Type result with conditional operator in C#

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The compiler does not infer the type of the result of the conditional operator from the usage of the result, but from the types of its arguments. The compiler fails when it sees this expression because it cannot deduce the type of the result:

IsDateTimeHappy(myDateTime) ? null : myDateTime;

Since null and DateTime are not compatible, you need to tell the compiler what the type should be. A cast should do the trick:

DateTime? x = IsDateTimeHappy(myDateTime) ? (DateTime?)null : myDateTime;
OutputDateTime(x);

Now the compiler will have no problems. You can also write the above on one line if you prefer (but I would probably not do this):

OutputDateTime(IsDateTimeHappy(myDateTime) ? (DateTime?)null : myDateTime);

Eric Lippert has a good answer that is also relevant here and goes into more details about how the compiler determines types.

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