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copied!<p>A standard approach, and one I take often, is to have a common library of object models representing my domain: Customer, Order, OrderLine, etc, that are shared across all layers. </p> <p>Better still, don't share the types across layers, just share the interfaces and have a factory available when instances are required.</p> <p>You can then have a pluggable DAL, but your DAL still needs to conform to the contract of returning an ICustomer.</p> <p>This doesn't stop references from being required, a reference to at least the interfaces are needed - as someone else has commented, other references, for the strong types or factories, could then be factored out - such as with IoC/DI frameworks.</p> <p>The way I see it: a commom model is a cross-cutting concern in your application design, and shouldn't be seen as breaking the layering.</p> <p><strong>Update:</strong> this is a very vague explanation of the solution, so I stand to have it built upon.</p> <p><strong>Update 2:</strong> and now of course to answer your question. A standard way to remove a reference to a DLL directly is to expose the contract via an interface, in your case a DAL method <code>GetCustomers</code>, your Service Layer talks to the interface, but requests a DAL instance via a Dependency Injection / Inversion of Control framework, or more easily, a factory. I usually go the factory route for small apps, it involves another DLL. Service Layer will reference interfaces and factory, DAL will reference interface, factory will reference interface and DAL.</p> <p><img src="https://i.stack.imgur.com/pI9WQ.png" alt="enter image description here"></p>

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