Side-by-side assembly

Side-by-side technology is a standard for executable files in Windows 98 Second Edition, Windows 2000, and later versions of Windows that attempts to alleviate problems (known as "DLL Hell") that arise from the use of dynamic-link libraries (DLLs) in Microsoft Windows. Such problems include version conflicts, missing DLLs, duplicate DLLs, and incorrect or missing registration. In side-by-side, Windows stores multiple versions of a DLL in the winsxs subdirectory of the Windows directory, and loads them on demand. This reduces dependency problems for applications that include a side-by-side manifest.

Side-by-side technology is also known as WinSxS or SxS, although technically WinSxS refers only to the global side-by-side store (officially called the "Windows component store"), which is conceptually the native equivalent of the .NET Global Assembly Cache. Executables that include an SxS manifest are designated SxS assemblies.

Microsoft Visual C++ 2005 and 2008 employ SxS with all C runtime libraries. However, runtime libraries in Visual C++ 2010 no longer use this technology; instead, they include the version number of a DLL in its file name, which means that different versions of one DLL will technically be completely different DLLs now.

SxS is also the technological basis for registration-free COM activation. Only in-process COM servers may be activated this way.

An application that employs SxS must have a manifest. Manifests are typically a section embedded in the application's executable file but may also be an external file. When the operating system loads the application and detects the presence of a manifest, the operating system DLL loader is directed to the version of the DLL corresponding to that listed in the manifest. If there is no manifest, the DLL loader loads a default version of all DLL dependencies. If the DLL is a COM server, it must have a manifest of its own for registration-free activation to succeed.

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