Autofac: Registering a Singleton and InstancePerDependency Dependency Relationship

Question

I'm using Autofac for dependency injection in my .NET application. I have a scenario where I want to register a service as a singleton and its dependency as InstancePerDependency. Here's a simplified example of what I'm trying to achieve:

builder.Register(c => new ClassA()).As<IClassA>().InstancePerDependency();

builder.Register(c => new ClassB(c.Resolve<IClassA>()))
       .As<IClassB>()
       .SingleInstance();

Will registering ClassB as a singleton affect IClassA's InstancePerDependency lifetime scope in this setup? If so, how can I achieve a singleton IClassA while ensuring that IClassA follows an InstancePerDependency lifetime scope?

Answer 1

In Autofac, when you register a service as a singleton and its dependency as InstancePerDependency, the singleton service (ClassB in your case) will hold onto a single instance of its dependency (ClassA in your case) throughout its lifetime. This means that every time ClassB is resolved, it will use the same instance of ClassA.

If you want ClassA to follow an InstancePerDependency lifetime scope while ensuring that ClassB remains a singleton, you can achieve this by registering ClassA as a factory delegate rather than a concrete instance. Here's how you can modify your registration:

builder.Register(c => new ClassA()).As<IClassA>().InstancePerDependency();

builder.Register(c =>
{
    var classAFactory = c.Resolve<Func<IClassA>>();
    return new ClassB(classAFactory());
})
.As<IClassB>()
.SingleInstance();