Smart Clients is a concept (not technology) of a Rich Internet enabled application. You can think of it as hybrid of thick & thin clients providing best of both worlds. Smart client applications provide richest user experience, intuitive/easier navigation, greater performance & throughput, leverage local computing horsepower, provide access to full keyboard (shortcuts), faciliate fast data-entry, provides easier/seamless integration with devices & local desktop applications (Office, LOB apps, etc), consume web-services (internet enabled), and can be designed to function in connected and disconnected/offline scenarios and still offer seamless/transparent deployment (main reason behind the adoption of web applications). Microsoft Smart Client solutions/applications may comprise one or more than one of these technologies (Windows Forms, InfoPath; XML Based Form Designer, Visual Studio Tools for Office, .NET Compact Framework based apps running on mobile devices). WinForms is the primary technology for building smart client applications/solutions, but other technologies may come into picture based on use case scenario, end-users and device selection.you Please note that you can embed web/browser based applications in smart client shell, so you are not giving up on either technology.
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