Emit with a human face

Introduction

The System.Reflection.Emit namespace provides classes to create dynamic assemblies at runtime. It allows compilers and tools to emit MSIL, execute it and store it to a disk. Although Emit is a powerful tool, it is also extremely hard to use.

Let us take a look at the following example, which demonstrates the "normal" way of Emit programming:

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

namespace EmitDemo
{
    public interface IHello
    {
        void SayHello(string toWhom);
    }

    class Program
    {
        static void Main(string[] args)
        {
            AssemblyName asmName = new AssemblyName();

            asmName.Name = "HelloWorld";

            AssemblyBuilder asmBuilder =
                Thread.GetDomain().DefineDynamicAssembly
            (asmName, AssemblyBuilderAccess.RunAndSave);

            ModuleBuilder modBuilder  = asmBuilder.DefineDynamicModule
                            ("HelloWorld");

            TypeBuilder   typeBuilder = modBuilder.DefineType(
                                    "Hello",
                                    TypeAttributes.Public,
                                    typeof(object),
                                    new Type[] { typeof(IHello) });

            MethodBuilder methodBuilder = typeBuilder.DefineMethod("SayHello",
                         MethodAttributes.Private | MethodAttributes.Virtual,
                         typeof(void),
                         new Type[] { typeof(string) });

            typeBuilder.DefineMethodOverride(methodBuilder, 
                    typeof(IHello).GetMethod("SayHello"));

            ILGenerator il = methodBuilder.GetILGenerator();

            // string.Format("Hello, {0} World!", toWhom)
            //
            il.Emit(OpCodes.Ldstr, "Hello, {0} World!");
            il.Emit(OpCodes.Ldarg_1);
            il.Emit(OpCodes.Call, typeof(string).GetMethod
        ("Format", new Type[] { typeof(string), typeof(object) }));

            // Console.WriteLine("Hello, World!");
            //
            il.Emit(OpCodes.Call, typeof(Console).GetMethod
            ("WriteLine", new Type[] { typeof(string) }));
            il.Emit(OpCodes.Ret);

            Type   type  = typeBuilder.CreateType();

            IHello hello = (IHello)Activator.CreateInstance(type);

            hello.SayHello("Emit");
        }
    }
}

Note that the Emit method takes one parameter as an OpCode and optionally another one, which does not depend on the current operation context. So, this way is not entirely type safe.

Alternative

Fortunately, there is an alternative way. Business Logic Toolkit for .NET provides a helper class, EmitHelper, which can make your life a little bit easy. It contains typed wrapper methods for almost all Emit commands and allows writing of source code that looks similar to MSIL.

The following examples show how to use the EmitHelper class with C#, VB.NET, and C++/CLI.

Examples

C#

using System;

using BLToolkit.Reflection;
using BLToolkit.Reflection.Emit;

namespace EmitHelperDemo
{
    public interface IHello
    {
        void SayHello(string toWhom);
    }

    class Program
    {
        static void Main(string[] args)
        {
            EmitHelper emit = new AssemblyBuilderHelper("HelloWorld.dll")
                .DefineType  ("Hello", typeof(object), typeof(IHello))
                .DefineMethod(typeof(IHello).GetMethod("SayHello"))
                .Emitter;

            emit
                // string.Format("Hello, {0} World!", toWhom)
                //
                .ldstr   ("Hello, {0} World!")
                .ldarg_1
                .call    (typeof(string), "Format", typeof(string), 
                                            typeof(object))

                // Console.WriteLine("Hello, World!");
                //
                .call    (typeof(Console), "WriteLine", typeof(string))
                .ret()
                ;

            Type   type  = emit.Method.Type.Create();

            IHello hello = (IHello)TypeAccessor.CreateInstance(type);

            hello.SayHello("C#");
        }
    }
}

VB.NET

Imports BLToolkit.Reflection
Imports BLToolkit.Reflection.Emit

Public Module Module1

    Public Interface IHello
        Sub SayHello(ByVal toWhom As String)
    End Interface

    Sub Main()
        Dim assemblyHelper As AssemblyBuilderHelper = _
                New AssemblyBuilderHelper("HelloWorld.dll")
        Dim typeHelper     As TypeBuilderHelper     = _
      assemblyHelper.DefineType("Hello", GetType(Object), GetType(IHello))
        Dim methodHelper   As MethodBuilderHelper   = _
      typeHelper.DefineMethod(GetType(IHello).GetMethod("SayHello"))
        Dim emit           As EmitHelper            = methodHelper.Emitter

        ' string.Format("Hello, {0} World!", toWhom)
        '
        emit _
            .ldstr("Hello, {0} World!") _
            .ldarg_1 _
            .call(GetType(String), "Format", GetType(String), GetType(Object))

        ' Console.WriteLine("Hello, World!");
        '
        emit _
            .call(GetType(Console), "WriteLine", GetType(String)) _
            .ret()

        Dim type  As Type   = typeHelper.Create()

        Dim hello As IHello = TypeAccessor.CreateInstance(type)

        hello.SayHello("VB")
    End Sub

End Module

C++/CLI

#include "stdafx.h"

using namespace System;

using namespace BLToolkit::Reflection;
using namespace BLToolkit::Reflection::Emit;

public interface class IHello
{
    void SayHello(String ^toWhom);
};

void main()
{
    AssemblyBuilderHelper ^assembly = gcnew AssemblyBuilderHelper
                                        ("HelloWorld.dll");

    EmitHelper ^emit = assembly
        ->DefineType  ("Hello", Object::typeid, IHello::typeid)
        ->DefineMethod(IHello::typeid->GetMethod("SayHello"))
        ->Emitter;

    emit
        // string.Format("Hello, {0} World!", toWhom)
        //
        ->ldstr   ("Hello, {0} World!")
        ->ldarg_1
        ->call    (String::typeid, "Format", String::typeid, Object::typeid)

        // Console.WriteLine("Hello, World!");
        //
        ->call    (Console::typeid, "WriteLine", String::typeid)
        ->ret()
        ;

    Type   ^type  = emit->Method->Type->Create();

    IHello ^hello = (IHello^)TypeAccessor::CreateInstance(type);

    hello->SayHello("C++");
}