System.Reflection.Emit Namespace

System.Reflection.Emit Namespace

This namespace provides classes that allow you to create dynamic assemblies and modules at runtime. It enables the generation of Intermediate Language (IL) instructions, types, and members on the fly, which is powerful for scenarios like Just-In-Time (JIT) compilation, serialization, and code generation.

Overview

The System.Reflection.Emit namespace is a cornerstone of dynamic code generation in .NET. It allows developers to emit code that is compiled and executed at runtime, rather than being statically compiled ahead of time. This is particularly useful when the exact structure of the code or types is not known until the application is running.

Key functionalities include:

• Creating dynamic assemblies and modules.
• Defining new types (classes, structs, interfaces, enums) dynamically.
• Defining members for these types, such as fields, properties, methods, and events.
• Generating the Intermediate Language (IL) instructions for method bodies.
• Emitting debugging information.

Key Classes

Here are some of the most important classes within the System.Reflection.Emit namespace:

AssemblyBuilder

Represents an assembly that is generated dynamically. You can define modules within this assembly.

ModuleBuilder

Represents a module within a dynamic assembly. You can define types within this module.

TypeBuilder

Represents a type that is being defined dynamically. You can add members to this type.

MethodBuilder

Represents a method that is being defined dynamically. You can emit IL instructions for its body.

ILGenerator

Provides methods for emitting the Intermediate Language (IL) instructions for a method.

Common Use Cases

• Code Generation Frameworks: Libraries that generate code based on user input or configuration.
• Serialization: Creating highly optimized serializers that generate custom serialization logic for specific types.
• Proxy Objects: Generating proxy classes for remote procedure calls (RPC) or for implementing design patterns like the Decorator or Proxy pattern.
• Runtime Scripting: Enabling scripting capabilities within an application by compiling and executing script code dynamically.
• Expression Trees Compilation: While often handled by System.Linq.Expressions, the underlying compilation can involve emit techniques.

Example: Creating a Simple Dynamic Type

The following example demonstrates how to create a simple class with a method dynamically using System.Reflection.Emit.

using System;
using System.Reflection;
using System.Reflection.Emit;

public class DynamicTypeGenerator
{
    public static void Main(string[] args)
    {
        // 1. Create a dynamic assembly
        AssemblyName assemblyName = new AssemblyName("MyDynamicAssembly");
        AssemblyBuilder assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(
            assemblyName,
            AssemblyBuilderAccess.RunAndSave); // Or AssemblyBuilderAccess.Run

        // 2. Create a dynamic module
        ModuleBuilder moduleBuilder = assemblyBuilder.DefineDynamicModule("MyDynamicModule");

        // 3. Define a new type (class)
        TypeBuilder typeBuilder = moduleBuilder.DefineType("MyDynamicClass", TypeAttributes.Public);

        // 4. Define a method
        MethodBuilder methodBuilder = typeBuilder.DefineMethod(
            "Greet",
            MethodAttributes.Public | MethodAttributes.Static,
            typeof(void), // Return type
            new Type[] { typeof(string) }); // Parameters

        // 5. Get an ILGenerator to emit instructions for the method body
        ILGenerator ilGenerator = methodBuilder.GetILGenerator();

        // Emit IL instructions: Console.WriteLine("Hello, " + name + "!");
        ilGenerator.Emit(OpCodes.Ldstr, "Hello, "); // Load the string "Hello, "
        ilGenerator.Emit(OpCodes.Ldarg_0);         // Load the first argument (the 'name' string)
        ilGenerator.Emit(OpCodes.Call, typeof(string).GetMethod("Concat", new Type[] { typeof(string), typeof(string) })); // Concat strings
        ilGenerator.Emit(OpCodes.Call, typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) })); // Call Console.WriteLine

        // Emit the end of the method
        ilGenerator.Emit(OpCodes.Ret);

        // 6. Create the type
        Type myDynamicType = typeBuilder.CreateType();

        // 7. Invoke the dynamic method
        Console.WriteLine("Invoking dynamic method...");
        MethodInfo greetMethod = myDynamicType.GetMethod("Greet");
        greetMethod.Invoke(null, new object[] { "World" });

        // 8. Optionally save the assembly
        // assemblyBuilder.Save("MyDynamicAssembly.dll");
    }
}
            

API Reference

The System.Reflection.Emit namespace contains a rich set of classes for fine-grained control over dynamic code generation. Here's a summary of some key types: