For months, various sites have been sharing bits and pieces about Microsoft's CShell, aka the Composable Shell that the Windows team has been building.
CShell is an evolution of the existing Windows Shell. The Windows Shell is the part of Windows that handles Start, window management, the Activity Center and Settings, and task switching.
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Currently, the Windows Shell is different across different Windows devices. In other words, the way the Shell works in Windows Phones and HoloLens is different from how it works with Windows 10 on PCs. There is a single core (OneCore) across the various Windows 10 flavors on different device types, but not a single Shell. That's where Microsoft is hoping CShell will fit in.
CShell is primarily something that will benefit Microsoft's own developers and its third-party developers interested in writing Universal Windows Platform apps. Some users who have more than one type of Windows 10 device -- a Windows 10 PC and Windows Phone or a Windows 10 PC and a HoloLens -- may notice the commonality in settings and behaviors across devices with future iterations of Windows 10, as Windows Central noted this week in its look at the CShell running in one of the newer Windows 10 Fall Creators Update test builds that Microsoft inadvertently released last week.
CShell will figure into whatever Microsoft does next in the mobile-device space. The rumored Microsoft mobile device that may be some kind of tablet that includes telephony capabilities, which is believed to be codenamed "Andromeda," will likely include the CShell as noted recently by Cassim Ketfi in a post on Numerama.com.
Perhaps the biggest reason that CShell ultimately may matter to non-developers is in how it will improve Microsoft's Continuum experience. Continuum for phones is Microsoft's technology that enables phones to connect to external keyboards, mice, and larger screen monitors, allowing phones to act as CPUs.
Microsoft officials have been promising for a while that Continuum actually would provide a true PC-like experience. But, as Windows Central pointed out this week, CShell will be what enables some of the coming Continuum features, such as context menus for apps and fast app switching.