[C#の黒魔術 IL Weaving] Unity で AOP(アスペクト指向プログラミング) をしよう

public class Test
{
    public static void Main()
    {
        new Test().DoSomething();
    }
    public void DoSomething()
    {
    }
}

using System;
using System.IO;
using System.Linq;
using Mono.Cecil;
using Mono.Cecil.Cil;
using Mono.Cecil.Pdb;
internal static class Program
{
    private static void Main(string[] args)
    {
        InjectCodeSample();
    }

    private static void InjectCodeSample()
    {
        const string dllPath = @"Path\To\Test.dll";
        // Test.dll を書き込みモードで開く
        using (var assemblyStream = new FileStream(dllPath, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite))
        {
            // Test.dll のモジュール定義を読み込む
            using (var moduleDefinition = ModuleDefinition.ReadModule(assemblyStream, new ReaderParameters
                   {
                       ReadingMode = ReadingMode.Immediate,
                       ReadWrite = true,  // dll を書き換える場合は true
                       ReadSymbols = true,
                       SymbolReaderProvider = new PdbReaderProvider()
                   }))
            {
                foreach (var typeDefinition in moduleDefinition.Types)
                foreach (var methodDefinition in typeDefinition.Methods)
                {
                    // DoSomething という名前をもつメソッドが見つかったらコードを書き換える
                    if (methodDefinition.Name == "DoSomething")
                        Inject(moduleDefinition, methodDefinition);
                }
                // 書き換えられた dll を上書き保存する
                moduleDefinition.Write(new WriterParameters()
                {
                    WriteSymbols = true,
                    SymbolWriterProvider = new PdbWriterProvider()
                });
            }
        }
    }

    // IL を変更する
    private static void Inject(ModuleDefinition moduleDefinition, MethodDefinition methodDefinition)
    {
        var processor = methodDefinition.Body.GetILProcessor();
        var injectionPoint = methodDefinition.Body.Instructions.First();
        var methodInfo = ((Action<string>)Console.WriteLine).Method;
        var methodRef = moduleDefinition.ImportReference(methodInfo);
        // 以下、 Console.WriteLine("Brain hacked! Thank you!") をメソッドの先頭に足す処理
        processor.InsertBefore(injectionPoint, Instruction.Create(OpCodes.Ldstr, "Brain hacked! Thank you!"));
        processor.InsertBefore(injectionPoint, Instruction.Create(OpCodes.Call, methodRef));
    }
}

        processor.InsertBefore(injectionPoint, Instruction.Create(OpCodes.Ldstr, "Brain hacked! Thank you!"));
        processor.InsertBefore(injectionPoint, Instruction.Create(OpCodes.Call, methodRef));

IL_0001: ldstr        "Brain hacked! Thank you!"
IL_0006: call         void [System.Console]System.Console::WriteLine(string)

> Brain hacked! Thank you!

using System;
using UnityEngine;

// インジェクトするコードをつくる
public class CodeToInject
{
    public static void SomethingDo()
    {
        Debug.Log("Injected!");
    }
}

// インジェクションポイントを指定するための Attribute を作る
[AttributeUsage(AttributeTargets.Method)] 
public class InjectCodeAttribute : Attribute
{
    public InjectCodeAttribute() { } 
}

using Mewlist.Weaver;

// IL Weaver を定義する
public class SampleWeaver : IWeaver
{
    public void Weave(AssemblyInjector assemblyInjector)
    {
        assemblyInjector
            .OnMainAssembly()
            .OnAttribute<InjectCodeAttribute>()
            .BeforeDo(CodeToInject.SomethingDo)
            .Inject();
    }
}

            .OnAttribute<InjectCodeAttribute>()

            .BeforeDo(CodeToInject.SomethingDo)

            .Inject()

using UnityEngine;

public class MyBehaviour : MonoBehaviour
{
    [InjectCode]
    void Start()
    {
    }
}