可以动态读取并修改已经生成的IL文件，这种在不修改源代码的情况下给程序集动态添加功能的技术称为面向切面编程（AOP）；
这里给出了一个注入使用Stopwatch来检测方法执行时间的代码，这里的Mono.Cecil类库可以通过nuget进行安装：
using System;
using System.IO;
using System.Linq;
using System.Diagnostics;
using Mono.Cecil;
using Mono.Cecil.Cil;
using Mono.Collections.Generic;
static void Main(string[] args)
{
for (int i = 0; i < args.Length; i++)
{
FileStream fileStream = new FileStream(args[i], FileMode.Open);
if (fileStream != null)
{
AssemblyDefinition aD = AssemblyDefinition.ReadAssembly(fileStream);
ModuleDefinition mD = aD.MainModule;
Collection<TypeDefinition> typeDefinition = mD.Types;
foreach (TypeDefinition type in typeDefinition)
{
if (type.IsClass)
{
foreach (MethodDefinition method in type.Methods)
{
if (method.IsPublic && !method.IsConstructor)
{
ILProcessor il = method.Body.GetILProcessor();
TypeReference stT = mD.ImportReference(typeof(Stopwatch));
VariableDefinition stV = new VariableDefinition(stT);
method.Body.Variables.Add(stV);
Instruction first = method.Body.Instructions.First();
il.InsertBefore(first, il.Create(OpCodes.Newobj, mD.ImportReference(typeof(Stopwatch).GetConstructor(new Type[] { }))));
il.InsertBefore(first, il.Create(OpCodes.Stloc_S, stV));
il.InsertBefore(first, il.Create(OpCodes.Ldloc_S, stV));
il.InsertBefore(first, il.Create(OpCodes.Callvirt, mD.ImportReference(typeof(Stopwatch).GetMethod("Start"))));
Instruction @return = method.Body.Instructions.Last();
il.InsertBefore(@return, il.Create(OpCodes.Ldloc_S, stV));
il.InsertBefore(@return, il.Create(OpCodes.Callvirt, mD.ImportReference(typeof(Stopwatch).GetMethod("Stop"))));
il.InsertBefore(@return, il.Create(OpCodes.Ldstr, $"{method.FullName} run time: "));
il.InsertBefore(@return, il.Create(OpCodes.Ldloc_S, stV));
il.InsertBefore(@return, il.Create(OpCodes.Callvirt, mD.ImportReference(typeof(Stopwatch).GetMethod("get_ElapsedMilliseconds"))));
il.InsertBefore(@return, il.Create(OpCodes.Box, mD.ImportReference(typeof(long))));
il.InsertBefore(@return, il.Create(OpCodes.Call, mD.ImportReference(typeof(string).GetMethod("Concat", new Type[] { typeof(object), typeof(object) }))));
il.InsertBefore(@return, il.Create(OpCodes.Call, mD.ImportReference(typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) }))));
}
}
}
}
FileInfo fileInfo = new FileInfo(args[i]);
string fileName = fileInfo.Name;
int pointIndex = fileName.LastIndexOf('.');
string frontName = fileName.Substring(0, pointIndex);
string backName = fileName.Substring(pointIndex, fileName.Length - pointIndex);
string writeFilePath = Path.Combine(fileInfo.Directory.FullName, frontName + "_inject" + backName);
aD.Write(writeFilePath);
Console.WriteLine($"Success! Output path: {writeFilePath}");
fileStream.Dispose();
}
}
Console.Read();
}
