Strategy

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();
    	}
    }
}