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copied!<p><code>List<T></code> is a specific implementation of <code>IList<T></code>, which is a container that can be addressed the same way as a linear array <code>T[]</code> using an integer index. When you specify <code>IList<T></code> as the type of the method's argument, you only specify that you need certain capabilities of the container. </p> <p>For example, the interface specification does not enforce a specific data structure to be used. The implementation of <code>List<T></code> happens to the same performance for accessing, deleting and adding elements as a linear array. However, you could imagine an implementation that is backed by a linked list instead, for which adding elements to the end is cheaper (constant-time) but random-access much more expensive. (Note that the .NET <code>LinkedList<T></code> does <em>not</em> implement <code>IList<T></code>.)</p> <p>This example also tells you that there may be situations when you need to specify the implementation, not the interface, in the argument list: In this example, whenever you require a particular access performance characteristic. This is usually guaranteed for a specific implementation of a container (<code>List<T></code> documentation: "It implements the <code>IList<T></code> generic interface using an array whose size is dynamically increased as required.").</p> <p>Additionally, you might want to consider exposing the least functionality you need. For example. if you don't need to change the content of the list, you should probably consider using <code>IEnumerable<T></code>, which <code>IList<T></code> extends.</p>

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