How to Ensure Multithreaded Safety When Dispatching Events with Null Checks?

Multithreaded Safety of Event Dispatching with Null Checks

In event-based programming, events allow different objects to communicate and react to certain occurrences. It's a common practice to dispatch events using a null check on the event handler delegate, as seen below:

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

However, in multithreaded environments, there's a potential race condition where another thread could modify the event's invocation list between the null check and the invocation. This can result in exceptions or incorrect behavior.

Addressing the Multithreaded Issue

To mitigate this, it's recommended to assign the event handler delegate to a local variable within the event dispatching method, as shown below:

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

By doing this, any subsequent modifications to the SomeEvent event handler list will not affect the copy stored in the ev variable. This ensures that the event will be invoked correctly, even if the event handler list changes in the meantime.

It's important to note that this solution addresses only one aspect of multithreaded event handling. It doesn't fully account for defunct event handlers or event handlers subscribing after the copy is taken. For a comprehensive guide on best practices in this area, refer to Eric Lippert's blog entry and the StackOverflow discussions on the topic.

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