Add SubscriberCount property to WeakEvent.

README.md

@@ -77,6 +77,8 @@ public record MyEventData(string Message);
 ## API
 
 ### `WeakEvent`
+ * `SubscriberCount`\
+   Number of alive subscribers currently registered to the event.
  * `Subscribe(Action handler)`\
    `Subscribe(Func<Task> handler)`\
    `Subscribe(Func<CancellationToken, Task> handler)`\
@@ -89,6 +91,8 @@ public record MyEventData(string Message);
    Raises the event by invoking all live subscribers. Dead subscribers (whose targets have been garbage-collected) are removed.
 
 ### `WeakEvent<TEvent>`
+ * `SubscriberCount`\
+   Number of alive subscribers currently registered to the event.
  * `Subscribe(Action<TEvent> handler)`\
    `Subscribe(Func<TEvent, Task> handler)`\
    `Subscribe(Func<TEvent, CancellationToken, Task> handler)`\

src/WeakEvent.Tests/WeakEventGenericTests.cs

@@ -116,7 +116,14 @@ public async Task DeadHandler_IsNotInvoked_AfterGarbageCollection()
 		var weakEvent = new WeakEvent<string>();
 		var callCount = 0;
 
-		await CreateSubscriberAndInvoke(weakEvent, () => callCount++);
+		static async Task createSubscriberAndInvoke(WeakEvent<string> weakEvent, Action onEvent)
+		{
+			var subscriber = new GenericSubscriber(onEvent);
+			weakEvent.Subscribe(subscriber.Handler);
+			await weakEvent.PublishAsync("Test");
+			// The subscriber goes out of scope after this method, allowing it to be GC’d.
+		}
+		await createSubscriberAndInvoke(weakEvent, () => callCount++);
 
 		// Force garbage collection to reclaim the subscriber instance.
 		GC.Collect();
@@ -129,12 +136,35 @@ public async Task DeadHandler_IsNotInvoked_AfterGarbageCollection()
 		Assert.Equal(1, callCount);
 	}
 
-	private static async Task CreateSubscriberAndInvoke(WeakEvent<string> weakEvent, Action onEvent)
+	[Fact]
+	public void MultipleHandlers_CorrectCount()
 	{
-		var subscriber = new GenericSubscriber(onEvent);
-		weakEvent.Subscribe(subscriber.Handler);
-		await weakEvent.PublishAsync("Test");
-		// The subscriber goes out of scope after this method, allowing it to be GC’d.
+		// Stage 1 - No subscribers yet
+		var weakEvent = new WeakEvent<string>();
+		Assert.Equal(0, weakEvent.SubscriberCount);
+
+		// Stage 2 - Add local handler
+		void syncHandler(string _)
+		{ }
+		weakEvent.Subscribe(syncHandler);
+
+		// Stage 3 - Add a garbage-collected handler
+		void createSubscriberAndAssert()
+		{
+			var subscriber = new GenericSubscriber(() => { });
+			weakEvent.Subscribe(subscriber.Handler);
+			Assert.Equal(2, weakEvent.SubscriberCount);
+		}
+		createSubscriberAndAssert();
+
+		// Stage 4 - Force garbage collection to reclaim the garbage-collectable subscriber instance.
+		GC.Collect();
+		GC.WaitForPendingFinalizers();
+		Assert.Equal(1, weakEvent.SubscriberCount);
+
+		// Stage 5 - Remove local handler
+		weakEvent.Unsubscribe(syncHandler);
+		Assert.Equal(0, weakEvent.SubscriberCount);
 	}
 
 	private class GenericSubscriber(Action onEvent)

src/WeakEvent.Tests/WeakEventNonGenericTests.cs

@@ -114,7 +114,14 @@ public async Task DeadHandler_IsNotInvoked_AfterGarbageCollection()
 		var weakEvent = new WeakEvent();
 		var callCount = 0;
 
-		await CreateSubscriberAndInvoke(weakEvent, () => callCount++);
+		static async Task createSubscriberAndInvoke(WeakEvent weakEvent, Action onEvent)
+		{
+			var subscriber = new NonGenericSubscriber(onEvent);
+			weakEvent.Subscribe(subscriber.Handler);
+			await weakEvent.PublishAsync();
+			// The subscriber goes out of scope after this method, allowing it to be GC’d.
+		}
+		await createSubscriberAndInvoke(weakEvent, () => callCount++);
 
 		// Force garbage collection to reclaim the subscriber instance.
 		GC.Collect();
@@ -127,12 +134,36 @@ public async Task DeadHandler_IsNotInvoked_AfterGarbageCollection()
 		Assert.Equal(1, callCount);
 	}
 
-	private static async Task CreateSubscriberAndInvoke(WeakEvent weakEvent, Action onEvent)
+	[Fact]
+	public void MultipleHandlers_CorrectCount()
 	{
-		var subscriber = new NonGenericSubscriber(onEvent);
-		weakEvent.Subscribe(subscriber.Handler);
-		await weakEvent.PublishAsync();
-		// The subscriber goes out of scope after this method, allowing it to be GC’d.
+		// Stage 1 - No subscribers yet
+		var weakEvent = new WeakEvent();
+		Assert.Equal(0, weakEvent.SubscriberCount);
+
+		// Stage 2 - Add local handler
+		void syncHandler()
+		{ }
+		weakEvent.Subscribe(syncHandler);
+		Assert.Equal(1, weakEvent.SubscriberCount);
+
+		// Stage 3 - Add a garbage-collected handler
+		void createSubscriberAndAssert()
+		{
+			var subscriber = new NonGenericSubscriber(() => { });
+			weakEvent.Subscribe(subscriber.Handler);
+			Assert.Equal(2, weakEvent.SubscriberCount);
+		}
+		createSubscriberAndAssert();
+
+		// Stage 4 - Force garbage collection to reclaim the garbage-collectable subscriber instance.
+		GC.Collect();
+		GC.WaitForPendingFinalizers();
+		Assert.Equal(1, weakEvent.SubscriberCount);
+
+		// Stage 5 - Remove local handler
+		weakEvent.Unsubscribe(syncHandler);
+		Assert.Equal(0, weakEvent.SubscriberCount);
 	}
 
 	private class NonGenericSubscriber(Action onEvent)

src/WeakEvent/PACKAGE.md

@@ -68,6 +68,8 @@ public record MyEventData(string Message);
 ## API Overview
 
 ### `WeakEvent`
+ * `SubscriberCount`\
+   Number of alive subscribers currently registered to the event.
  * `Subscribe(Action handler)`\
    `Subscribe(Func<Task> handler)`\
    `Subscribe(Func<CancellationToken, Task> handler)`\
@@ -80,6 +82,8 @@ public record MyEventData(string Message);
    Raises the event by invoking all live subscribers. Dead subscribers (whose targets have been garbage-collected) are removed.
 
 ### `WeakEvent<TEvent>`
+ * `SubscriberCount`\
+   Number of alive subscribers currently registered to the event.
  * `Subscribe(Action<TEvent> handler)`\
    `Subscribe(Func<TEvent, Task> handler)`\
    `Subscribe(Func<TEvent, CancellationToken, Task> handler)`\

src/WeakEvent/WeakEvent.cs

@@ -150,6 +150,11 @@ public abstract class WeakEventBase
 
 	private readonly SemaphoreSlim _lock = new(1, 1);
 
+	/// <summary>
+	/// Number of alive subscribers currently registered to the event.
+	/// </summary>
+	public int SubscriberCount => _handlers.Count(x => x.IsAlive);
+
 	/// <summary>
 	/// Subscribes the specified delegate handler to the event.
 	/// </summary>