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copied!<blockquote> <p>If I wasn't using a DI container, I wouldn't have to reference EntityFramework library in my MVC3 app, only my business layer which would reference my DAL/Repo layer.</p> </blockquote> <p>Yes, that's exactly the situation DI works so hard to avoid :)</p> <p>With tightly coupled code, each library may only have a few references, but these again have other references, creating a deep graph of dependencies, like this:</p> <p><img src="https://i.stack.imgur.com/6HBLN.png" alt="Deep Graph"></p> <p>Because the dependency graph is deep, it means that most libraries drag along a lot of other dependencies - e.g. in the diagram, <em>Library C</em> drags along <em>Library H, Library E, Library J, Library M, Library K</em> and <em>Library N</em>. This makes it harder to reuse each library independently from the rest - for example <a href="https://blog.ploeh.dk/2011/11/10/TDDImprovesReusability.aspx" rel="nofollow noreferrer">in unit testing</a>.</p> <p>However, in a loosely coupled application, by moving all the references to the <a href="https://blog.ploeh.dk/2011/07/28/CompositionRoot.aspx" rel="nofollow noreferrer">Composition Root</a>, the <strong>dependency graph is severely flattened</strong>:</p> <p><img src="https://i.stack.imgur.com/y2xO2.png" alt="Shallow Graph"></p> <p>As illustrated by the green color, it's now possible to reuse <em>Library C</em> without dragging along any unwanted dependencies.</p> <p>However, all that said, with many DI Containers, you don't <em>have</em> to add hard references to all required libraries. Instead, you can use <strong>late binding</strong> either in the form of convention-based assembly-scanning (preferred) or XML configuration.</p> <p>When you do that, however, you must remember to copy the assemblies to the application's bin folder, because that no longer happens automatically. Personally, I rarely find it worth that extra effort.</p> <p>A more elaborate version of this answer can be found in <a href="https://freecontent.manning.com/dependency-injection-in-net-2nd-edition-understanding-the-composition-root/" rel="nofollow noreferrer">this excerpt</a> from <a href="https://manning.com/seemann2/" rel="nofollow noreferrer">Dependency Injection in .NET, second edition</a>.</p>

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