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IT Forum >> Lesson: Strategy Design Pattern
| 10/23/05 5:15 PM | |
theseanster
156 
Edited: 23-Oct-05 Member Since: 05/13/2002 Posts: 8637 |
The Strategy (GoF) pattern is basically: giving the ability to provide an algorithm (implementation) to another class so it can handle a specific operation. The best anology I have seen was in MSDN magazine when they talked about using IComparer. You've probably already used this pattern without know its formal name. Just so I can walk you through it "codified" I'll do something that MSDN magazine didn't do...show the code. If you take a look at the code below we have a Person class and a PersonCollection. The SortByName method on the PersonCollection class wraps an ArrayList.Sort() call. ArrayList.Sort lets you provide an IComparer implementation...or strategy. To be clear, I've derived my own class from IComperer called "Strategy". This contains a very simple strategy algorithm which is defaulting to use the String.Compare operation so we can sort alphabetically by name. The call to this.InnerList.Sort passes an instance of the Strategy class. Quite simply, this is the GoF strategy pattern. public class Person { //should be properties, but to be brief we'll use fields public string Name; public int Age; public Person(string name, int age) { Name = name; Age = age; } } public class PersonCollection : CollectionBase { class Strategy : IComparer { public int Compare(object x, object y) { Person px = x as Person; Person py = y as Person; return String.Compare(px.Name, py.Name); } } public void SortByName() { this.InnerList.Sort(new Strategy()); } public void Add(Person value) { this.InnerList.Add(value); } } class Program { static void Main(string[] args) { PersonCollection people = new PersonCollection(); people.Add(new Person("Bubba", 20)); people.Add(new Person("Al", 22)); people.Add(new Person("Cooter", 99)); people.SortByName(); //prints: Al, Bubba, Cooter foreach(Person p in people) Console.WriteLine(p.Name); } } |
| 10/23/05 5:23 PM | |
|
theseanster
156
Edited: 23-Oct-05 Member Since: 05/13/2002 Posts: 8638 |
Note that in C# 2.0, which will RTM in about 2 weeks, you can simplify the above example quite a bit using anonymous methods. It turns out the to sort a generic List<T>, you can pass in a System.Comparison delegate like so...
static void Main(string[] args) { List<Person> people = new List<Person>(); people.Add(new Person("Bubba", 20)); people.Add(new Person("Al", 22)); people.Add(new Person("Cooter", 99)); //use anonymous method here... people.Sort(delegate(Person x, Person y) { return String.Compare(x.Name, y.Name); }); //prints: Al, Bubba, Cooter foreach(Person p in people) Console.WriteLine(p.Name); } |
| 10/23/05 5:37 PM | |
|
theseanster
156
Edited: 23-Oct-05 05:37 PM Member Since: 05/13/2002 Posts: 8639 |
While we're on the subject, in C# 3.0 you can make this example even more simple using lambdas. The whole strategy pattern for sorting because a single lambda expression like so... static void Main(string[] args) { List<Person> people = new List<Person>(); people.Add(new Person("Bubba", 20)); people.Add(new Person("Al", 22)); people.Add(new Person("Cooter", 99)); //use the C# 3.0 lambda feature people.Sort(((x, y) => String.Compare(x.Name, y.Name))); //prints: Al, Bubba, Cooter foreach(Person p in people) Console.WriteLine(p.Name); } |
| 11/20/05 3:56 PM | |
|
theseanster
156
Edited: 20-Nov-05 Member Since: 05/13/2002 Posts: 8866 |
Is this too much for everyone??? :-) |
| 11/20/05 7:46 PM | |
|
theseanster
156
Edited: 20-Nov-05 Member Since: 05/13/2002 Posts: 8870 |
I've already mastered notepad, and for the last time...I'm not interested in animal porn or Joe Ray's anal videos. Sheesh. |
| 1/27/06 9:23 PM | |
|
theseanster
156
Edited: 27-Jan-06 Member Since: 05/13/2002 Posts: 9515 |
TTT by request... |
| 1/28/06 12:16 AM | |
Revolver of Reason
Edited: 28-Jan-06 Member Since: 01/01/2001 Posts: 29679 |
ttt for a good'un |
| 11/13/12 11:37 AM | |
|
theseanster
156
Member Since: 5/13/02 Posts: 21201 |
bump to bring back from the dead |
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