Should I use IDisposable for purely managed resources?

This was discussed before . Your case is much easier though, you are also implementing the finalizer. That's fundamentally wrong, you are hiding a bug in the client code.

Beware that finalizers run on a separate thread. Debugging a consistent deadlock is much easier than dealing with locks that disappear randomly and asynchronously.

This is possible by keeping a _disposed boolean field in Transaction and exposing it through an IsDisposed read-only property. This is standard practice.

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