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项目地址：Mono.Cecil
项目描述：In simple English, with Cecil, you can load existing managed assemblies, browse all the contained types, modify them on the fly and save back to the disk the modified assembly.
类似项目：Microsoft CCI
Common Compiler Infrastructure: Metadata API
Common Compiler Infrastructure: Code Model and AST API
Common Compiler Infrastructure: Sample applications
Common Compiler Infrastructure - Contrib
对比评价：来自StackOverflow
Mono.Cecil has better, more understandable and easy in use object model. However, I had an ugly bug when used it in my program (reference to the wrong method was saved in the assembly; I think there was some bug with metadata tokens handling)
Microsoft.CCI has an ugly, utterly over-designed object model in the same time lacking many simple features; however, it's more mature than Mono.Cecil. Finally, I abandoned Mono.Cecil and used Microsoft.CCI for my program.

基本示例
Cecil是对已编译生成IL的程序集进行操作，所以先写一个简单的Console exe程序，这里项目名称使用Cecil.Program：

using System;
using System.Reflection;
namespace Cecil.Program
{
    class Program
    {
        static void Main(string[] args)
        {
            TestType tt = new TestType();
            tt.SayHello();
            tt.AboutMe();
            Console.ReadKey();
        }
    }
    public class TestType
    {
        [Obsolete]
        public void SayHello()
        {
            Console.WriteLine("\tHello Cecil !");
        }
        public void AboutMe()
        {
            Type type = typeof(TestType);
            MethodInfo method = type.GetMethod("SayHello");
            if (method.IsVirtual)
                Console.WriteLine("\tI'm a virtual method");
            else
                Console.WriteLine("\tI'm a non-virtual method");
            object[] attributes = method.GetCustomAttributes(false);
            if (attributes != null && attributes.Length > 0)
            {
                Console.WriteLine("\tI have the following attributes:");
                foreach (object attr in attributes)
                    Console.WriteLine("\t\t" + attr.GetType().Name);
            }
        }
    }
}

这个程序集的运行结果如下：
    

方法SayHello的IL代码如下：
    

接下来使用另外一个Console exe程序来修改Cecil.Program.exe，项目名称使用Cecil：

using Mono.Cecil;
using Mono.Cecil.Cil;
AssemblyDefinition assembly = AssemblyFactory.GetAssembly("Cecil.Program.exe");
TypeDefinition type = assembly.MainModule.Types["Cecil.Program.TestType"];
MethodDefinition sayHello = null;
foreach (MethodDefinition md in type.Methods)
    if (md.Name == "SayHello") sayHello = md;
//Console.WriteLine(string value)方法
MethodInfo writeLine = typeof(Console).GetMethod("WriteLine"
    , new Type[] { typeof(string) });
//Console.WriteLine方法导入MainModule，并返回在AssemblyDefinition中的引用方式
MethodReference writeLineRef = assembly.MainModule.Import(writeLine);
//在SayHello方法开始位置插入一条trace语句
//  Console.WriteLine(">>Intercepting ");
//如果插入的语句需要使用函数入参，则必须插入在OpCodes.Ldarg等指令之后
CilWorker worker = sayHello.Body.CilWorker;
Instruction ldstr = worker.Create(OpCodes.Ldstr, ">>Intercepting " + sayHello.Name);
Instruction call = worker.Create(OpCodes.Call, writeLineRef);
Instruction first = sayHello.Body.Instructions[0];
worker.InsertBefore(first, call);
worker.InsertBefore(call, ldstr);
//在SayHello方法结束位置插入一条trace语句
//  Console.WriteLine(">>Intercepted ");
//语句必须插入在OpCodes.Ret指令的前面
int offset = sayHello.Body.Instructions.Count - 1;
Instruction last = sayHello.Body.Instructions[offset--];
while (last.OpCode == OpCodes.Nop || last.OpCode == OpCodes.Ret)
    last = sayHello.Body.Instructions[offset--];
ldstr = worker.Create(OpCodes.Ldstr, ">>Intercepted " + sayHello.Name);
worker.InsertAfter(last, ldstr);
worker.InsertAfter(ldstr, call);
//把SayHello方法改为虚方法
sayHello.IsVirtual = true;
//给SayHello方法添加一个SerializableAttribute
CustomAttribute attribute = new CustomAttribute(
    assembly.MainModule.Import(
        typeof(SerializableAttribute).GetConstructor(Type.EmptyTypes)
));
sayHello.CustomAttributes.Add(attribute);
AssemblyFactory.SaveAssembly(assembly, "Cecil.Program.modified.exe");
Console.WriteLine("Assembly modified successfully!");
Console.ReadKey();

编译生成Cecil.exe，然后把Cecil.Program.exe拷贝到这个目录下，运行Cecil.exe，便会在当前目录生成Cecil.Program.modified.exe，运行Cecil.Program.modified.exe结果如下：
    
修改后的方法SayHello的IL代码如下：
    

从上面的基本使用方法可以看出，Cecil的确是易于使用，对象模型结构非常实用，这里是官方网站的一个主要对象结构图：
    

IL指令的复杂性
在assembly、type、method级别上对程序集做修改是非常简单的，但是如果要修改方法体的IL代码，则可能会遇到一些较麻烦的事情，需要细致的处理
例如上面的SayHello方法如果是这样：

public void SayHello(bool print)
{
    if (print)
        Console.WriteLine("\tHello Cecil !");
}

测试代码这样来调用：

TestType2 tt2 = new TestType2();
tt2.SayHello(true);
tt2.SayHello(false);
Console.ReadKey();

其运行结果只会输出一条Hello Cecil !消息，仍然使用Cecil.exe来修改这个程序集，其运行结果如下图：
    
调用tt2.SayHello(false);时，应该也会有一个>>Intercepted SayHello消息，但是没有输出，对比一下IL代码就清楚了：
    
修改后的IL代码如下：
    
IL_000b那一句，为false时就直接跳转到IL_0021这个返回指令上了，不会输出Intercepted的消息

使用Mono.Cecil也可以修改这个跳转地址，例如：

//得到指令brfalse.s
Instruction jmp = sayHello.Body.Instructions[1];
....
//把跳转的目标地址改成IL_0017 ldstr指令位置
jmp.Operand = ldstr;