From 64cdc622fd37d4e1c4ef5b3c9b9fc2d8f6eb2d76 Mon Sep 17 00:00:00 2001
From: Mason <awesomest.mason@gmail.com>
Date: Fri, 17 Apr 2026 14:41:22 -0400
Subject: [PATCH] fix: 120 ObserveCountChanged misses count if observer has
 side effect

---
 .../ObservableCollectionR3Extensions.cs       |  2 +-
 .../ObservableCollectionExtensionsTest.cs     | 25 +++++++++++++++++--
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/src/ObservableCollections.R3/ObservableCollectionR3Extensions.cs b/src/ObservableCollections.R3/ObservableCollectionR3Extensions.cs
index 16d681b..2b4c0d3 100644
--- a/src/ObservableCollections.R3/ObservableCollectionR3Extensions.cs
+++ b/src/ObservableCollections.R3/ObservableCollectionR3Extensions.cs
@@ -438,10 +438,10 @@ protected override void Handler(in NotifyCollectionChangedEventArgs<T> eventArgs
                 case NotifyCollectionChangedAction.Add:
                 case NotifyCollectionChangedAction.Remove:
                 case NotifyCollectionChangedAction.Reset when countPrev != collection.Count:
+                    countPrev = collection.Count;
                     observer.OnNext(collection.Count);
                     break;
             }
-            countPrev = collection.Count;
         }
     }
 }
diff --git a/tests/ObservableCollections.R3.Tests/ObservableCollectionExtensionsTest.cs b/tests/ObservableCollections.R3.Tests/ObservableCollectionExtensionsTest.cs
index 85f9355..ff26392 100644
--- a/tests/ObservableCollections.R3.Tests/ObservableCollectionExtensionsTest.cs
+++ b/tests/ObservableCollections.R3.Tests/ObservableCollectionExtensionsTest.cs
@@ -161,7 +161,7 @@ public void ObserveReplace()
         collection[1] = 444;
         events.Count.Should().Be(1);
     }
-    
+
     [Fact]
     public void ObserveDictionaryReplace()
     {
@@ -247,6 +247,27 @@ public void ObserveCountChanged()
         events.Count.Should().Be(3);
     }
 
+    [Fact]
+    public void ObserveCountChanged_WithSideEffect()
+    {
+        var events = new List<int>();
+        var collection = new ObservableList<int>([]);
+
+        using var _ = collection.ObserveCountChanged().Subscribe(count =>
+        {
+            events.Add(count);
+            // Side effect - when count is 1, clear the list
+            if(count == 1) collection.Clear();
+        });
+
+        events.Should().BeEmpty();
+
+        collection.Add(12);
+
+        collection.Count.Should().Be(0);
+        events.Should().BeEquivalentTo([1, 0]);
+    }
+
     [Fact]
     public void ObserveCountChanged_NotifyCurrent()
     {
@@ -257,4 +278,4 @@ public void ObserveCountChanged_NotifyCurrent()
             .Subscribe(count => events.Add(count));
         events[0].Should().Be(3);
     }
-}
\ No newline at end of file
+}