带有表达式的属性中使用的 Nullable DateTime 返回意外的默认值

Question

I have the following two method extensions:

public static class DateTimeConverter
{
    public static DateTime? Convert(this DateTime? time) =>
        time != null
        ? new DateTime(time.Value.Ticks) :
        default;
}

And the other one:

public static class DateTimeConverterExplicit
{
    public static DateTime? Convert(this DateTime? time) =>
        time != null
        ? new DateTime(time.Value.Ticks) :
        (DateTime ?)default;
}

When running the following code:

DateTime? dateTime = default;

var first = DateTimeConverter.Convert(dateTime);
var second = DateTimeConverterExplicit.Convert(dateTime);

Console.WriteLine($"First: {first}");
Console.WriteLine($"Second: {second}");

I get the following output:

First: 1-1-0001 00:00:00
Second:

I wonder why C# thinks it should return the default value of DateTime when using the first DateTimeConverter (and not the default value of Nullable<DateTime> ). When I explicit cast it to a DateTime? the default value is as expected.

When I re-write the first one as follows:

public static DateTime? Convert(this DateTime? time)
{
    if (time != null)
    {
        return new DateTime(time.Value.Ticks);
    }
    else
    {
        return default;
    }
}

It also works as expected. So somehow C# is not able to determine the right type. Does anyone knows why this is happening?

Answer 1

The rule for types used in ternary expression is that both sub-expressions must be the same type, OR one of the types must be a legal target type of the other.

In the first method, your two sub-expressions are DateTime , not DateTime? . default here picks up the type from the other sub-expression and thus becomes DateTime , it does not pick up the type from the target, the return statement and thus the method return type.

Instead, the ternary expression is evaluated, to a DateTime , and then this value is silently promoted to a DateTime? , but then the damage has already been done.

In your last example, with two return statements, default picks up the required type from the method since it is being used in a return statement, and becomes DateTime? , whereas in the other return statement, the DateTime value is again silently promoted to DateTime? .

Answer 2

In addition to Lasse's answer I just wanted to share you how you could have looked it up yourself in Visual Studio.

In your first example you could write default(DateTime) instead of only default and than notice that Visual Studio will tell you that the DateTime can be omiited and you should only write default .

but when you write default(DateTime?) Visual Studio will not produce an hint.

This tells you that only writing default produces a DateTime object and not DateTime? .

Or you could have simply mouse overed default keyword and it would provide a tooltip that tells what it represents a System.DateTime , and not a System.Nullable<T> where T is DateTime .

And now that you know that default results in a DateTime it should be obvious, IMHO, why it displays the MinValue instead of null .