Global Assembly Cache

The Global Assembly Cache (GAC) is a machine-wide CLI assembly cache for the Common Language Infrastructure (CLI) in Microsoft's .NET Framework. The approach of having a specially controlled central repository addresses the flaws in the shared library concept and helps to avoid pitfalls of other solutions that led to drawbacks like DLL hell.

Assemblies residing in the GAC must adhere to a specific versioning scheme which allows for side-by-side execution of different code versions. Specifically, such assemblies must be strongly named.

There are two ways to interact with the GAC: the Global Assembly Cache Tool (gacutil.exe) and the Assembly Cache Viewer (shfusion.dll).

gacutil.exe is an older command-line utility that shipped with .NET 1.1 and is still available with the .NET SDK.

One can check the availability of a shared assembly in GAC by using the command:

One can register a shared assembly in the GAC by using the command:

Or by copying an assembly file into the following location:

Note that for .NET 4.0 the GAC location is now:

Other options for this utility will be briefly described if you use the /? flag, i.e.:

The newer interface, the Assembly Cache Viewer, is integrated into Windows Explorer. Browsing %windir%\assembly\ (for example, C:\WINDOWS\assembly) or  %WINDIR%\Microsoft.NET\assembly, displays the assemblies contained in the cache along with their versions, culture, public key token, and processor architecture. Assemblies are installed by dragging and dropping and uninstalled by selecting and pressing the delete key or using the context menu.

With the launch of the .NET Framework 4, the Assembly Cache Viewer shell extension is obsolete.

A computer has two CLI assemblies both named AssemblyA, but one is version 1.0 and the other is version 2.0. Since it is required that both be compiled to a file named AssemblyA, they cannot exist in the same directory within the FAT32 file system. Instead, the virtual file system of the GAC can be used by programs that need to use each version of the assembly specifically.

...
Wikipedia