Question

Is it possible to do something like the following:

public class ChildClass : BaseClass
{
    public ChildClass(BaseClass o)
    {
        base = o;
    }
}

Basically, I want a transparent way to wrap a base class inside of other functionality. One example I’ve thought of is a custom Settings Provider which transparently audits the settings passed through it.

public class SettingsAuditor : SettingsProvider
{
    public SettingsAuditor(SettingsProvider o)
    {
        base = o;
    }

    public override void SetPropertyValues(SettingsContext context, SettingsPropertyValueCollection propvals)
    {
        // Log the property change to a file
        base.SetPropertyValues(context, propvals);
    }
}

Then I could do the following:

mySettingsProvider = new SettingsAuditor(mySettingsProvider);

And all changes would go through the overridden SetPropertyValues before passing to the original object.

I could use a private SettingsProvider member, but then I either cannot inherit from SettingsProvider, or have an entire SettingsProvider (base) not being used at all.

I’m using C# 4.0 and .Net 4.0.

Answer 1

1. You cannot do base = o;
2. What you’re looking for is the Decorator Pattern), which is a way to compositionally add functionality at runtime (vs. inheritance).

Instead of trying to set the base, you just contain the inner member. As long as the wrapper implements the same interface or base class as the inner object, you can pass back the new wrapper. You can wrap as many decorators as you want.

Consider:

public interface ICar
{
    void Drive();
}

public class Car : ICar
{
    public void Drive()
    {
        Console.WriteLine("vroom");
    }
}

public class BuckleUp : ICar
{
    ICar car;

    public BuckleUp(ICar car) { this.car = car; }

    public void Drive()
    {
        Console.WriteLine("click!");
        car.Drive();
    }
}

public class CheckMirrors : ICar
{
    ICar car;
    public CheckMirrors(ICar car) { this.car = car; }

    public void Drive()
    {
        Console.WriteLine("mirrors adjusted");
        car.Drive();
    }
}

Now consider you have a method that accepts an ICar and tells it to drive. You could give it a Car, and it would work, but you could also wrap that car in a BuckleUp and a CheckMirrors and you wouldn’t have to change that method at all. You’ve modified functionality through composition using the Decorator Pattern.