Question

我正在尝试编写一个表达式树，它可以使用EventInfo给出的方法订阅MethodInfo给出的事件。表达式树应该编译成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。

这是我到目前为止所做的：

// 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);

在致电MakeMemberAccess时，我收到以下异常：

ArgumentException：Member＆＃39; void theMethodName（）＆＃39;不是字段或属性

此处的目标是targetMethodRef基本上代表在使用方法订阅事件时+=右侧显示的内容。

TLDR：如何创建表达式，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？

Answer 1

应该是这样。这里的复杂性是您必须使用CreateDelegate在lambda方法中创建委托。遗憾的是，似乎没有可能创建一个开放的委托（没有target的委托）在lambda方法中编译然后＆＃34;关闭＆＃34;执行lambda方法时，它在lambda方法中。或者至少我不知道该怎么做。 CreateDelegate遗憾的是有点慢。

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（没有找到关于此的大量文档）：

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;
}

Delegate有一个带有两个参数的构造函数，目标object和IntPtr，它是该方法的本机函数指针。它通常由CIL使用ldftn / ldvirtftn，但.MethodHandle.GetFunctionPointer()是相同的＆＃34;事物＆＃34;。所以我们在我们构建的lambda表达式中调用这个构造函数。

表达式树中的访问方法组

1 个答案: