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    <p>The main reason to use DI is that you want to put the responsibility of the knowledge of the implementation where the knowledge is there. The idea of DI is very much inline with encapsulation and design by interface. If the front end asks from the back end for some data, then is it unimportant for the front end how the back end resolves that question. That is up to the requesthandler.</p> <p>That is already common in OOP for a long time. Many times creating code pieces like:</p> <pre><code>I_Dosomething x = new Impl_Dosomething(); </code></pre> <p>The drawback is that the implementation class is still hardcoded, hence has the front end the knowledge which implementation is used. DI takes the design by interface one step further, that the only thing the front end needs to know is the knowledge of the interface. In between the DYI and DI is the pattern of a service locator, because the front end has to provide a key (present in the registry of the service locator) to lets its request become resolved. Service locator example:</p> <pre><code>I_Dosomething x = ServiceLocator.returnDoing(String pKey); </code></pre> <p>DI example:</p> <pre><code>I_Dosomething x = DIContainer.returnThat(); </code></pre> <p>One of the requirements of DI is that the container must be able to find out which class is the implementation of which interface. Hence does a DI container require strongly typed design and only one implementation for each interface at the same time. If you need more implementations of an interface at the same time (like a calculator), you need the service locator or factory design pattern.</p> <p>D(b)I: Dependency Injection and Design by Interface. This restriction is not a very big practical problem though. The benefit of using D(b)I is that it serves communication between the client and the provider. An interface is a perspective on an object or a set of behaviours. The latter is crucial here. </p> <p>I prefer the administration of service contracts together with D(b)I in coding. They should go together. The use of D(b)I as a technical solution without organizational administration of service contracts is not very beneficial in my point of view, because DI is then just an extra layer of encapsulation. But when you can use it together with organizational administration you can really make use of the organizing principle D(b)I offers. It can help you in the long run to structure communication with the client and other technical departments in topics as testing, versioning and the development of alternatives. When you have an implicit interface as in a hardcoded class, then is it much less communicable over time then when you make it explicit using D(b)I. It all boils down to maintenance, which is over time and not at a time. :-)</p>
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    1. POWhy does one use dependency injection?
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    1. USLoek Bergman
    1. USLoek Bergman
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    1. CO"The drawback is that the implementation class is still hardcoded" <-- most of the time there is only one implementation and like I said I can't think of nongame code that requires an interface that isn't already built in (.NET).
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    2. COI have edited my answer and deleted my previous comment.
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    3. CO@acidzombie24 May be ... but compare the effort to implement a solution using DI from the beginning to the effort to change a non-DI solution later if you need interfaces. I'd nearly always go with the first option. It's better to pay now 100$ instead of having to pay 100.000$ tomorrow.
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