Injecting a nullable dependency via constructor requires unity to register that nullable dependency

Question

Just wondering if anyone had this scenario with Unity(as an IoC container) where the class has two injected dependencies(interfaces) where one dependency can be null. For example:

public MyServiceDll(IRepository repository, ICanBeNullDependency canBeNullDependency = null)
{
    _repository = repository;
    _canBeNullDependency = canBeNullDependency;
}

ICanBeNullDependency is from another assembly. MyServiceDll is another assembly. MyServiceDll is referenced by web api and injected its interface in one of the controllers. ICanBeNullDependency can be null so I don't need to register an implementation of this interface in unityconfig.cs but when the controller gets called it will error saying:

The current type, ICanBeNullDependency, is an interface and cannot be constructed. Are you missing a type mapping?

NightOwl888 · Accepted Answer · 2017-01-19 18:41:39Z

3

For dependencies that are not always required, you can use the null object pattern.

public interface ICanBeNullDependency
{
    void DoSomething();
}

public class AcutallyDoSomething : ICanBeNullDependency
{
    public void DoSomething()
    {
         Console.WriteLine("something done");
    }
}

public class NullDoSomething : ICanBeNullDependency
{
    public void DoSomething()
    {
        // Do nothing - this class represents a null
    }
}

Then if you want a "null state", you inject an instance of the null class.

The main advantage over actually making the variable null is that you don't need any conditional logic in your service to deal with a null, and of course it is DI-friendly.

edited Jan 19, 2017 at 18:41

answered Jan 19, 2017 at 16:44

NightOwl888

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1 Comment

Steven Over a year ago

I would even say "you should use the null object pattern."

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