RegisterInstance & TransientLifetimeManager, is it even possible?

Question

Recently one of our development teams started with dependency injection.

One thing I noticed is that (when initializing the Container) the code has a lot of places, where it uses a TransientLifetimeManager on a instance. The instance has been created explicitly by the developer a few lines before.

KalenderView kalenderView = new KalenderView(new KalenderViewModel());
unityContainer.RegisterInstance(kalenderView, new TransientLifetimeManager());

Personally I would rewrite it into two RegisterType calls, one for the ViewModel, one for the View, but this got my attention.

My question is: Will the code even do, what I would expect it to do? It looks like it's creating a new object everytime a instance needs to be resolved (roughly explained). But that can't be right, can it? It would be "magic" for me, I don't think, unity can "analyse" and save the state of an object during registration, right?

I found nothing on whether it's possible to use TransientLifetimeManager on RegisterInstance. So, will it ignore the given manager and use the ContainerControlledLifetimeManager instead?

Answer 1

Let's write simple test:

namespace UnityTest
{
    using System;
    using Unity;

    class Program
    {
        public class TestClass
        {
            public static int Version = 0;

            public TestClass()
            {
                Version++;
                Console.WriteLine(Version);
            }
        }

        static void Main(string[] args)
        {
            var container = new UnityContainer();

            var obj = new TestClass();

            container.RegisterInstance(obj, new TransientLifetimeManager());

            container.Resolve<TestClass>();
            container.Resolve<TestClass>();
            container.Resolve<TestClass>();
        }
    }
}

Will output:

1
2
3
4

So, container creates new instance per resolve.

You can even remove registration and it will work:

namespace UnityTest
{
    using System;
    using Unity;

    class Program
    {
        public class TestClass
        {
            public static int Version = 0;

            public TestClass()
            {
                Version++;
                Console.WriteLine(Version);
            }
        }

        static void Main(string[] args)
        {
            var container = new UnityContainer();

            container.Resolve<TestClass>();
            container.Resolve<TestClass>();
            container.Resolve<TestClass>();
        }
    }
}

Will output:

1
2
3