可空-如何在C＃中的DateTime类型中仅比较没有日期的Date？

Question

如何在C＃中的DateTime类型中仅比较没有时间的日期，其中一个日期可以为空，我该怎么做？

Answer 1

DateTime val1;
DateTime? val2;

if (!val2.HasValue)
    return false;

return val1.Date == val2.Value.Date;

Answer 2

You can use the Date property of DateTime object

Datetime x;
Datetime? y;

if (y != null && y.HasValue && x.Date == y.Value.Date)
{
 //DoSomething
}

Answer 3

This uses short-circuit to avoid comparisons if nullable date is null, then uses the DateTime.Date property to determine equivalency.

bool Comparison(DateTime? nullableDate, DateTime aDate) {
    if(nullableDate != null && aDate.Date == nullableDate.Value.Date) {
        return true;
    }

    return false;
}

Answer 4

bool DatesMatch(DateTime referenceDate, DateTime? nullableDate)
{
    return (nullableDate.HasValue) ? 
        referenceDate.Date == nullableDate.Value.Date : 
        false;
}

Answer 5

If you want a true comparison, you can use:

    Datetime dateTime1
    Datetime? dateTime2

    if(dateTime2.Date != null)
       dateTime1.Date.CompareTo(dateTime2.Date);

Hope it helps...

Answer 6

The only challenging aspect here is the fact that you want something which is both DateTime and nullable.

Here is the solution for standard DateTime: How to compare only Date without Time in DateTime types in C#?

if(dtOne.Date == dtTwo.Date)

For nullable, it's just a matter of choice. I would choose an extension method.

class Program
{
    static void Main(string[] args)
    {
        var d1 = new DateTime(2000, 01, 01, 12, 24, 48);
        DateTime? d2 = new DateTime(2000, 01, 01, 07, 29, 31);

        Console.WriteLine((d1.Date == ((DateTime)d2).Date));

        Console.WriteLine((d1.CompareDate(d2)));
        Console.WriteLine((d1.CompareDate(null)));

        Console.WriteLine("Press enter to continue...");
        Console.ReadLine();
    }
}

static class DateCompare
{
    public static bool CompareDate(this DateTime dtOne, DateTime? dtTwo)
    {
        if (dtTwo == null) return false;
        return (dtOne.Date == ((DateTime)dtTwo).Date);
    }
}

Answer 7

You could create a method like the one below, following the similar return values as the .net framework comparisons, giving -1 when the left side is smallest, 0 for equal dates, and +1 for right side smallest:

    private static int Compare(DateTime? firstDate, DateTime? secondDate)
    {
        if(!firstDate.HasValue && !secondDate.HasValue)
            return 0;
        if (!firstDate.HasValue)
            return -1;
        if (!secondDate.HasValue)
            return 1;
        else 
            return DateTime.Compare(firstDate.Value.Date, secondDate.Value.Date);
    }

Of Course a nicer implementation would be to create an extension method for this.