在Autofac中注册时如何避免两次使用new

Question

I want to register two dependencies using constructors with parameters but I want only to make 'new' on the 'Dependency' class one time. Instead of this:

builder.RegisterInstance(new ClassA(new Dependency())).As<IClassA>().SingleInstance();
builder.RegisterInstance(new ClassB(new Dependency())).AsSelf().SingleInstance();

I want something like this:

Dependency dependency = new Dependency();
builder.RegisterInstance(new ClassA(dependency)).As<IClassA>().SingleInstance();
builder.RegisterInstance(new ClassB(dependency)).AsSelf().SingleInstance();

Answer 1

Assuming that there is a reason you want to instantiate the type instead of registering it with Autofac which will do it for you then you should use Register instead of RegisterInstance .

builder.RegisterType<Dependency>().As<Dependency>().SingleInstance();
builder.Register(t => new ClassA(t.Resolve<Dependency>())).As<IClassA>().SingleInstance();

You can then use the passed in IComponentContext to resolve any required dependencies. Do keep in mind that with SingleInstance your instance is only created once so dependencies should be scoped similarly (don't expect a per request dependency to "renew" itself with every call to your singleton).

Answer 2

If you register the dependency first it should automatically get passed to the new instances if they require it.

Assumptions:

public class ClassA : IClassA
{
    public ClassA(Dependency dependency)
    {
    }
 }

public class ClassB
{
    public ClassB(Dependency dependency)
    {
    }
 }

Then register dependency

var dependency = new Dependency();
builder.RegisterInstance(dependency).As<Dependency>();
builder.RegisterType<ClassA>().As<IClassA>().SingleInstance();
builder.RegisterType<ClassB>().AsSelf().SingleInstance();