What are the different types of assemblies in .NET?

In .NET, an assembly is the fundamental unit of deployment, version control, reuse, and security. It is a compiled code library typically stored as an EXE or DLL file that contains IL (Intermediate Language) code, metadata, and resources.

Assembly Classification

.NET assemblies can be categorized in several ways:

1. By File Type

// Executable assembly with entry point
public class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello World!");
    }
}

2. By Accessibility

// Private assembly reference
<Reference Include="PrivateLibrary" />

// Shared assembly reference (strong-named, in GAC)
<Reference Include="SharedLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />

3. By Creation Method

// Creating a dynamic assembly at runtime
AssemblyName assemblyName = new AssemblyName("DynamicAssembly");
AssemblyBuilder assemblyBuilder = AssemblyBuilder.DefineDynamicAssembly(
    assemblyName, AssemblyBuilderAccess.Run);

4. By Composition

// Creating a multi-file assembly (using compiler options)
csc /target:library /out:MyAssembly.dll File1.cs File2.cs

5. By Reference Type

// Strong naming an assembly
[assembly: AssemblyKeyFile("MyKey.snk")]

// In modern .NET projects
<PropertyGroup>
  <SignAssembly>true</SignAssembly>
  <AssemblyOriginatorKeyFile>MyKey.snk</AssemblyOriginatorKeyFile>
</PropertyGroup>

Assembly Structure

Every .NET assembly contains:

1. Assembly Manifest: Metadata about the assembly (version, dependencies)
2. Type Metadata: Information about types defined in the assembly
3. IL Code: Compiled code in Intermediate Language format
4. Resources: Embedded files (images, strings, etc.)

Assembly Loading

Assemblies are loaded into the CLR using different strategies:

• Load-by-name: CLR searches for the assembly in predefined locations
• Explicit loading: Using Assembly.Load() or Assembly.LoadFrom()
• Reflection-only loading: For inspection without execution

// Explicitly loading an assembly
Assembly assembly = Assembly.LoadFrom("MyLibrary.dll");

// Creating an instance from a loaded assembly
Type type = assembly.GetType("MyLibrary.MyClass");
object instance = Activator.CreateInstance(type);

Modern .NET Assembly Considerations

In .NET Core and .NET 5+:

• No GAC: Shared assemblies use NuGet packages instead
• Self-contained deployment: All dependencies included with the app
• Assembly trimming: Unused parts can be removed
• Single-file applications: All assemblies bundled into one executable