Nullable type issue with ?: Conditional Operator

c# generics nullable conditional-operator
Nick Gotch picture Nick Gotch · Nov 17, 2008 · Viewed 57.4k times · Source

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

Stewart Johnson picture Stewart Johnson · Nov 17, 2008

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.

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