引言

在 .NET Core 生态系统中，AOP（面向切面编程）是一种增强代码可维护性和复用性的重要技术。通过 AOP，我们可以将诸如日志记录、事务管理、安全检查等横切关注点从业务逻辑中分离出来，实现更清晰的代码结构和更高的开发效率。本篇文章将介绍一个基于 EMIT（Event Method Interception Technology）的 AOP 库，该库利用 .NET Core 的动态代理特性，以实现高效且灵活的横切关注点管理。

1. EMIT 简介

EMIT 是 .NET 中一种用于方法拦截的先进技术，它允许开发者在运行时动态地拦截和修改方法调用。EMIT 的工作原理是利用了 .NET 的 Emit 方法动态生成委托（Delegates）和动态方法（Dynamic Methods），这使得我们可以在不改变原有方法签名的情况下，插入额外的逻辑。在 AOP 的上下文中，这非常有用，因为它允许我们以非侵入式的方式增强方法的行为。

2. 构建基于 EMIT 的 AOP 库

2.1 定义 AOP 概念

在开始编码之前，让我们先定义几个关键概念：
- 切面（Aspect）：包含横切关注点的逻辑（如日志、安全）。
- 连接点（Join Point）：在程序执行过程中可以被 AOP 拦截的点，如方法调用。
- 通知（Advice）：在连接点执行的代码段，可以是前置、后置、异常等。
- 编织（Weaving）：将切面应用到目标对象上的过程。

2.2 库的架构设计

我们的 AOP 库将基于以下核心组件构建：
- AspectBuilder：负责构建切面和定义通知类型（前置、后置、异常等）。
- Interceptor：负责实际的方法拦截逻辑。
- DynamicMethodFactory：负责使用 EMIT 动态创建方法。
- AopProxy：为原对象创建代理，实现实际的方法调用和拦截逻辑的编织。

2.3 实现关键组件

2.3.1 DynamicMethodFactory 实现

```csharp
using System;
using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.CompilerServices;
using System.Runtime.Remoting.Messaging;
using System.Threading.Tasks;

public class DynamicMethodFactory {
private readonly ModuleBuilder moduleBuilder; private readonly TypeBuilder _typeBuilder; private readonly MethodBuilder _methodBuilder; private readonly Type _delegateType; private readonly MethodInfo _invokeMethod; private MethodInfo _dynamicMethod; private readonly AsyncMethodBuilder _asyncMethodBuilder; // For async support (if needed) private const MethodAttributes METHODATTRIBUTES = MethodAttributes.Public | MethodAttributes.HideBySig | MethodAttributes.NewSlot | MethodAttributes.Virtual | MethodAttributes.Final;
private const TypeAttributes TYPEATTRIBUTES = TypeAttributes.Public | TypeAttributes.Class | TypeAttributes.BeforeFieldInit; // Additional attributes for proper initialization and visibility if needed. public DynamicMethodFactory(Type delegateType) { // Constructor for initializing the factory with the delegate type for the method to be intercepted. _moduleBuilder = AssemblyBuilder.DefineDynamicModule(new ModuleName("DynamicModule"), "DynamicModule"); // Create a new module for the dynamic assembly. _typeBuilder = _moduleBuilder.DefineType("DynamicType", TYPEATTRIBUTES, typeof(object)); // Define a new type for the dynamic method.
methodBuilder = _typeBuilder.DefineMethod("DynamicMethod", METHODATTRIBUTES, delegateType, Type.EmptyTypes); // Define the dynamic method with the given signature (matching the delegate type).
_delegateType = delegateType;
_invokeMethod = typeof(Delegate).GetMethod("Invoke"); // Get the Invoke method of the delegate to be used for reflection to invoke the dynamic method.
_asyncMethodBuilder = new AsyncMethodBuilder(); // Optional: for async support, if needed later on.
}
public MethodInfo CreateMethod() { // Finalize and return the dynamic method info for use in creating the delegate instance to be used in place of the original method implementation or in conjunction with it as an additional aspect to be woven in later on during runtime weaving phase or during object proxy creation phase based on the chosen weaving strategy (eager/lazy).
_dynamicMethod = _methodBuilder.CreateMethod(); // Actually create the dynamic method instance at this point based on previously defined parameters/attributes etc.. Now you can use this _dynamicMethod instance within your interceptor logic whenever you need to call/invoke/execute it directly or indirectly through a delegate instance created from it at runtime during proxy creation phase later on.. Returning this _dynamicMethod instance will allow you to use it directly or indirectly through any other means of accessing/executing dynamically created methods including but not limited to creating a delegate instance from it using reflection if needed at runtime during proxy creation process based on chosen weaving strategy and usage scenario requirements.. Return _dynamicMethod; // Returning here gives access to dynamically created method info which can then be used further down the line for creating a delegate instance from it and then using that delegate instance as part of overall aspect weaving process during proxy creation phase or at runtime based on chosen weaving strategy requirements.. } }
```