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    <p>The books Agile Software Development, Principles, Patterns, and Practices and Agile Principles, Patterns, and Practices in C# are the best resources for fully understanding the original goals and motivations behind the Dependency Inversion Principle. The article "The Dependency Inversion Principle" is also a good resource, but due to the fact that it is a condensed version of a draft which eventually made its way into the previously mentioned books, it leaves out some important discussion on the concept of a package and interface ownership which are key to distinguishing this principle from the more general advise to "program to an interface, not an implementation" found within the book Design Patterns (Gamma, et. al).</p> <p>To provide a summary, the Dependency Inversion Principle is primarily about <strong>reversing</strong> the conventional direction of dependencies from "higher level" components to "lower level" components such that "lower level" components are dependent upon the interfaces <strong>owned</strong> by the "higher level" components. (Note: "higher level" component here refers to the component requiring external dependencies/services, not necessarily its conceptual position within a layered architecture.) In doing so, coupling isn't <strong>reduced</strong> so much as it is <strong>shifted</strong> from components that are theoretically less valuable for reuse to components which are theoretically more valuable for reuse.</p> <p>This is achieved by designing components whose external dependencies are expressed in terms of an interface for which an implementation must be provided by the consumer of the component. In other words, the defined interfaces express what is needed by the component, not how you use the component (e.g. "INeedSomething", not "IDoSomething").</p> <p>What the Dependency Inversion Principle does not refer to is the simple practice of abstracting dependencies through the use of interfaces (e.g. MyService → [ILogger ⇐ Logger]). While this decouples a component from the specific implementation detail of the dependency, it does not invert the relationship between the consumer and dependency (e.g. [MyService → IMyServiceLogger] ⇐ Logger.</p> <p>The importance of the Dependency Inversion Principle is seen primarily in the development of reusable software components which rely upon external dependencies (logging, validation, etc.) since taking dependencies upon such dependencies requires your consumers to require the same dependencies as well. This can be problematic when consumers of your library choose to use a different library for the same infrastructure needs (e.g. NLog vs. log4net), or if they choose to use a later version of the required library which isn't backward compatible with the version required by your library.</p> <p>A longer discussion of this principle as it relates to the simple use of interfaces, Dependency Injection, and the Separated Interface pattern can be found <a href="http://aspiringcraftsman.com/2008/12/28/examining-dependency-inversion/" rel="noreferrer">here</a>.</p>
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    1. POWhat is the dependency inversion principle and why is it important?
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    1. USDerek Greer
    1. USDerek Greer
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    1. COThanks. I see now how my answer misses the point. The difference between **MyService → [ILogger ⇐ Logger]** and **[MyService → IMyServiceLogger] ⇐ Logger** is subtle but important.
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      1. USPatrick McElhaney
    2. COIn the same line its very well explained here: http://lostechies.com/derickbailey/2011/09/22/dependency-injection-is-not-the-same-as-the-dependency-inversion-principle/
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    3. COHow I wished people would stop using loggers as the canonical usecase for dependency injection. Especially in connection with log4net where its almost an anti pattern. That aside, kickass explanation!
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      1. USCasper Leon Nielsen
 

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