Why copy an event handler into another variable

The problem is that between this line

if (ThresholdReached != null)

and this line

ThresholdReached(this, e);

a different thread could have removed the handlers from the ThresholdReached event. So it would be null and an exception would be thrown.

By storing the value in a local variable, you make the call thread-safe.

Since C# 6 you can shorten the code to this

ThresholdReached?.Invoke(this, e);

Now the compiler takes care of storing the value in a temporary variable.

There's a theoretical risk of a thread race in the second version where someone unsubscribes the event between the check and the invoke, causing a NullReferenceException in the invoke step. Capturing the value into a local and testing/invoking that prevents this. However, perhaps use the third version, possible with C# 6 or above (thanks @Cid):

ThresholdReached?.Invoke(this, e);

This is basically a short-hand version of the first version - all the safety, but now with terseness.