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Emit with a human face

, 25 Oct 2006
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A wrapper for the System.Reflection.Emit namespace


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 =
            (asmName, AssemblyBuilderAccess.RunAndSave);

            ModuleBuilder modBuilder  = asmBuilder.DefineDynamicModule

            TypeBuilder   typeBuilder = modBuilder.DefineType(
                                    new Type[] { typeof(IHello) });

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


            ILGenerator il = methodBuilder.GetILGenerator();

            // string.Format("Hello, {0} World!", toWhom)
            il.Emit(OpCodes.Ldstr, "Hello, {0} World!");
            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) }));

            Type   type  = typeBuilder.CreateType();

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


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.


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.



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))

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

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

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

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



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   = _
        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)) _

        Dim type  As Type   = typeHelper.Create()

        Dim hello As IHello = TypeAccessor.CreateInstance(type)

    End Sub

End Module


#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

    EmitHelper ^emit = assembly
        ->DefineType  ("Hello", Object::typeid, IHello::typeid)

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

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

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

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



This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Igor Tkachev
United States United States
No Biography provided

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Comments and Discussions

GeneralThanks for the article Pin
Miguel Barros31-Aug-09 11:54
memberMiguel Barros31-Aug-09 11:54 
QuestionJust instance of type IHello ? Pin
Stuard25-Aug-09 16:16
memberStuard25-Aug-09 16:16 
QuestionHow to emit a generic type? Pin
Liu Junfeng6-Jun-06 20:12
memberLiu Junfeng6-Jun-06 20:12 
AnswerRe: How to emit a generic type? [modified] Pin
Igor Tkachev8-Jun-06 8:25
memberIgor Tkachev8-Jun-06 8:25 
Generalvery clever Pin
flaming red dingo6-Jun-06 14:04
memberflaming red dingo6-Jun-06 14:04 
I like how you return an instance of the same class on every call in order to make the
.call, etc syntax work. Very clever. =)

props, and regards.

flaming red dingo

GeneralBrilliant! [modified] Pin
Mike_V21-May-06 16:57
memberMike_V21-May-06 16:57 
GeneralRe: Brilliant! [modified] Pin
BillWoodruff27-May-06 23:26
memberBillWoodruff27-May-06 23:26 
GeneralRe: Brilliant! [modified] Pin
Igor Tkachev29-May-06 10:37
memberIgor Tkachev29-May-06 10:37 
GeneralRe: Brilliant! [modified] Pin
Mike_V29-May-06 12:09
memberMike_V29-May-06 12:09 

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