Adapters and Decorators


The adapter pattern takes one service contract and adapts it (like a wrapper) to another.

This introductory article describes a concrete example of the adapter pattern and how you can work with it in Autofac.

Autofac provides built-in adapter registration so you can register a set of services and have them each automatically adapted to a different interface.

var builder = new ContainerBuilder();

// Register the services to be adapted
       .WithMetadata("Name", "Save File");
       .WithMetadata("Name", "Open File");

// Then register the adapter. In this case, the ICommand
// registrations are using some metadata, so we're
// adapting Meta<ICommand> instead of plain ICommand.
builder.RegisterAdapter<Meta<ICommand>, ToolbarButton>(
   cmd => new ToolbarButton(cmd.Value, (string)cmd.Metadata["Name"]));

var container = builder.Build();

// The resolved set of buttons will have two buttons
// in it - one button adapted for each of the registered
// ICommand instances.
var buttons = container.Resolve<IEnumerable<ToolbarButton>>();


The decorator pattern is somewhat similar to the adapter pattern, where one service “wraps” another. However, in contrast to adapters, decorators expose the same service as what they’re decorating. The point of using decorators is to add functionality to an object without changing the object’s signature.

This article has some details about how decorators work in Autofac.

Autofac provides built-in decorator registration so you can register services and have them automatically wrapped with decorator classes.

var builder = new ContainerBuilder();

// Register the services to be decorated. You have to
// name them rather than register them As<ICommandHandler>()
// so the *decorator* can be the As<ICommandHandler>() registration.

// Then register the decorator. The decorator uses the
// named registrations to get the items to wrap.
    (c, inner) => new CommandHandlerDecorator(inner),
    fromKey: "handler");

var container = builder.Build();

// The resolved set of commands will have two items
// in it, both of which will be wrapped in a CommandHandlerDecorator.
var handlers = container.Resolve<IEnumerable<ICommandHandler>>();

You can also use open generic decorator registrations.

var builder = new ContainerBuilder();

// Register the open generic with a name so the
// decorator can use it.
       .Named("handler", typeof(ICommandHandler<>));

// Register the generic decorator so it can wrap
// the resolved named generics.
        fromKey: "handler");

var container = builder.Build();

// You can then resolve closed generics and they'll be
// wrapped with your decorator.
var mailHandlers = container.Resolve<IEnumerable<ICommandHandler<EmailCommand>>>();

If you are using decorators on a WCF service implementation class, there is some additional information on the WCF integration page about some special considerations.