Wrapper Patterns in C#, Part II: The Proxy Pattern

Did you ever wish for a superhuman power to be impervious to bullets or travel outside your body? How about the superpower to be able to breathe underwater or fly? Or how about a changing the way you look so you can disguise yourself as anyone, or anything? In this series of four articles, we will travel down the C# rabbit hole and see how it is all possible with some wrapper patterns: Proxy, Decorator, and Adapter.

Part II. The Proxy

When we use our wrapper powers to have wrappers make objects appear where they are not or shield a wrapped object, we are really talking about the GOF Proxy pattern. Let us first look at how our newly discovered wrapper power can stop bullets and give us the ability to travel outside our bodies.

The force shield wrapper (protection by proxy)

Let's say we have a contract for a person IDude. All objects that implement the IDude interface have a name and can get shot.

public interface IDude

{

string Name { get; set; }

void GotShot(string typeOfGun);

}

Here is a NormalDude. If any NormalDude gets shot, they get hurt.

public class NormalDude: IDude

{

private string m_Name = string.Empty;

private bool m_IsHurt = false;

public string Name

{

get{ return m_Name;}

set{ m_Name = value; }

}

public void GotShot(string typeOfGun)

{

m_IsHurt = true;

Console.WriteLine(m_Name + " got shot by a " + typeOfGun + " gun.");

}

public override string ToString()

{

StringBuilder result = new StringBuilder();

result.Append(m_Name);

if(m_IsHurt) { result.Append(" is hurt"); }

else { result.Append(" is as healthy as a clam"); }

return result.ToString();

}

And here is a wrapper for our NormalDude: a SuperDude. If a SuperDude gets shot, he acts as a shield for the NormalDude he wraps. Any NormalDude wrapped in a SuperDude can't get hurt by getting shot. This is an example the Proxy pattern because we are limiting access to the wrapped object.

public class SuperDude : IDude

{

private IDude m_dude;

public SuperDude(IDude dude)

{

m_dude = dude;

}

#region IDude Members

public string Name

{

get { return "Super" + m_dude.Name; }

set { m_dude.Name = value; }

}

public void GotShot(string typeOfGun)

{

StringBuilder result = new StringBuilder();

result.Append(this.Name).Append(" got shot by a ").Append(typeOfGun);

result.Append(" gun but it bounced off!! \nYou can't hurt ").Append(this.Name);

result.Append("\n\n");

Console.WriteLine(result.ToString());

// NOTICE: THE GotShot() METHOD WAS NOT CALLED ON THE WRAPPED OBJECT

}

#endregion

public override string ToString()

{

StringBuilder result = new StringBuilder();

result.Append(Name).Append(" can't get hurt!");

result.Append(" (").Append(Name).Append(" is a super-hero proxy, you know).\n");

return result.ToString();

}

Here's what happens when the bad-guys come along:

IDude Joe = new NormalDude();
Joe.Name = "Average Joe";
IDude Clark = new NormalDude(); // MILD MANNERED CLARK (You know the one...)
Clark.Name = "Clark";
Clark = new SuperDude(Clark); // HERE IS WHERE WE WRAP Clark (He puts the suit on)
Console.WriteLine("HERE COME THE BAD-GUYS -- BULLETS ARE FLYING EVERYWHERE\n\n");
Joe.GotShot("Pea Shooter");
Clark.GotShot("Bazooka");
Console.WriteLine(Joe.ToString());
Console.WriteLine(Clark.ToString());

Here are the results:

Projection (interaction by proxy)

If use a wrapper to remotely view and interact with things, then it is a remote proxy. Let's look at another example. We can also use a proxy as a stand-in for the real object it is wrapping.

Let's say we have an interface that defines perceiving and changing a string.

public interface IInteractor

{

void Percieve(string percievedThing);

void Change(ref string perceivedThing);

}

And a Me class that implements the interface:

class Me: IInteractor

{

private string m_LookingAt;

private string m_ChangeTo;

public string ChangeTo

{

get { return m_ChangeTo; }

set { m_ChangeTo = value; }

}

#region IInteractor Members

public void Percieve(string percievedThing)

{

m_LookingAt= percievedThing;

}

public void Change(ref string perceivedThing)

{

Console.WriteLine("I'm changing " + perceivedThing + " to a " + m_ChangeTo);

perceivedThing = m_LookingAt = m_ChangeTo;

}

#endregion

public override string ToString()

{

return "I'm looking at a " + m_LookingAt;

}

A house class in which something can be changed:

class House

{

private string m_thing = "vase";

public void LookAtThing(IInteractor person)

{

person.Percieve(m_thing);

}

public void ChangeThing(IInteractor person)

{

person.Change(ref m_thing);

}

public override string ToString()

{

return "This house has a " + m_thing;

}

And a proxy for the Me class:

class MeProxy : IInteractor

{

private Me m_wrappedObject;

public MeProxy(Me wrappedObject)

{

m_wrappedObject = wrappedObject;

}

#region IInteractor Members

public void Percieve(string percievedThing)

{

Console.WriteLine("Perception by Proxy");

m_wrappedObject.Percieve(percievedThing);

}

public void Change(ref string perceivedThing)

{

Console.WriteLine("Change by Proxy");

m_wrappedObject.Change(ref perceivedThing);

}

#endregion

public override string ToString()

{

return "I'm an apparition. The real me says:" + m_wrappedObject.ToString();

}

Here's the story. Joe has a favorite vase at his house. We thought it would be funny to put a duck in place of the vase. When Joe found out, he was pretty angry so we use our secret proxy power to send an apparition into his house and change the duck into a bag of money (so he can buy a new vase).

IDude Joe = new NormalDude();
Joe.Name = "Average Joe";
Me Matt = new Me();
House JoesHouse = new House();
JoesHouse.LookAtThing(Matt); // I'm looking at what's in JoesHouse
Console.WriteLine(Matt.ToString());
Console.WriteLine(JoesHouse.ToString());
// I'll change whatever I find to a duck
Matt.ChangeTo = "duck";
JoesHouse.ChangeThing(Matt);
Console.WriteLine(Matt.ToString());
Console.WriteLine(JoesHouse.ToString());
Console.WriteLine("\nJoe is upset -- his vase is a duck.\n----------------------------------\n\n");
// Because Joe is upset, we'll use our superpower to change the duck to a bag-o-money
// We dont' want to go in ourselves, so we'll send the proxy.
MeProxy Apparition = new MeProxy(Matt);
Matt.ChangeTo = "bag-o-money";
JoesHouse.ChangeThing(Apparition);
Console.WriteLine(Matt.ToString());
Console.WriteLine(JoesHouse.ToString());
Console.WriteLine(Apparition.ToString());

This example demonstrates how we can use the proxy pattern to interact with things through a proxy by passing method calls through the proxy.

Next article we'll look at the powers a decorator can give us.

Until then

-Only use your powers for good.