What is the GAC in .NET?

Question

Right, so basically it's a way to keep DLLs globally accessible without worrying about conflicts. No more DLL Hell. Each architecture and version gets it's own place to live.

It also gets it own way to browse it in Explorer, so if you go to

C:\Windows\assembly

In windows explorer it lists all the DLLs.

But if you fire up cmd, you can see how it's really structured:

C:\Users\tritter>cd C:\Windows\assembly

C:\Windows\assembly>dir

 Directory of C:\Windows\assembly

07/20/2009  02:18 PM    <DIR>          GAC
06/17/2009  04:22 PM    <DIR>          GAC_32
06/17/2009  04:22 PM    <DIR>          GAC_64
06/17/2009  04:22 PM    <DIR>          GAC_MSIL
 ...snip...
               0 File(s)              0 bytes
               9 Dir(s)  90,538,311,680 bytes free

C:\Windows\assembly>cd GAC_64

C:\Windows\assembly\GAC_64>dir

 Directory of C:\Windows\assembly\GAC_64

06/17/2009  04:22 PM    <DIR>          .
06/17/2009  04:22 PM    <DIR>          ..
01/19/2008  09:54 AM    <DIR>          blbproxy
 ...snip...
01/19/2008  09:54 AM    <DIR>          srmlib
01/19/2008  06:11 AM    <DIR>          System.Data
01/19/2008  06:11 AM    <DIR>          System.Data.OracleClient
 ...snip...
               0 File(s)              0 bytes
              34 Dir(s)  90,538,311,680 bytes free

C:\Windows\assembly\GAC_64>cd System.Data

C:\Windows\assembly\GAC_64\System.Data>dir
 Directory of C:\Windows\assembly\GAC_64\System.Data

01/19/2008  06:11 AM    <DIR>          .
01/19/2008  06:11 AM    <DIR>          ..
04/11/2009  12:20 PM    <DIR>          2.0.0.0__b77a5c561934e089
               0 File(s)              0 bytes
               3 Dir(s)  90,538,311,680 bytes free

C:\Windows\assembly\GAC_64\System.Data>cd 2.0.0.0__b77a5c561934e089

C:\Windows\assembly\GAC_64\System.Data\2.0.0.0__b77a5c561934e089>dir

 Directory of C:\Windows\assembly\GAC_64\System.Data\2.0.0.0__b77a5c561934e089

04/11/2009  12:20 PM    <DIR>          .
04/11/2009  12:20 PM    <DIR>          ..
04/11/2009  12:12 PM         3,008,512 System.Data.dll
               1 File(s)      3,008,512 bytes
               2 Dir(s)  90,538,311,680 bytes free

C:\Windows\assembly\GAC_64\System.Data\2.0.0.0__b77a5c561934e089>

Here you can see version 2.0.0.0__b77a5c561934e089 of System.Data.

A DLL is identified by 5 parts:

1. Name
2. Version
3. Architecture
4. Culture
5. Public Key

Although the first 3 are generally the big ones.

GAC = Global Assembly Cache

Let's break it down:

• global - applies to the entire machine
• assembly - what .NET calls its code-libraries (DLLs)
• cache - a place to store things for faster/common access

So the GAC must be a place to store code libraries so they're accessible to all applications running on the machine.

Global Assembly Cache

Each computer where the common language runtime is installed has a machine-wide code cache called the global assembly cache. The global assembly cache stores assemblies specifically designated to be shared by several applications on the computer.

You should share assemblies by installing them into the global assembly cache only when you need to. As a general guideline, keep assembly dependencies private, and locate assemblies in the application directory unless sharing an assembly is explicitly required. In addition, it is not necessary to install assemblies into the global assembly cache to make them accessible to COM interop or unmanaged code.

The things MSDN contains may surprise you... you can usually read it like an article. The straightforward and most important bits at the top, the intricate details deeper down. It certainly explains it better than I could.

Note that Visual Studio displays all the DLLs in the GAC in the .NET tab of the References window. (Right-click on a project in Solution Explorer and select Add Reference.) This should give you a more tangeable idea.

Centralized DLL library.

The Global Assembly Cache (GAC) is a folder in Windows directory to store the .NET assemblies that are specifically designated to be shared by all applications executed on a system. Assemblies can be shared among multiple applications on the machine by registering them in global Assembly cache(GAC). GAC is a machine wide a local cache of assemblies maintained by the .NET Framework.