Nullable type issue with ?: Conditional Operator

Question

Could someone explain why this works in C#.NET 2.0:

 Nullable<DateTime> foo;
 if (true)
  foo = null;
 else
  foo = new DateTime(0);

...but this doesn't:

  Nullable<DateTime> foo;
 foo = true ? null : new DateTime(0);

The latter form gives me an compile error "Type of conditional expression cannot be determined because there is no implicit conversion between '<null>' and 'System.DateTime'."

Not that I can't use the former, but the second style is more consistent with the rest of my code.

Answer 1

This question has been asked a bunch of times already. The compiler is telling you that it doesn't know how convert null into a DateTime.

The solution is simple:

DateTime? foo;
foo = true ? (DateTime?)null : new DateTime(0);

Note that Nullable<DateTime> can be written DateTime? which will save you a bunch of typing.

Answer 2

It's because in a ternary operator, the two values must be the same type.

Answer 3

Check John Skeets answer here for a good explanation (no credit taken)

Answer 4

FYI (Offtopic, but nifty and related to nullable types) we have a handy operator just for nullable types called the null coalescing operator

??

Used like this:

// Left hand is the nullable type, righthand is default if the type is null.
Nullable<DateTime> foo;
DateTime value = foo ?? new DateTime(0);