Question

I have started to use Autofac following this tutorials:
http://flexamusements.blogspot.com/2010/09/dependency-injection-part-3-making-our.html

Simple class with no parameter in the constructor

builder.RegisterType<ConsoleOutputService>().As<IOutputService>();

As explained in the tutorial, the code above can be read as: setup ConsoleOutputService as the implementation of IOutputService

Simple class with one parameter in the constructor

builder.Register(c => new MultipleOutputService(outputFilePath)).As<IOutputService>();

I don’t understand why are we using a lambda expression to register this class (and what does this expression exactly does) and why we can’t type this code

builder.RegisterType<MultipleOutputService(outputFilePath)>().As<IOutputService>();

Thanks in advance for your help

Answer 1

You can’t write that code because it doesn’t make sense in C#.
RegisterType is a generic method; generic methods must take types as generic parameters.

You’re trying to register a type with a custom way to create it (inyour case, a constructor parameter); the only way that C# supports to specify such a thing is a lambda expression (or other delegate).