?: 条件运算符的可空类型问题

Question

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 1

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.

Answer 2

FYI (Offtopic, but nifty and related to nullable types) we have a handy operator just for nullable types called the null coalescing operator

??

Used like this:

// Left hand is the nullable type, righthand is default if the type is null.
Nullable<DateTime> foo;
DateTime value = foo ?? new DateTime(0);

Answer 3

这是因为在三元运算符中，两个值必须解析为相同的类型。

Answer 4

Another solution similar to the accepted is to use C#'s default keyword. While defined using generics, it is actually applicable to any type.

Example usage applied to the OP's question:

Nullable<DateTime> foo;
foo = true ? default(DateTime) : new DateTime(0);

Example usage with the current accepted answer:

DateTime? foo;
foo = true ? default(DateTime) : new DateTime(0);

Also, by using default , you do not need to specify the variable as nullable in order to assign it a null value. The compiler will auto-assign the specific variable-type's default value and no error will be encountered. Example:

DateTime foo;
foo = true ? default(DateTime) : new DateTime(0);

Answer 5

I know this question was asked in 2008 and it is now 5 years later but the answer marked as an answer does not satisfy me. The real answer is that DateTime is a struct, and as a struct it is not compatible with null. You have two ways of solving that:

First is to make null compatible with DateTime (for instance, cast null to DateTime? as the gentleman with 70 upvotes suggests, or cast null to Object or to ValueType).

The second is to make the DateTime compatible with null (for instance, cast DateTime to DateTime?).