Understanding Assemblies and the GAC

What is an Assembly?

An assembly is a compiled code library used for deployment, versioning, and security in .NET. It can be an executable (.exe) or a library (.dll).

Every assembly contains:

• MSIL Code: Intermediate Language compiled from C# or other .NET languages
• Metadata: Information about the types, methods, references, and versioning
• Manifest: The identity of the assembly (name, version, culture, public key token)
• Optional resources: Images, localization data, embedded files

.NET uses assemblies to isolate code, promote reuse, and simplify version control.

Creating and Referencing Assemblies

In Visual Studio:

1. Create a Class Library project (generates a .dll)
2. Create a Console App project
3. Right-click the Console App → Add Reference → Choose your Class Library

// In Library Project
public class Calculator
{
    public static int Multiply(int a, int b) => a * b;
}

// In Console Project
using MyLibrary;

Console.WriteLine(Calculator.Multiply(3, 4)); // Output: 12

Types of Assemblies

• Private Assemblies – Stored in the application folder, used only by that app
• Shared Assemblies – Installed in the GAC for global reuse
• Satellite Assemblies – Contain localized resources for different cultures/languages

Global Assembly Cache (GAC)

The GAC is a central repository for shared assemblies that can be used by multiple .NET applications on a single system. It ensures:

• Assembly sharing: Avoid code duplication
• Versioning: Support side-by-side versions
• Security: Strong-named assemblies only

Location: C:\Windows\Microsoft.NET\assembly

Adding an Assembly to the GAC

Use the gacutil command (from Developer Command Prompt):

gacutil -i MyLibrary.dll

You can also check installed assemblies using:

gacutil -l

Note: Assemblies in the GAC must have a strong name (signed using sn.exe).

Strong Naming an Assembly

Before placing an assembly in the GAC, it must be strongly named. Use the sn tool to create a key pair and sign your assembly:

// Create a key file
sn -k MyKey.snk

// Add to AssemblyInfo.cs
[assembly: AssemblyKeyFile("MyKey.snk")]

Assemblies are the foundation of deployment and modularity in .NET. The GAC makes it easy to share libraries securely and consistently across apps. Next, we’ll dive into the .NET Base Class Library (BCL) to explore built-in functionality.