Samual");
      studentRecords.Add("Jimmy");
      studentRecords.Add("Sandra");
      studentRecords.Add("Vivek");
      studentRecords.Add("Anna");
 
      studentRecords.SetSortStrategy(new QuickSort());
      studentRecords.Sort();
 
      studentRecords.SetSortStrategy(new ShellSort());
      studentRecords.Sort();
 
      studentRecords.SetSortStrategy(new MergeSort());
      studentRecords.Sort();
 
      // Wait for user
      Console.ReadKey();
    }
  }
 

  /// <summary>
  /// The 'Strategy' abstract class
  /// </summary>
  abstract class SortStrategy
  {
    public abstract void Sort(List<string> list);
  }
 
  /// <summary>
  /// A 'ConcreteStrategy' class
  /// </summary>
  class QuickSort : SortStrategy
  {
    public override void Sort(List<string> list)
    {
      list.Sort(); // Default is Quicksort
      Console.WriteLine("QuickSorted list ");
    }
  }
 
  /// <summary>
  /// A 'ConcreteStrategy' class
  /// </summary>
  class ShellSort : SortStrategy
  {
    public override void Sort(List<string> list)
    {
      //list.ShellSort(); not-implemented
      Console.WriteLine("ShellSorted list ");
    }
  }
 
  /// <summary>
  /// A 'ConcreteStrategy' class
  /// </summary>
  class MergeSort : SortStrategy
  {
    public override void Sort(List<string> list)
    {
      //list.MergeSort(); not-implemented
      Console.WriteLine("MergeSorted list ");
    }
  }
 
  /// <summary>
  /// The 'Context' class
  /// </summary>
  class SortedList
  {
    private List<string> _list = new List<string>();
    private SortStrategy _sortstrategy;
 
    public void SetSortStrategy(SortStrategy sortstrategy)
    {
      this._sortstrategy = sortstrategy;
    }
 
    public void Add(string name)
    {
      _list.Add(name);
    }
 
    public void Sort()
    {
      _sortstrategy.Sort(_list);
 
      // Iterate over list and display results
      foreach (string name in _list)
      {
        Console.WriteLine(" " + name);
      }
      Console.WriteLine();
    }
  }

 

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