使用IL Emit调用现有对象的方法 [英] Calling a method of existing object using IL Emit

问题描述

我正在尝试编写基于属性的拦截器（类似于 DynamicProxy ）。这个想法是基于某些自定义属性，将调用该属性内部的方法，即

I am attempting to write an attribute based interceptor (something similar to DynamicProxy). The idea being, based on certain custom attributes, a method inside that attribute would be called, i.e.

• 在实际使用之前在属性类中调用一个方法方法被调用。


• 调用实际方法。

我能够覆盖现有方法使用 MethodBuilder 和 TypeBuilder 。但是，我不知道如何在Attribute中调用该方法。

I am able to override existing method using MethodBuilder and TypeBuilder. However, I can't figure out how to call the method inside the Attribute.

我的代码：

static void CreateMethods<T>(TypeBuilder tb)
        {
            foreach (var methodToOverride in typeof(T).GetMethods(BindingFlags.Instance | BindingFlags.Public | BindingFlags.DeclaredOnly))
            {
                var attribute = (ProxyMethod)methodToOverride.GetCustomAttribute(typeof(ProxyMethod));
                if (attribute == null)
                    continue;

                MethodBuilder methodBuilder = tb.DefineMethod(
                    methodToOverride.Name,
                    MethodAttributes.Public
                    | MethodAttributes.HideBySig
                    | MethodAttributes.NewSlot
                    | MethodAttributes.Virtual
                    | MethodAttributes.Final,
                    CallingConventions.HasThis,
                    methodToOverride.ReturnType,
                    Type.EmptyTypes
                );

                ILGenerator il = methodBuilder.GetILGenerator();

                il.Emit(OpCodes.Ldstr, "The I.M implementation of C"); //step1
                il.Emit(OpCodes.Call, typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) })); //step1

                il.Emit(OpCodes.Ldarg_0);
                il.Emit(OpCodes.Call, methodToOverride);
                il.Emit(OpCodes.Ret);

                tb.DefineMethodOverride(methodBuilder, methodToOverride);
            }
        }

我认为我应该做的是加载 attribute 放到堆栈上，然后通过发出对 MethodInfo attribute.attributeMethod（） c $ c>。但是，无论到哪里看，我都能找到使用 OpCodes.NewObj 创建对象的新实例的示例。我不想使用它，因为属性可能有参数。

What I think I should do is load attribute onto stack, then call attribute.attributeMethod() by emitting a call to the MethodInfo. However, everywhere I look, I can find examples of creating a new instance of the objected using OpCodes.NewObj. I don't want to use this, because attributes may have parameters.

我想不出在属性类内部调用方法的方法（它将代替step1注释） ）。

I can't think of any of calling the method inside attribute class (which would replace step1 comment).

编辑：基于评论，我试图将IL中代码的 GetCustomAttribute 部分移至。这就是我现在拥有的

Based on comments, I'm trying to move the GetCustomAttribute part of the code in IL. This is what I have right now

il.Emit(OpCodes.Ldtoken, methodToOverride);
il.Emit(OpCodes.Ldtoken, typeof(ProxyMethod));
il.Emit(OpCodes.Call, typeof(Attribute).GetMethod("GetCustomAttribute", new [] { typeof(MemberInfo), typeof(Type) }));

这对我来说是一个错误。有技巧吗？

It throws an error for me. Any tips?

推荐答案

动态代码生成总是很烦人。让我们先准备一些辅助方法来摆脱所有 .GetMethod 的东西：

Dynamic code generation is always kind of annoying. Let's cook up some helper methods first to get rid of all the .GetMethod stuff:

static class Method {
    public static MethodInfo Of<TResult>(Expression<Func<TResult>> f) => ((MethodCallExpression) f.Body).Method;
    public static MethodInfo Of<T>(Expression<Action<T>> f) => ((MethodCallExpression) f.Body).Method;
    public static MethodInfo Of(Expression<Action> f) => ((MethodCallExpression) f.Body).Method;
}

现在让我们说我们有 ProxyMethodAttribute 及其方法 MyMethod -这就是我们如何检索并调用它的方法：

Now let's say we have ProxyMethodAttribute and its method MyMethod -- here's how we'd retrieve that and call it:

il.Emit(OpCodes.Ldtoken, methodToOverride);
il.Emit(OpCodes.Call, Method.Of(() => MethodBase.GetMethodFromHandle(default(RuntimeMethodHandle))));
il.Emit(OpCodes.Ldtoken, typeof(ProxyMethodAttribute));
il.Emit(OpCodes.Call, Method.Of(() => Type.GetTypeFromHandle(default(RuntimeTypeHandle))));
il.Emit(OpCodes.Ldc_I4_1);
il.Emit(OpCodes.Call, Method.Of(() => Attribute.GetCustomAttribute(default(MemberInfo), default(Type), default(bool))));
il.Emit(OpCodes.Callvirt, Method.Of((ProxyMethodAttribute a) => a.MyMethod()));

请注意我们如何调用 GetXXXFromHandle 从生成的元数据令牌中生成实际实例，以及我们如何需要对 Attribute.GetCustomAttribute （ MethodInfo.GetCustomAttribute 实际上并不存在，它是作为扩展方法实现的。）

Note how we need to call GetXXXFromHandle to produce actual instances from the metadata tokens generated, and how we need a more involved call to Attribute.GetCustomAttribute (MethodInfo.GetCustomAttribute doesn't actually exist, this is implemented as an extension method.)

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