DI with Unity when multiple instances of the same type is needed

Question

I need help with this. I\'m using Unity as my container and I want to inject two different instances of the same type into my constructor.

class Example
{
           


        

Answer 1

There are many ways to achieve the results you want (as evidenced by the multiple answers). Here is another way using named registrations (without attributes):

IUnityContainer container = new UnityContainer();

container.RegisterType("ReceiveQueue", 
    new InjectionConstructor("receivePath"));

container.RegisterType("SendQueue",
    new InjectionConstructor("sendPath"));

container.RegisterType(
    new InjectionConstructor(
        new ResolvedParameter("ReceiveQueue"),
        new ResolvedParameter("SendQueue")));

Example example = container.Resolve();

The downside of this approach is that if the Example constructor is changed then the registration code must also be modified to match. Also, the error would be a runtime error and not a more preferable compile time error.

You could combine the above with an InjectionFactory to invoke the constructor manually to give compile time checking:

IUnityContainer container = new UnityContainer();

container.RegisterType("ReceiveQueue",
    new InjectionConstructor("receivePath"));

container.RegisterType("SendQueue",
    new InjectionConstructor("sendPath"));

container.RegisterType(new InjectionFactory(c =>
    new Example(c.Resolve("ReceiveQueue"),
                c.Resolve("SendQueue"))));

Example example = container.Resolve();

If you are using a composition root then the use of the magic strings ("ReceiveQueue" and "SendQueue") would be limited to the one registration location.