Why does this call to Trigger keySet() throw a null pointer exception?

Question

I have a simple trigger like:

trigger littleTrigger on Custom__c (before insert) {
    Set<Id> idSet = Trigger.newMap.keySet();
    //Some stuff
}

Why could the second line throw a null pointer exception?

When I code it like:

trigger littleTrigger on Custom__c (before insert) {
    Set<Id> idSet = new Set<Id>();

    for (Custom__c d : Trigger.New)
        idSet.add(d.Id);
}

It works just fine.

Answer 1

From the Trigger Context Variables documentation:

• Trigger.new: this sObject list is only available in insert and update triggers
• Trigger.newMap: this map is only available in before update, after insert, and after update triggers

In a "before insert" trigger, the sObjects don't yet have ID values so a Trigger.newMap keyed by the sObject ID can't be made available and Trigger.newMap returns null. But a list of the sObjects (again without IDs) can be made available.