What is the Global Assembly Cache (GAC)?

The Global Assembly Cache (GAC) is a machine-wide cache for .NET Framework assemblies that are specifically designated to be shared by multiple applications on a computer. It provides a centralized repository for storing and managing shared assemblies, ensuring version control and security.

In modern .NET (Core, 5+), the traditional GAC has been replaced with alternative sharing mechanisms like NuGet packages and the runtime assembly store, emphasizing application-local deployment rather than machine-wide sharing.

Key Characteristics

GAC Locations

• .NET Framework (up to 4.0): %windir%\assembly
• .NET Framework 4.0+: %windir%\Microsoft.NET\assembly
• .NET Core/5+: No traditional GAC (uses different mechanisms)

Working with the GAC

Installing Assemblies

# Using gacutil.exe (requires .NET Framework SDK)
gacutil /i SharedLibrary.dll

# Uninstalling
gacutil /u SharedLibrary

Referencing GAC Assemblies

<!-- Strong-named reference to a GAC assembly -->
<Reference Include="SharedLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />

Strong Naming (Required for GAC)

// Generate key pair (using Developer Command Prompt)
// sn -k MyKeyPair.snk

// Sign the assembly in project file
<PropertyGroup>
  <SignAssembly>true</SignAssembly>
  <AssemblyOriginatorKeyFile>MyKeyPair.snk</AssemblyOriginatorKeyFile>
</PropertyGroup>

Modern Alternatives (.NET Core/5+)

Since .NET Core and .NET 5+, the GAC concept has changed:

1. NuGet Packages: Primary mechanism for sharing code

   <PackageReference Include="SharedLibrary" Version="1.0.0" />

2. Runtime Store: For shared framework components

   dotnet store --manifest packages.manifest.csproj --runtime win-x64

3. Assembly Load Context: For more controlled assembly loading

When to Use the GAC

Appropriate Use Cases

• Shared components used by multiple applications
• Common libraries across an organization
• Security-sensitive code requiring higher trust

When to Avoid

• Application-specific assemblies
• Frequently updated code
• Modern .NET Core/5+ applications
• When deployment simplicity is important

Common Issues

1. Assembly Binding Redirects: Required when using different versions

   <bindingRedirect oldVersion="1.0.0.0-1.9.9.9" newVersion="2.0.0.0" />

2. Deployment Complexity: Requires administrative access and careful versioning

3. DLL Hell: The GAC was designed to solve “DLL Hell” but can create its own versioning challenges

Best Practices

1. For .NET Framework: Use the GAC only for truly shared components
2. For .NET Core/5+: Use NuGet packages instead of the GAC
3. Always strong-name assemblies intended for the GAC
4. Use assembly binding redirects when needed
5. Consider deployment implications when deciding to use the GAC