The
Global Assembly Cache or
GAC is a machine-wide .NET assemblies
cacheIn computer science, a cache is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch or to compute, compared to the cost of reading the cache. In other words, a cache is a temporary storage area where frequently...
for
MicrosoftMicrosoft Corporation is a multinational computer technology corporation that develops, manufactures, licenses, and supports a wide range of software products for computing devices...
's
CLRThe Common Language Runtime is a core component of Microsoft's .NET initiative. It is Microsoft's implementation of the Common Language Infrastructure standard, which defines an execution environment for program code...
platformIn computing, a platform describes some sort of hardware architecture or software framework , that allows software to run...
. The approach of having a specially controlled central repository addresses the shared library concept and helps to avoid pitfalls of other solutions that lead to drawbacks like
DLL hellIn computing, DLL hell is a colloquial term for the complications that arise when working with dynamic link libraries used with Microsoft Windows operating systems, particularly legacy 16-bit editions....
.
Assemblies that should be used by all applications can be put in the global assembly cache.
The
Global Assembly Cache or
GAC is a machine-wide .NET assemblies
cacheIn computer science, a cache is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch or to compute, compared to the cost of reading the cache. In other words, a cache is a temporary storage area where frequently...
for
MicrosoftMicrosoft Corporation is a multinational computer technology corporation that develops, manufactures, licenses, and supports a wide range of software products for computing devices...
's
CLRThe Common Language Runtime is a core component of Microsoft's .NET initiative. It is Microsoft's implementation of the Common Language Infrastructure standard, which defines an execution environment for program code...
platformIn computing, a platform describes some sort of hardware architecture or software framework , that allows software to run...
. The approach of having a specially controlled central repository addresses the shared library concept and helps to avoid pitfalls of other solutions that lead to drawbacks like
DLL hellIn computing, DLL hell is a colloquial term for the complications that arise when working with dynamic link libraries used with Microsoft Windows operating systems, particularly legacy 16-bit editions....
.
Shared location
Assemblies that should be used by all applications can be put in the global assembly cache. For example, if all applications should use an assembly located in the global assembly cache, a version policy statement can be added to the Machine.config file that redirects references to the assembly.
Requirements
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 assembles must be strong named.
Usage
gacutil.exe is the .NET utility used to work with the GAC.
One can check the availability of a shared assembly in GAC by using the command:
gacutil.exe /l
One can register a shared assembly in the GAC by using the command:
gacutil.exe /i
Or by dropping an assembly file into the following location using the GUI:
%windir%\assembly\
Other options for this utility will be briefly described if you use the /? flag, i.e.:
gacutil.exe /?
Example of use
A computer has two .NET 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
FAT32File Allocation Table or FAT is a computer file system architecture now widely used on most computer systems and most memory cards, such as those used with digital cameras.It was developed by Bill Gates and Marc McDonald during 1976–1977....
file systemIn computing, a file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them...
. Instead, the virtual file system of the GAC can be used by programs that need to use each version of the assembly specifically.
Implementation
The GAC as a construct does not actually exist within the Windows OS. It is implemented and managed by the .NET Framework. The folder within
%systemroot% named
assembly contains all globally-available assemblies with managed filenames so that the version and public key tokens can be included. Each version can therefore exist within the same location and be called without requiring subsequent versions to preserve code entry point locations as usual. Explorer allows the drag-and-drop installation of assemblies into this folder only if they would otherwise be permitted to be installed from the command line.
A calling application may specify specific versions of an assembly when referencing them, so the
Common Language RuntimeThe Common Language Runtime is a core component of Microsoft's .NET initiative. It is Microsoft's implementation of the Common Language Infrastructure standard, which defines an execution environment for program code...
can simply refer to the filename to use the correct one.
Pitfalls
The Global Assembly Cache mechanism helps to avoid older
DLL hellIn computing, DLL hell is a colloquial term for the complications that arise when working with dynamic link libraries used with Microsoft Windows operating systems, particularly legacy 16-bit editions....
, but it still has some drawbacks, as for example:
- By default, applications will only run with the version of the .NET Framework
The Microsoft .NET Framework is a software framework that can be installed on computers running Microsoft Windows operating systems. It includes a large library of coded solutions to common programming problems and a virtual machine that manages the execution of programs written specifically for...
used to compile it, which can cause the application to fail on machines with newer versions of the .NET Framework installed — even when the application would normally run properly with the newer version.
- It is sometimes necessary to use Conditional compilation if some of the core .NET calls (used in the application) are only supported for some versions of the framework.
- .NET applications that rely on native code risk incompatibilities, even with the GAC mechanism.
- Every assembly that is added to the GAC must be strongly named
Strong Key is a naming convention used in computer programming. There can be more than one component with the same naming, but with different versions. This can lead to many conflicts...
. The process of making an assembly "strongly named" can be quite painful in some situations. For example, if an assembly depends on another assembly that is not strongly named, it can not be registered in the GAC. In cases where the code of the 3rd party assembly is not in the programmer's propriety, transforming the assembly to be strongly named can in fact be impossible.
External links