Why can't I use a ternary operator with this expression? [duplicate]

问题

This question already has answers here:

Nullable types and the ternary operator: why is `? 10 : null` forbidden? [duplicate] (9 answers)

Conditional operator cannot cast implicitly? (3 answers)

Closed 6 years ago.

var dict = new Dictionary<string, object>();
DateTime? myDate;

/*Next line gives: Type of conditional expression cannot be 
determined because there is no implicit conversion between 'System.DateTime?' 
and 'System.DBNull' */

dict.Add("breakit", myDate.HasValue ? myDate.Value : DBNull.Value);

I don't understand why there needs to be an implicit conversion if one or the other is going into a dictionary expecting type Object.

回答1:

In C#, every conditional expression must have a type. What type is your expression of?

I understand your concern, the conversion is not needed for your particular case, but this is how C# compiler works, so you have to obey its rules.

This should work instead (I didn't check though):

dict.Add("breakit", myDate.HasValue ? (object)myDate.Value : (object)DBNull.Value);

回答2:

Did you try:

DateTime? date = myDate.HasValue ? myDate.Value : null;

dict.Add("breakit", date);

来源：https://stackoverflow.com/questions/6507715/why-cant-i-use-a-ternary-operator-with-this-expression