Question

I’m working with a nullable DateTime object and ran into some strange behavior. Here’s a sample function:

    public DateTime? Weird()
    {
        DateTime check = DateTime.Now;
        DateTime? dt;
        if (check == DateTime.MinValue)
            dt = null;
        else
            dt = Viewer.ActiveThroughUTC.ToLocalTime();

        //this line give a compile error
        dt = (check == DateTime.MinValue) ? (null) : (Viewer.ActiveThroughUTC.ToLocalTime());
        return dt;
    }

As far as I know, the line with the ternary operator should be the same as the preceding four lines, but VS2010 is giving me a compile error, saying that no conversion exists between <null> and DateTime (even though the object in question is a ‘DateTime?’). Is there something I should know about the ternary operator or is this (gasp?) a bug?

Answer 1

Both elements in the ?: operator should be of the same type (but don’t have to be – see the details below). Cast null to DateTime?:

dt = (check == DateTime.MinValue) ? (DateTime?)null : ...

From the spec:

The second and third operands of the ?: operator control the type of the conditional expression. Let X and Y be the types of the second and third operands. Then,

If X and Y are the same type, then this is the type of the conditional expression.

• Otherwise, if an implicit conversion (Section 6.1) exists from X to Y, but not from Y to X, then Y is the type of the conditional expression.
• Otherwise, if an implicit conversion (Section 6.1) exists from Y to X, but not from X to Y, then X is the type of the conditional expression.
• Otherwise, no expression type can be determined, and a compile-time error occurs.

(Interestingly, it’s not actually called the “ternary” operator. It’s one possible ternary (three-valued) operator, and I’m not aware of any others in C#. It’s called the “?:” operator, which is a little hard to pronounce. Also called the “conditional” operator.)