c# – 表达式树中的访问方法组

2024年1月28日 136次阅读

我正在尝试编写一个表达式树,它可以使用MethodInfo给出的方法订阅EventInfo给出的事件.表达式树应编译成Action< object,object>其中参数是事件源对象和订阅对象. EventInfo和MethodInfos保证兼容.

这是我到目前为止：

// Given the following
object Source = /**/;           // the object that will fire an event
EventInfo SourceEvent = /**/;   // the event that will be fired
object Target = /**/;           // the object that will subscribe to the event
MethodInfo TargetMethod = /**/; // the method that will react to the event

// setting up objects involved
var sourceParam = Expression.Parameter(typeof(object), "source");
var targetParam = Expression.Parameter(typeof(object), "target");
var sourceParamCast = Expression.Convert(sourceParam, SourceEvent.DeclaringType);
var targetParamCast = Expression.Convert(targetParam, TargetMethod.DeclaringType);

// Get subscribing method group. This is where things fail
var targetMethodRef = Expression.MakeMemberAccess(targetParamCast, TargetMethod);
// Subscribe to the event
var addMethodCall = Expression.Call(sourceParamCast, SourceEvent.AddMethod, targetMethodRef);

var lambda = Expression.Lambda<Action<object, object>>(addMethodCall, sourceParam, targetParam);
var subscriptionAction = lambda.Compile();

// And then later, subscribe to the event
subscriptionAction(Source, Target);

在调用MakeMemberAccess时,我得到以下异常：

ArgumentException: Member ‘void theMethodName()’ not field or property

这里的目标是targetMethodRef基本上表示在使用方法订阅事件时在=的右侧显示的内容.

TLDR：如何创建表达式以将对象上的方法组作为参数传递给表达式树中的函数调用？

最佳答案 应该是这样.这里的复杂性是你必须使用CreateDelegate在lambda方法中创建一个委托.遗憾的是,似乎不可能在lambda方法中创建一个open委托(没有目标的委托),然后在执行lambda方法时在lambda方法中“关闭”它.或者至少我不知道该怎么做.可悲的是,CreateDelegate有点慢.

static Action<object, object> MakeFunc(EventInfo sourceEvent, MethodInfo targetMethod)
{
    // setting up objects involved
    var sourceParam = Expression.Parameter(typeof(object), "source");
    var targetParam = Expression.Parameter(typeof(object), "target");
    var sourceParamCast = Expression.Convert(sourceParam, sourceEvent.DeclaringType);
    var targetParamCast = Expression.Convert(targetParam, targetMethod.DeclaringType);
    var createDelegate = typeof(Delegate).GetMethod(nameof(Delegate.CreateDelegate), BindingFlags.Static | BindingFlags.Public, null, new[] { typeof(Type), typeof(object), typeof(MethodInfo) }, null);

    // Create a delegate of type sourceEvent.EventHandlerType
    var createDelegateCall = Expression.Call(createDelegate, Expression.Constant(sourceEvent.EventHandlerType), targetParam, Expression.Constant(targetMethod));

    // Cast the Delegate to its real type
    var delegateCast = Expression.Convert(createDelegateCall, sourceEvent.EventHandlerType);

    // Subscribe to the event
    var addMethodCall = Expression.Call(sourceParamCast, sourceEvent.AddMethod, delegateCast);

    var lambda = Expression.Lambda<Action<object, object>>(addMethodCall, sourceParam, targetParam);
    var subscriptionAction = lambda.Compile();

    return subscriptionAction;
}

嗯……可以通过调用委托构造函数来完成.通过试用建立(没有找到很多关于此的文档)：

static Action<object, object> MakeFunc(EventInfo sourceEvent, MethodInfo targetMethod)
{
    // setting up objects involved
    var sourceParam = Expression.Parameter(typeof(object), "source");
    var targetParam = Expression.Parameter(typeof(object), "target");
    var sourceParamCast = Expression.Convert(sourceParam, sourceEvent.DeclaringType);
    var targetParamCast = Expression.Convert(targetParam, targetMethod.DeclaringType);

    ConstructorInfo delegateContructror = sourceEvent.EventHandlerType.GetConstructor(BindingFlags.Instance | BindingFlags.Public, null, new[] { typeof(object), typeof(IntPtr) }, null);
    IntPtr fp = targetMethod.MethodHandle.GetFunctionPointer();

    // create the delegate
    var newDelegate = Expression.New(delegateContructror, targetParam, Expression.Constant(fp));

    // Subscribe to the event
    var addMethodCall = Expression.Call(sourceParamCast, sourceEvent.AddMethod, newDelegate);

    var lambda = Expression.Lambda<Action<object, object>>(addMethodCall, sourceParam, targetParam);
    var subscriptionAction = lambda.Compile();

    return subscriptionAction;
}

委托有一个带有两个参数的构造函数,即目标对象和IntPtr,它是该方法的本机函数指针.它通常由CIL用于ldftn / ldvirtftn,但.MethodHandle.GetFunctionPointer()是相同的“东西”.所以我们在我们构建的lambda表达式中调用这个构造函数.

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