Question

I’ve seen many developers when wanting to invoke an Event they assign it to a local variable named handler and invoke handler instead of invoking Event directly.
Why we are not invoking Events directly?

private void OnSomethingChanged(EventArgs e)
{
    if (SomethingEvent != null)
    {
        SomethingEvent(this, e);
    }
}

Answer 1

The code you’ve posted isn’t thread-safe, basically. If the final subscriber unsubscribes in a different thread after the if check but before the invocation, you’ll get a NullReferenceException.

One option is to write an extension method:

public static void NullSafeInvoke(this EventHandler handler,
                                  object sender, EventArgs e)
{
    if (handler != null)
    {
        handler(this, e);
    }
}

You can then write:

private void OnSomethingChanged(EventArgs e)
{
    SomethingEvent.NullSafeInvoke(this, e);
}

You’d probably want another overload for EventHandler<T>, too.