Strategy

The strategy pattern

The strategy pattern is a behavioral pattern and is a very simple pattern. Essentially, we choose an object based on some strategy.
Without further ado lets start with an example of the strategy abstraction:

abstract class GamePlan {
    public virtual void Scheme() {
    Console.WriteLine("{0} is scheming.", this.GetType().Name);
    }
}

We then have a class that has to use our strategy:

class Scenario {
    private GamePlan gamePlan;
    private Scenario(GamePlan gamePlan) {
        this.gamePlan = gamePlan;
    }
    public static Scenario CreateScenarioWithGamePlan(GamePlan gamePlan) {
        return new Scenario(gamePlan);
    }
    public void Maneuver() {
        gamePlan.Scheme();
    }
}

Pretty basic stuff, we can inject out strategy at runtime.
I've constructed a few simple classes that would enact a specific strategy:

class SimpleTactics : GamePlan {
}

class FancyTactics : GamePlan {
}

class ComplexTactics : GamePlan {
    public override void Scheme() {
        base.Scheme();
        Console.WriteLine("Adding more for ComplexTactics...");
    }
}

Then a very simple test program, now I've not introduced any criteria for our strategy changes but imagine some illustrious decision making engine:

static void Main(string[] args) {
    GamePlan simpleTactics = new SimpleTactics();
    GamePlan fancyTactics = new FancyTactics();
    GamePlan complexTactics = new ComplexTactics();

    Scenario scenario = Scenario.CreateScenarioWithGamePlan(simpleTactics);
    scenario.Maneuver();

    scenario = Scenario.CreateScenarioWithGamePlan(fancyTactics);
    scenario.Maneuver();

    scenario = Scenario.CreateScenarioWithGamePlan(complexTactics);
    scenario.Maneuver();

    Console.ReadKey();
}

And then the output is predictably:

SimpleTactics is scheming.
FancyTactics is scheming.
ComplexTactics is scheming.
Adding more for ComplexTactics...

Summary

That's the strategy pattern, I think it's one of the simpler patterns and is used ubiquitously, but the developer that used the pattern may not have known that it was used.

References

wiki
Design Patterns [GOF]
Adaptive Code Pg 133
dofactory strategy pattern
oodesign adapter pattern