Core Concepts: Assemblies

Introduction to Assemblies

In .NET, an assembly is a compiled code library used for deployment, versioning, and security. An assembly is a building block of .NET applications; it can be a .dll or .exe file. Assemblies contain one or more managed types along with metadata about those types.

Key Features of Assemblies

Self-describing via metadata
Versioning and deployment unit
Security boundary
Encapsulation of code

Example: Creating a Simple Assembly

Here's an example of how to create a simple assembly in C#:


// File: HelloWorld.cs
using System;

namespace HelloWorld
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello, World!");
        }
    }
}

Compile this file using the C# compiler:

csc /target:library /out:HelloWorld.dll HelloWorld.cs

This command generates an assembly named HelloWorld.dll.

Components of an Assembly

An assembly consists of four key components:

Manifest: Contains metadata about the assembly.
Type Metadata: Information about types defined in the assembly.
MSIL Code: Intermediate language code executed by the CLR.
Resources: Additional files like images, strings, etc., required by the assembly.

Example: Viewing Assembly Metadata

You can use tools like ILDASM (Intermediate Language Disassembler) to view the metadata of an assembly. Here is how you can use ILDASM:

ildasm HelloWorld.dll

This command opens a window displaying the structure and metadata of the HelloWorld.dll assembly.

Strong-Named Assemblies

A strong-named assembly has a unique identity that consists of its name, version number, culture information, and a public key token. Strong names provide a unique identity to the assembly.

Example: Creating a Strong-Named Assembly

First, generate a key pair using the sn utility:

sn -k MyKey.snk

Next, compile the assembly with the strong name:

csc /target:library /out:HelloWorld.dll /keyfile:MyKey.snk HelloWorld.cs

This command creates a strong-named assembly HelloWorld.dll signed with MyKey.snk.

Global Assembly Cache (GAC)

The Global Assembly Cache (GAC) is a machine-wide code cache for storing assemblies that are intended to be shared by multiple applications on the computer. Assemblies must have a strong name to be placed in the GAC.

Example: Installing an Assembly to the GAC

Use the gacutil tool to install an assembly to the GAC:

gacutil -i HelloWorld.dll

This command installs the HelloWorld.dll assembly to the GAC, making it available for shared use by multiple applications.

Conclusion

Assemblies are a fundamental part of the .NET framework, serving as the building blocks for applications and libraries. They provide a way to encapsulate and manage code, ensure versioning and deployment consistency, and enforce security boundaries.