9.67 Resolvers

Windsor gives you ability to override/extend its default resolution strategy for components/dependencies by using sub dependency resolvers.

How does it work

When resolving components Windsor will ask all ISubDependencyResolvers registered with its IDependencyResolver if they would like to provide the component themselves.

:information_source: See How dependencies are resolved for more details.

The ISubDependencyResolver interface

Resolvers are types implementing ISubDependencyResolver interface, which exposes two methods:

bool CanResolve(CreationContext context, ISubDependencyResolver contextHandlerResolver, ComponentModel model, DependencyModel dependency);
object Resolve(CreationContext context, ISubDependencyResolver contextHandlerResolver, ComponentModel model, DependencyModel dependency);

First CanResolve is called to see if the resolver would like to provide given component, then if it returns true, Resolve will be called to let the resolver provide the value.

To plug your own custom resolver you need to add it to kernel's IDependencyResolver's collection of sub-resolvers:

container.Kernel.Resolver.AddSubResolver(new MyOwnResolver());

Standard resolvers

Windsor provides few resolvers that you can plug into the container out of the box.

CollectionResolver

:information_source: This is a new type in Windsor 2.5

The CollectionResolver is used to satisfy dependency on a collection of components, for example:

:information_source: Supported types: There are many collection types that CollectionResolver supports. Namely IEnumerable<T>, ICollection<T>, IList<T> and T[]. This makes it the preferred choice over the other resolver types discussed below

public class Foo
{
    public Foo(IEnumerable<IBar> bars)
    {
        // something
    }
}

It has the same constructors as array resolver discussed below.

ArrayResolver

The ArrayResolver is used to satisfy dependency on an array of components, for example:

public class Foo
{
    public Foo(IBar[] bars)
    {
    }
}

Without using ArrayResolver Windsor will try to locate a component for IBar[], whereas what you would most likely have is multiple components for IBar. If you want to inject all the IBar components as the array, you use ArrayResolver. The resolver will resolve all IBar components it can, and then construct an array, put them in the array and provide the array as the value for the dependency.

ArrayResolver constructors

The ArrayResolver type exposes two constructors:

public ArrayResolver(IKernel kernel);
public ArrayResolver(IKernel kernel, bool allowEmptyArray);

When you call the first one, or the second one with allowEmptyArray set to false, and no instance of IBar can be resolved the resolver will throw an exception. If you set allowEmptyArray to true an empty array will be provided instead.

You simply register it with the container, passing the kernel as argument to its constructor:

container.Kernel.Resolver.AddSubResolver(new ArrayResolver(container.Kernel, true));

ListResolver

The ListResolver is used to satisfy dependency on a generic list of components, for example:

public class Foo
{
    public Foo(IList<IBar> bars)
    {
    }
}

It works identically to ArrayResolver.