c# - 通过 Linq 表达式树识别事件

Question

当事件没有出现在 += 或 -= 旁边时，编译器通常会卡住，所以我不确定这是否可能。

我希望能够通过使用表达式树来识别事件，因此我可以为测试创建一个事件观察器。语法看起来像这样:

using(var foo = new EventWatcher(target, x => x.MyEventToWatch) {
    // act here
}   // throws on Dispose() if MyEventToWatch hasn't fired

我的问题有两个:

1. 编译器会卡住吗？如果是这样，关于如何防止这种情况有什么建议吗？
2. 如何从构造函数中解析 Expression 对象，以便附加到 target 的 MyEventToWatch 事件？

Answer 1

编辑:作为Curt已经指出，我的实现是相当有缺陷的，因为它只能在声明事件的类中使用 :) 而不是“x => x.MyEvent”返回事件，它正在返回支持字段，只能由类访问。

由于表达式不能包含赋值语句，像“( x, h ) => x.MyEvent += h”这样的修饰表达式不能用于检索事件，因此需要反射代替使用。正确的实现需要使用反射来检索事件的 EventInfo(不幸的是，它不会是强类型的)。​​

否则，唯一需要做的更新是存储反射的EventInfo，并使用AddEventHandler/RemoveEventHandler方法注册监听器(而不是手动 Delegate Combine/Remove 调用和字段集)。实现的其余部分不需要更改。祝你好运:)

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注意:这是演示质量的代码，对访问器的格式做出了多项假设。正确的错误检查、静态事件的处理等留给读者作为练习；)

public sealed class EventWatcher : IDisposable {
  private readonly object target_;
  private readonly string eventName_;
  private readonly FieldInfo eventField_;
  private readonly Delegate listener_;
  private bool eventWasRaised_;

  public static EventWatcher Create<T>( T target, Expression<Func<T,Delegate>> accessor ) {
    return new EventWatcher( target, accessor );
  }

  private EventWatcher( object target, LambdaExpression accessor ) {
    this.target_ = target;

    // Retrieve event definition from expression.
    var eventAccessor = accessor.Body as MemberExpression;
    this.eventField_ = eventAccessor.Member as FieldInfo;
    this.eventName_ = this.eventField_.Name;

    // Create our event listener and add it to the declaring object's event field.
    this.listener_ = CreateEventListenerDelegate( this.eventField_.FieldType );
    var currentEventList = this.eventField_.GetValue( this.target_ ) as Delegate;
    var newEventList = Delegate.Combine( currentEventList, this.listener_ );
    this.eventField_.SetValue( this.target_, newEventList );
  }

  public void SetEventWasRaised( ) {
    this.eventWasRaised_ = true;
  }

  private Delegate CreateEventListenerDelegate( Type eventType ) {
    // Create the event listener's body, setting the 'eventWasRaised_' field.
    var setMethod = typeof( EventWatcher ).GetMethod( "SetEventWasRaised" );
    var body = Expression.Call( Expression.Constant( this ), setMethod );

    // Get the event delegate's parameters from its 'Invoke' method.
    var invokeMethod = eventType.GetMethod( "Invoke" );
    var parameters = invokeMethod.GetParameters( )
        .Select( ( p ) => Expression.Parameter( p.ParameterType, p.Name ) );

    // Create the listener.
    var listener = Expression.Lambda( eventType, body, parameters );
    return listener.Compile( );
  }

  void IDisposable.Dispose( ) {
    // Remove the event listener.
    var currentEventList = this.eventField_.GetValue( this.target_ ) as Delegate;
    var newEventList = Delegate.Remove( currentEventList, this.listener_ );
    this.eventField_.SetValue( this.target_, newEventList );

    // Ensure event was raised.
    if( !this.eventWasRaised_ )
      throw new InvalidOperationException( "Event was not raised: " + this.eventName_ );
  }
}

用法与建议的略有不同，以便利用类型推断:

try {
  using( EventWatcher.Create( o, x => x.MyEvent ) ) {
    //o.RaiseEvent( );  // Uncomment for test to succeed.
  }
  Console.WriteLine( "Event raised successfully" );
}
catch( InvalidOperationException ex ) {
  Console.WriteLine( ex.Message );
}