How to Avoid Race Conditions When Dispatching Events in C#?

Race Condition in Event Dispatching

An event in C# is often dispatched using the following code:

public event EventHandler SomeEvent;
...
{
    ....
    if(SomeEvent!=null)SomeEvent();
}

However, in a multithreaded environment, this approach can lead to a race condition. Here's how it can happen:

1. Thread 1 checks if SomeEvent is not null.
2. Thread 2 removes the only registered delegate from SomeEvent.
3. Thread 1 proceeds to invoke SomeEvent, which causes an exception due to the null invocation list.

To address this concurrency issue, a best practice is to copy the invocation list to a temporary variable before checking for null:

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

This approach is thread-safe because:

• Delegate.Combine and Delegate.Remove return new delegate instances instead of modifying existing ones.
• Assignment of object references in .NET is atomic.
• The event accessors (add/remove) are synchronized.

By copying the invocation list, we ensure that the event handlers are invoked even if they are removed after the copy is taken. However, it's important to note that this solution does not address potential state issues with defunct event handlers.

The above is the detailed content of How to Avoid Race Conditions When Dispatching Events in C#?. For more information, please follow other related articles on the PHP Chinese website!