Nullable DateTime conversion [duplicate]

Answer 1

You can try this

var lastPostDate = reader[3] == DBNull.Value ?
                                default(DateTime?): 
                                Convert.ToDateTime(reader[3]);

Answer 2

You might want to do it like this:

DateTime? lastPostDate =  (DateTime?)(reader.IsDbNull(3) ? null : reader[3]); 

The problem you are having is that the ternary operator wants a viable cast between the left and right sides. And null can't be cast to DateTime.

Note the above works because both sides of the ternary are object's. The object is explicitly cast to DateTime? which works: as long as reader[3] is in fact a date.

Answer 3

Cast the null literal: (DateTime?)null or (Nullable<DateTime>)null.

You can also use default(DateTime?) or default(Nullable<DateTime>)

And, as other answers have noted, you can also apply the cast to the DateTime value rather than to the null literal.

EDIT (adapted from my comment to Prutswonder's answer):

The point is that the conditional operator does not consider the type of its assignment target, so it will only compile if there is an implicit conversion from the type of its second operand to the type of its third operand, or from the type of its third operand to the type of its second operand.

For example, this won't compile:

bool b = GetSomeBooleanValue();
object o = b ? "Forty-two" : 42;

Casting either the second or third operand to object, however, fixes the problem, because there is an implicit conversion from int to object and also from string to object:

object o = b ? "Forty-two" : (object)42;

or

object o = b ? (object)"Forty-two" : 42;

Answer 4

Make sure those two types are nullable DateTime

var lastPostDate = reader[3] == DBNull.Value ?
                                        null : 
                                   (DateTime?) Convert.ToDateTime(reader[3]);

• Usage of DateTime? instead of Nullable<DateTime> is a time saver...
• Use better indent of the ? expression like I did.

---

I have found this excellent explanations in Eric Lippert blog:

The specification for the ?: operator states the following:

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 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 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.

The compiler doesn't check what is the type that can "hold" those two types.

In this case:

• null and DateTime aren't the same type.
• null doesn't have an implicit conversion to DateTime
• DateTime doesn't have an implicit conversion to null

So we end up with a compile-time error.