New version of FastSmartWeakEvent for http://www.codeproject.com/Articles/29922/Weak-Events-in-C

FastSmartWeakEvent.cs

// Copyright (c) 2013 Daniel Grunwald
// 
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without
// restriction, including without limitation the rights to use,
// copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following
// conditions:
// 
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.

using System;
using System.Collections.Generic;
using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.CompilerServices;
using System.Threading;

namespace SmartWeakEvent
{
  /// <summary>
	/// A class for managing a weak event.
	/// See http://www.codeproject.com/Articles/29922/Weak-Events-in-C
	/// </summary>
	/// <typeparam name="T">The delegate type of the event handlers.</typeparam>
	public sealed class FastSmartWeakEvent<T> where T : class
	{
		static FastSmartWeakEvent()
		{
			if (!typeof(T).IsSubclassOf(typeof(Delegate)))
				throw new ArgumentException("T must be a delegate type");
			MethodInfo invoke = typeof(T).GetMethod("Invoke");
			if (invoke == null)
				throw new ArgumentException("T must be a delegate type");
			if (invoke.ReturnType != typeof(void))
				throw new ArgumentException("The delegate return type must be void.");
			foreach (var p in invoke.GetParameters()) {
				if (p.IsOut && !p.IsIn)
					throw new ArgumentException("The delegate type must not have out-parameters");
			}
		}
		
		class HandlerEntry
		{
			public readonly FastSmartWeakEvent<T> ParentEventSource;
			readonly WeakReference weakReference;
			public readonly MethodInfo TargetMethod;
			public Delegate WrappingDelegate;
			
			public HandlerEntry(FastSmartWeakEvent<T> parentEventSource, object targetInstance, MethodInfo targetMethod)
			{
				this.ParentEventSource = parentEventSource;
				this.weakReference = new WeakReference(targetInstance);
				this.TargetMethod = targetMethod;
			}
			
			// This property is accessed by the generated IL method
			public object TargetInstance {
				get { return weakReference.Target; }
			}
			
			// This method is called by the generated IL method
			public void CalledWhenDead()
			{
				ParentEventSource.RemoveFromRaiseDelegate(this.WrappingDelegate);
			}
			
			/*
			 A wrapper method like this is generated using IL.Emit and attached to this object.
			 The signature of the method depends on the delegate type T.
			this.WrappingDelegate = delegate(object sender, EventArgs e)
			{
				object target = this.TargetInstance;
				if (target == null)
					this.CalledWhenDead();
				else
					((TargetType)target).TargetMethod(sender, e);
			}
			 */
		}
		
		volatile Delegate _raiseDelegate;
		
		Delegate GetRaiseDelegateInternal()
		{
			return _raiseDelegate;
		}
		
		#pragma warning disable 420 // CS0420 - a reference to a volatile field will not be treated as volatile
		// can be ignored because CompareExchange() treats the reference as volatile
		void AddToRaiseDelegate(Delegate d)
		{
			Delegate oldDelegate, newDelegate;
			do {
				oldDelegate = _raiseDelegate;
				newDelegate = Delegate.Combine(oldDelegate, d);
			} while (Interlocked.CompareExchange(ref _raiseDelegate, newDelegate, oldDelegate) != oldDelegate);
		}
		
		void RemoveFromRaiseDelegate(Delegate d)
		{
			Delegate oldDelegate, newDelegate;
			do {
				oldDelegate = _raiseDelegate;
				newDelegate = Delegate.Remove(oldDelegate, d);
			} while (Interlocked.CompareExchange(ref _raiseDelegate, newDelegate, oldDelegate) != oldDelegate);
		}
		#pragma warning restore 420
		
		public void Add(T eh)
		{
			if (eh != null) {
				Delegate d = (Delegate)(object)eh;
				RemoveDeadEntries();
				object targetInstance = d.Target;
				if (targetInstance != null) {
					MethodInfo targetMethod = d.Method;
					var wd = new HandlerEntry(this, targetInstance, targetMethod);
					var dynamicMethod = GetInvoker(targetMethod);
					wd.WrappingDelegate = dynamicMethod.CreateDelegate(typeof(T), wd);
					AddToRaiseDelegate(wd.WrappingDelegate);
				} else {
					// delegate to static method: use directly without wrapping delegate
					AddToRaiseDelegate(d);
				}
			}
		}
		
		/// <summary>
		/// Removes dead entries from the handler list.
		/// You normally do not need to invoke this method manually, as dead entry removal runs automatically as part of the normal operation of the FastSmartWeakEvent.
		/// </summary>
		public void RemoveDeadEntries()
		{
			Delegate raiseDelegate = GetRaiseDelegateInternal();
			if (raiseDelegate == null)
				return;
			foreach (var d in raiseDelegate.GetInvocationList()) {
				var wd = d.Target as HandlerEntry;
				if (wd != null && wd.TargetInstance == null) {
					RemoveFromRaiseDelegate(d);
				}
			}
		}
		
		public void Remove(T eh)
		{
			if (eh == null)
				return;
			Delegate d = (Delegate)(object)eh;
			object targetInstance = d.Target;
			if (targetInstance == null) {
				// delegate to static method: use directly without wrapping delegate
				RemoveFromRaiseDelegate(d);
				return;
			}
			MethodInfo targetMethod = d.Method;
			// Find+Remove the last copy of a delegate pointing to targetInstance/targetMethod
			Delegate raiseDelegate = GetRaiseDelegateInternal();
			if (raiseDelegate == null)
				return;
			Delegate[] invocationList = raiseDelegate.GetInvocationList();
			for (int i = invocationList.Length - 1; i >= 0; i--) {
				var wrappingDelegate = invocationList[i];
				var weakDelegate = wrappingDelegate.Target as HandlerEntry;
				if (weakDelegate == null)
					continue;
				object target = weakDelegate.TargetInstance;
				if (target == null) {
					RemoveFromRaiseDelegate(wrappingDelegate);
				} else if (target == targetInstance && weakDelegate.TargetMethod == targetMethod) {
					RemoveFromRaiseDelegate(wrappingDelegate);
					break;
				}
			}
		}
		
