C# Design patterns: the Strategy pattern

January 28, 2009

Summary

Multiple classes implement an interface, handling an algorithm in a different way.

Example

Sorting is the clearest working example of the Strategy pattern. In .NET you’ll find it in the IComparer interface alongside Array.Sort/LINQ, see the links section for an article on this on MSDN.

	namespace DesignPatterns
	{
	public class User
	{
	public string Name { get; set; }
	public int Age { get; set; }
	public override string ToString()
	{
	return string.Format("{0} {1}", Name, Age);
	}
	}

	/// <summary>
	/// Compares two users based on name.
	/// </summary>
	public class NameSorter : IComparer<User>
	{
	public int Compare(User x, User y)
	{
	return x.Name.CompareTo(y.Name);
	}
	}

	/// <summary>
	/// Compares two Users based on their age.
	/// </summary>
	public class AgeSorter : IComparer<User>
	{
	public int Compare(User x, User y)
	{
	return x.Age.CompareTo(y.Age);
	}
	}

	/// <summary>
	/// A simple extension method class for pretty-printing the
	/// List<Users> collection.
	/// </summary>
	public static class ListExtension
	{
	public static string Print(this List<User> list)
	{
	StringBuilder builder = new StringBuilder();
	foreach (User user in list)
	{
	builder.AppendLine(user.ToString());
	}

	return builder.ToString();
	}
	}
	}

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I'm Chris Small, a software engineer working in London. This is my tech blog. Find out more about me via Github, Stackoverflow, Resume