C# Mono.Cecil 注入的 IL 代码未执行

Question

I want to make a little sample code with Mono.Cecil The goal is to manipulate the IL. The method X does print something on the console. To test the manipulation of the IL code i remove all instructions from the method. But when I run External2.dll the Console Output is still printed (but the instructions to do the Console output are removed from the External2.dll). How can I fix this problem?

var module = ModuleDefinition.ReadModule("External.dll");
var types = module.Types;
foreach (var type in types) {
    foreach (var method in type.Methods) {
        if (method.Name == "X") {
            Console.WriteLine(method.Body.Instructions.Count);
            method.Body.Instructions.Clear();
            Console.WriteLine(method.Body.Instructions.Count);
        }
    }
}
module.Write("External2.dll");

Answer 1

I think you need to use AssemblyDefinition instead of ModuleDefinition

Also, you cannot simply remove all instructions from your method body. You need to have at least one ret . For instance the code below does work:

using System;
using System.Linq;
using Mono.Cecil;
using System.IO;

namespace cecil
{
    class Program
    {
        static void Main(string[] args)
        {
            X();

            var assembly = AssemblyDefinition.ReadAssembly(typeof(Program).Assembly.Location);
            var method = assembly.MainModule.Types.SelectMany(t => t.Methods).SingleOrDefault(m => m.Name == "X");

            if (method == null)
            {
                System.Console.WriteLine("Method X not found.");
            }
            else
            {
                Console.WriteLine(method.Body.Instructions.Count);
                method.Body.Instructions.Clear();
                var ilProcessor = method.Body.GetILProcessor();
                ilProcessor.Append(ilProcessor.Create(Mono.Cecil.Cil.OpCodes.Ret));

                Console.WriteLine(method.Body.Instructions.Count);

                var newAssemblyFileName= "External2.dll";
                assembly.Write(newAssemblyFileName);
                System.Console.WriteLine($"Assembly saved to {Path.GetFullPath(newAssemblyFileName)}");
            }
        }

        private static void X()
        {
            System.Console.WriteLine("From X");
        }
    }
}

btw, you can always use https://cecilifier.me to help you out.