In WPF and Silverlight, Dispatcher is an object that is used to execute work on a specific thread. In another meaning, dispatcher defines an object that receives requests from the client and sends them to any resource (such as a servlet, HTML file, or JSP file) on the server.

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Officially the open/close principle states that: Software entities should be open for extension, but closed for modification. The idea here is that code that is tried and tested, shouldn't have to change because you want to add some functionality to it.