= Mono Cecil =
Cecil is a library written by Jb Evain to generate and inspect programs and libraries in the ECMA CIL format. It has full support for generics, and support some debugging symbol format.

http://www.mono-project.com/Cecil

http://www.mono-project.com/Cecil:FAQ

https://github.com/jbevain/cecil/wiki/Importing

Download Cecil assembly http://obfuscar.googlecode.com/svn-history/r83/trunk/ThirdParty/Mono/Mono.Cecil.dll

== IL Code rewriting ==
Sample program to change with Mono.Cecil
{{{#!highlight csharp
/*
mcs samples.cs
mono sample.exe
*/
public class Acme{
    public int Add(int arg1,int arg2){
      return arg1 + arg2;  
    }
}

public class Sample{
    public static void Main(string []args){
      new Acme().Add(4,9);  
    }
}
}}}

Program to rewrite the sample program
{{{#!highlight csharp
/*
mcs rewrite.cs /r:Mono.Cecil.dll
mono rewrite.exe 
*/
using Mono.Cecil;
using Mono.Cecil.Cil;

public class RewriteIL{
  public static void Main(string []args){
    AssemblyDefinition ad = AssemblyDefinition.ReadAssembly("sample.exe");
    
    foreach(TypeDefinition td in ad.MainModule.Types){
      
      foreach(MethodDefinition md  in td.Methods){
	
	if(md.Name=="Add"){
	  System.Console.WriteLine(td+"."+md);

          System.Console.WriteLine("Original IL Code:");
	  foreach(Instruction inst in md.Body.Instructions){
	    System.Console.WriteLine(inst);
	  }

          var il = md.Body.GetILProcessor();
          var arg1 = il.Create (OpCodes.Ldarg_1);
          var arg2 = il.Create (OpCodes.Ldarg_2);
	  MethodReference writeline = md.Module.Import( typeof(System.Console).GetMethod("WriteLine", new [] { typeof (int) }));
          var call = il.Create(OpCodes.Call,writeline);

          // insert the IL code in reversed order, at top of the method.
          // "normal" order would be ldarg.2 call ldarg.1 call ...
          il.InsertBefore (md.Body.Instructions[0], call);
          il.InsertBefore (md.Body.Instructions[0], arg2);
          il.InsertBefore (md.Body.Instructions[0], call);
	  il.InsertBefore (md.Body.Instructions[0], arg1);

          System.Console.WriteLine("New IL Code:");
	  foreach(Instruction inst in md.Body.Instructions){
	    System.Console.WriteLine(inst);
	  }

	}
      }
      
    }

    ad.Write("sampleRewrite.exe");
  }
}
}}}

In the rewritten program the arguments for the Add method will appear on the console.