Can Thread Safety Issues Arise When Dispatching Events in Multi-Threaded .NET Applications?

Event Dispatching Safety in Multi-Threaded Environments

When using events in multi-threaded applications, it is crucial to ensure thread safety to avoid potential race conditions. One common approach to event dispatching involves checking if an event is null before invoking it. However, this raises the question:

Can another thread alter the event invocation list between the null check and the event invocation?

To address this concern, a more robust solution is to use the following pattern:

protected virtual void OnSomeEvent(EventArgs args)
{
    EventHandler ev = SomeEvent;
    if (ev != null) ev(this, args);
}

This technique works because:

• Delegate assignments are atomic in .NET.
• The default implementations of event accessors (add/remove) are synchronized.
• By creating a copy of the multicast delegate, any subsequent changes to the original event will not affect the invocation.

While this solution addresses the scenario of null events, it does not handle the possibility of defunct event handlers or event subscriptions after the copy is taken. For more comprehensive solutions and further discussion, please refer to the provided external resources.

Additionally, in C# 6.0, Krzysztof's solution, as described in the answer, presents a viable alternative.

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