How to convert System.DateTime? to System.DateOnly?

Question

How can I convert nullable DateTime to nullable DateOnly? So DateTime? to DateOnly?

The error says:

Error CS0029 Cannot implicitly convert type 'System.DateOnly?' to 'System.DateTime?'

I can convert from DateTime to DateOnly by doing:

DateOnly mydate = DateOnly.FromDateTime(mydatetime);

but what about nullables?

I have a way but I don't think is the best idea...

Heinzi · Accepted Answer · 2022-05-04T20:38:39.893

Let's make an method that does exactly the same as FromDateTime, just invoked as an extension on DateTime:

public static DateOnly ToDateOnly(this DateTime datetime) 
    => DateOnly.FromDateTime(datetime);

Now you can use the null-conditional member access operator ?. to lift this method to its nullable version:

var myNullableDateOnly = myNullableDateTime?.ToDateOnly();

Unfortunately, C# has no "null-conditional static method call operator". Thus, we need this "extension method workaround".

score 2 · Answer 2 · answered May 04 '22 at 20:22

2

You can create a DateTime extension methods:

public static class DateTimeExtends
{
    public static DateOnly ToDateOnly(this DateTime date)
    {
        return new DateOnly(date.Year, date.Month, date.Day);
    }

    public static DateOnly? ToDateOnly(this DateTime? date)
    {
        return date != null ? (DateOnly?)date.Value.ToDateOnly() : null;
    }
}

And use on any DateTime instance:

DateOnly date = DateTime.Now.ToDateOnly();

NOTE: Not tested, maybe have a tipo error...

answered May 04 '22 at 20:22

Victor

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I like the approach in one line. ToDateOnly is already covered – Leandro Bardelli May 04 '22 at 20:24
The first method is losing the TimeZone info, and the second does unnecessary casting. Downvoted. – funatparties May 04 '22 at 20:27
@funatparties It's has been written from my smartphone (NOTE: Not tested...). I can't do any better. Best regards. – Victor May 04 '22 at 20:46

score 2 · Answer 3 · answered May 04 '22 at 20:24

2

public static DateOnly? ToNullableDateOnly(this DateTime? input) 
{
    if (input == null) return null;
    return DateOnly.FromDateTime(input.Value);
}

answered May 04 '22 at 20:24

Joel Coehoorn

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I think something like this in my answer. There is no other way? No function inside the framework to do this? – Leandro Bardelli May 04 '22 at 20:25
1

It's similar, but extends yours to work as an extension (so it looks like it's part of the type), and with less code and fewer operations (the `??` expands into something much larger that isn't needed). – Joel Coehoorn May 04 '22 at 20:27
Oh sorry didn't see the change. Thanks indeed! – Leandro Bardelli May 04 '22 at 20:28

Leandro Bardelli · Answer 4 · 2022-05-04T20:17:44.303

0

I just create this method, if someone has another one better I'll glad to accept it as answer

    public DateOnly? NullableDateTime_To_NullableDateOnly(DateTime? input) {
        if (input != null)
        {
            return DateOnly.FromDateTime(input ?? DateTime.Now); //this is irrelevant but...
        }
        return null;
    }

edited May 04 '22 at 20:17

answered May 04 '22 at 20:08

Leandro Bardelli

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2

dateOnly = dateTime.HasValue ? DateOnly.FromDateTime(dateTime.Value) : null; – funatparties May 04 '22 at 20:24

How to convert System.DateTime? to System.DateOnly?

4 Answers4