Pattern for dealing with null handler for events in C#

If you’ve used events in C# before, you’ve probably written code like this too:

public event EventHandler Started;
// make sure Started is not null before firing the event
// else, NullReferenceException will be thrown
if (Started != null)
    Started(this, some_event_args);

This is perfectly ok and normal to do, but can quickly become tiresome if you have to fire events in multiple places in your code and have to do a null reference check every time!

So instead, I have been using this pattern for a while:

// initialise with empty event hanlder so there's no need for Null reference check later
public event EventHandler Started = (s, e) => { };
// no need for Null reference check anymore as Started is never null
Started(this, some_event_args);

If all you need is the ability to add/remove handlers, then this pattern would do you fine as the event will never be null because there’s no way for you to remove the anonymous method the event was initialised with unless you set the event to null.

However, if you occasionally need to clear ALL the event handlers and set the event to null, then don’t use this pattern as you might start seeing NullReferenceException being thrown before you add event handlers back in.

Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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