How can I sort on nullable DateTime fields? [duplicate]

Question

I have the following line of C# code:

dueEvents.Sort((e1, e2) => e1.EventDateGmt.CompareTo(e2.EventDateGmt));

dueEvents is a List. The EventDateGmt fields are each a DateTime. This compiles.

I change it to this:

dueEvents.Sort((e1, e2) => e1.EventDateLocal.CompareTo(e2.EventDateLocal));

EventDateLocal is a DateTime?. This causes the following compiler error:

CS7036: There is no argument given that corresponds to the required comparison type...

How can I sort on EventDateLocal?

Answer 1

Since DateTime? values can be null we can't compare them directly. We should decide if null value is less (nulls first) or greater (nulls last) than any other value. For instance:

// Nulls First
dueEvents.Sort((left, right) => {
  // We have to deal with nulls... 
  if (!left.HasValue) 
    return !right.HasValue ? 0 : -1;
  else if (!right.HasValue)
    return 1;

  // Both left and right are not null, that's why we can do business as usual
  // and compare Values which are not null
  return left.Value.CompareTo(right.Value);  
});

Or

// Nulls Last
dueEvents.Sort((left, right) => {
  if (!left.HasValue) 
    return !right.HasValue ? 0 : 1;
  else if (!right.HasValue)
    return -1;

  return left.Value.CompareTo(right.Value);  
});