[armadavalor]

Fixes the lost count change in ObserveCountChanged described in #132.

Cause: the base observer constructor attached the CollectionChanged handler before the derived constructor initialized countPrev. A Reset raised in that window was compared against the default 0 and dropped, and the derived constructor then overwrote countPrev with the new count, so nothing could detect it afterwards.

Fix:

• ObservableCollectionObserverBase gets a protected virtual OnBeforeAttach() hook that runs before the handler is attached. countPrev is initialized there.
• The subscription is created under lock (collection.SyncRoot), so a change from another thread cannot land between attaching the handler and the notifyCurrentCount: true notification. This also covers the duplicated or out-of-order first value mentioned in the issue.

Verification: added ObserveCountChanged_ChangeWhileSubscribing, which uses the decorator from the issue to clear the collection right after the handler is attached. It fails without the change (nothing emitted) and passes with it. I only had a .NET 10 SDK with the runtime rolled forward (DOTNET_ROLL_FORWARD=Major), so the net8.0 test project ran on that:

Passed! - Failed: 0, Passed: 11, Skipped: 0, Total: 11 - ObservableCollections.R3.Tests.dll (net8.0)

Closes #132

[Copilot]

🟡 Changes recommended

With notifyCurrentCount enabled, a reentrant attachment-time reset produces duplicate initial count notifications ([0, 0]) that the outer lock does not prevent.

1 open issue

• Avoid duplicate initial notification on reentrant attach · New

What changed in this PR

This PR addresses lost count-change notifications during R3 ObserveCountChanged subscriptions, as reported in #132.

Changes:

• Initializes the previous count through a new pre-attachment hook.
• Locks subscription setup to prevent concurrent mutations between handler attachment and the initial notification.
• Adds a regression test that clears the collection during handler attachment.

File	Description
tests/​ObservableCollections.R3.Tests/​ObservableCollectionExtensionsTest.cs	Adds an attach-time mutation helper and regression test.
src/​ObservableCollections.R3/​ObservableCollectionR3Extensions.cs	Initializes count tracking before attachment and locks subscription setup.

_{Review effort: Balanced}