designpatterns.md

Design Patterns

LightInject has built-in support for the Decorator pattern and the Composite pattern.

Decorator Pattern

A decorator is a class that implements the same interface as the type it is decorating and takes the target instance as a constructor argument.

public class FooDecorator : IFoo
{     
    public FooDecorator(IFoo foo)
    {     
    }
} 

Decorators are applied using the Decorate method.

container.Register<IFoo, Foo>();
container.Decorate(typeof(IFoo), typeof(FooDecorator));

var instance = container.GetInstance<IFoo>();

Assert.IsInstanceOfType(instance, typeof(FooDecorator));

Decorators can be nested and they are applied in the same sequence as they are registered.

container.Register<IFoo, Foo>();            
container.Decorate(typeof(IFoo), typeof(FooDecorator));
container.Decorate(typeof(IFoo), typeof(AnotherFooDecorator));

var instance = container.GetInstance<IFoo>();

Assert.IsInstanceOfType(instance, typeof(AnotherFooDecorator));

If we have multiple services implementing the same interface, we can apply the decorator to implementations matching the given predicate.

container.Register<IFoo, Foo>();
container.Register<IFoo, AnotherFoo>("AnotherFoo");
container.Decorate(typeof(IFoo), typeof(FooDecorator), service => service.ServiceName == "AnotherFoo");

var instance = container.GetInstance<IFoo>();
var decoratedInstance = container.GetInstance<IFoo>("AnotherFoo");

Assert.IsInstanceOfType(instance, typeof(Foo));
Assert.IsInstanceOfType(decoratedInstance, typeof(FooDecorator));

Decorators can have their own dependencies in addition to the target instance it is decorating.

public class FooDecoratorWithDependency : IFoo
{        
    public FooDecoratorWithDependency(IFoo foo, IBar bar)
    {
        Foo = foo;
        Bar = bar;
    }

    public IFoo Foo { get; private set; }

    public IBar Bar { get; private set; }
}

The dependencies of the decorator can be implicitly resolved.

container.Register<IFoo, Foo>();
container.Register<IBar, Bar>();
container.Decorate(typeof(IFoo), typeof(FooDecoratorWithDependency));

var instance = (FooDecoratorWithDependency)container.GetInstance<IFoo>();

Assert.IsInstanceOfType(instance.Foo, typeof(IFoo));
Assert.IsInstanceOfType(instance.Bar, typeof(IBar));

By using a function factory, we can explicitly specify the depenendecies of the decorator.

container.Register<IFoo, Foo>();
container.Register<IBar, Bar>();
container.Decorate<IFoo>((serviceFactory, target) 
    => new FooDecoratorWithDependency(target, serviceFactory.GetInstance<IBar>()));

var instance = (FooDecoratorWithDependency)container.GetInstance<IFoo>();

Assert.IsInstanceOfType(instance.Foo, typeof(IFoo));
Assert.IsInstanceOfType(instance.Bar, typeof(IBar));

Note: The target instance is available through the function delegate so that we can pass it to the constructor of the decorator.

Decorators can also be applied to open generic types.

container.Register(typeof(IFoo<>), typeof(Foo<>));
container.Decorate(typeof(IFoo<>), typeof(FooDecorator<>));

var instance = container.GetInstance<IFoo<int>>();

Assert.IsInstanceOfType(instance, typeof(FooDecorator<int>));

Lazy Decorators

A lazy decorator is a decorator that creates its target only when and if one of its methods are invoked.

public class LazyFooDecorator : IFoo
{
    private Lazy<IFoo> lazyFoo;
    public LazyFooDecorator(Lazy<IFoo> lazyFoo)
    {
        this.lazyFoo = lazyFoo;
    }            

    public void Execute()
    {
        lazyFoo.Value.Execute;
    }
}

This decorator then postpones the creation of the target IFoo instance until the Execute method is invoked.

Since LightInject has native support for Lazy<T> this becomes very easy to configure.

container.Register(typeof(IFoo), typeof(Foo));
container.Decorate(typeof(IFoo), typeof(LazyFooDecorator));    

var instance = container.GetInstance<IFoo>();    

Assert.IsInstanceOfType(instance, typeof(LazyFooDecorator));

Composite Pattern

The composite pattern is a simple pattern that lets a class implement an interface and then delegates invocation of methods to a set other classes implementing the same interface.

public class FooWithEnumerableIFooDependency : IFoo
{
    public IEnumerable<IFoo> FooList { get; private set; }

    public FooWithEnumerableIFooDependency(IEnumerable<IFoo> fooList)
    {
        FooList = fooList;
    }
}

While this looks like a recursive dependency, LightInject detects this and removes the FooWithEnumerableIFooDependency from the IEnumerable<IFoo> beeing injected.

container.Register(typeof(IFoo), typeof(Foo), "Foo");
container.Register(typeof(IFoo), typeof(AnotherFoo), "AnotherFoo");
container.Register(typeof(IFoo), typeof(FooWithEnumerableIFooDependency));            
var instance = (FooWithEnumerableIFooDependency)container.GetInstance<IFoo>();
Assert.IsInstanceOfType(instance.FooList.First(), typeof(Foo));
Assert.IsInstanceOfType(instance.FooList.Last(), typeof(AnotherFoo));