		/// <summary>
		/// Retrieves a delegate that can be used to raise the event.
		/// The delegate will contain a copy of the current invocation list. If handlers are added/removed from the event, GetRaiseDelegate() must be called
		/// again to retrieve a delegate that invokes the up-to-date invocation list.
		/// 
		/// If the invocation list is empty, this method will return null.
		/// </summary>
		public T GetRaiseDelegate()
		{
			return (T)(object)GetRaiseDelegateInternal();
		}
		
		/// <summary>
		/// Gets whether the event has listeners that were not cleaned up yet.
		/// </summary>
		public bool HasListeners {
			get { return GetRaiseDelegateInternal() != null; }
		}
		
		#region Code Generation
		static readonly MethodInfo getTargetMethod = typeof(HandlerEntry).GetMethod("get_TargetInstance");
		static readonly MethodInfo calledWhileDeadMethod = typeof(HandlerEntry).GetMethod("CalledWhenDead");
		
		static readonly Dictionary<MethodInfo, DynamicMethod> invokerMethods = new Dictionary<MethodInfo, DynamicMethod>();
		
		static DynamicMethod GetInvoker(MethodInfo method)
		{
			DynamicMethod dynamicMethod;
			lock (invokerMethods) {
				if (invokerMethods.TryGetValue(method, out dynamicMethod))
					return dynamicMethod;
			}
			
			if (method.DeclaringType.GetCustomAttributes(typeof(CompilerGeneratedAttribute), false).Length != 0)
				throw new ArgumentException("Cannot create weak event to anonymous method with closure.");
			
			var parameters = method.GetParameters();
			var dynamicMethodParameterTypes = new Type[parameters.Length + 1];
			dynamicMethodParameterTypes[0] = typeof(HandlerEntry);
			for (int i = 0; i < parameters.Length; i++) {
				dynamicMethodParameterTypes[i + 1] = parameters[i].ParameterType;
			}
			
			dynamicMethod = new DynamicMethod("FastSmartWeakEvent", typeof(void), dynamicMethodParameterTypes, typeof(HandlerEntry), true);
			ILGenerator il = dynamicMethod.GetILGenerator();
			il.Emit(OpCodes.Ldarg_0);
			il.EmitCall(OpCodes.Call, getTargetMethod, null);
			il.Emit(OpCodes.Dup);
			Label label = il.DefineLabel();
			// Exit if target is null (was garbage-collected)
			il.Emit(OpCodes.Brtrue, label);
			il.Emit(OpCodes.Pop); // pop the duplicate null target
			il.Emit(OpCodes.Ldarg_0);
			il.EmitCall(OpCodes.Call, calledWhileDeadMethod, null);
			il.Emit(OpCodes.Ret);
			il.MarkLabel(label);
			il.Emit(OpCodes.Castclass, method.DeclaringType);
			for (int i = 0; i < parameters.Length; i++) {
				il.Emit(OpCodes.Ldarg, i + 1);
			}
			il.EmitCall(OpCodes.Call, method, null);
			il.Emit(OpCodes.Ret);
			
			lock (invokerMethods) {
				invokerMethods[method] = dynamicMethod;
			}
			return dynamicMethod;
		}
		#endregion
	}
	
	/// <summary>
	/// Strongly-typed raise methods for FastSmartWeakEvent
	/// </summary>
	public static class FastSmartWeakEventRaiseExtensions
	{
		public static void Raise(this FastSmartWeakEvent<EventHandler> ev, object sender, EventArgs e)
		{
			var d = ev.GetRaiseDelegate();
			if (d != null)
				d(sender, e);
		}
		
		public static void Raise<T>(this FastSmartWeakEvent<EventHandler<T>> ev, object sender, T e) where T : EventArgs
		{
			var d = ev.GetRaiseDelegate();
			if (d != null)
				d(sender, e);
		}
	}
}

I found two places for improvement, which make a huge difference when there are lots of subscribers:

• In the Add() method, you call RemoveDeadEntries(). However, that method is relatively slow, and is not really needed. Cleanup is also done when raising/invoking the event, so at best this cleans up the list a bit earlier
• In AddToRaiseDelegate() and RemoveFromRaiseDelegate(), you compare the values using the != operator. However, System.MulticastDelegate overrides the operator to perform value equality. That also is very slow when there are many entries in the delegate. Instead, use an explicit call to ReferenceEquals, i.e. while (!ReferenceEquals(Interlocked.CompareExchange(ref this.raiseDelegate, newDelegate, oldDelegate), oldDelegate))

In my case I have a weak-event with approx. 14000 subscribers. With those changes, I improved the runtime by 3 orders of magnitude, from 22s to 90ms.

Is it not possible to implement anonymous method support? If it is used with FluentAssertions and Monitor, this fails.
"Cannot create weak event to anonymous method with closure"