Is there a way to specify ToUpper or ToLower when formatting data in an interpolated string?

[tacosontitan]

I want to print today's date in the format d MMM yyyy and it's easy to do with interpolation:

var output = $"Today is {DateTime.Now:d MMM yyyy}.";
Console.WriteLine(output);

Assuming today's date is January 1st, 2024, the expected output of the above snippet is:

Today is 1 Jan 2024.

However, I have an added requirement that I want to display this in uppercase, but that currently (to my knowledge) requires manual invocations of ToString and ToUpper to achieve:

var output = $"Today is {DateTime.Now.ToString("d MMM yyyy".ToUpper()}.";
Console.WriteLine(output);

This achieves the desired effect, but I can't help but feel like this is something that would have support without those manual invocations. Is there a way to specify ToUpper or ToLower when formatting data in an interpolated string?

Important

I'm not looking to format the entire output variable, simply the date within it; this makes the desired output:

Today is 1 JAN 2024.

[danmoseley]

Are you trying to avoid allocations? Ie this path is very hot.

[tacosontitan]

In this particular case, yes, but I've had this exact desire on more than a handful of occasions some hot, some not.

[Joe4evr]

Looks like you could create an instance of DateTimeFormatInfo where you set AbbreviatedMonthNames to the desired values, and pass that along with the format string. Maybe look at some documentation for specifics because it looks like you must set an array of exactly 13 elements (with index 0 null?)

[Clockwork-Muse]

.... Sure, but if it's for display it's bug-for-bug compatible behavior.

There shouldn't normally be a reason to customize the output this way, since this ignores any potential user format settings.
In the rare case that this may be some sort of spreadsheet program, then I really hope they're doing some sort of culture merging for driving display values with custom formats ....

[Joe4evr]

There shouldn't normally be a reason to customize the output this way, since this ignores any potential user format settings.

Here's a first-hand experience reason: As of the start of this year, my workplace mandated the same format for any date placed in documents sent to customers in order to be unambiguous all around the world. (I'm personally less affected because I don't interact with the company's customers, but it was something everyone had to be notified of.)

OP's "added requirement" is likely similar.

[tacosontitan]

While I appreciate the feedback to be culture cognizant, this is a case where I've done what I can to sway the decision and just have to implement it at this point, and my current solution works fine, it just seems redundant.

That being said, my question is likely too targeted on DateTime consequently causing people to treat it as an X/Y problem. The real question here is if casing can be specified when formatting data in an interpolated string.

[Joe4evr]

The real question here is if casing can be specified when formatting data in an interpolated string.

That's a no, sadly. My other answer would work for anything ISpanFormattable if you're willing to change from interpolation to like a StringBuilder or something, or just live with the intermediate allocation.

[Joe4evr]

Another option (which is a bit more involved since I believe it doesn't work nicely with string interpolation) would be:

• Create a Span<char> buffer that fits your format (looks like 11 chars should be sufficient, so this could even be stackalloced)
• Fill the buffer using TryFormat
• Use ToUpper() on the span which I believe does the conversion in-place

Then just pick whatever method you like to copy the contents of the buffer into your final string (the real minimal-alloc way is String.Create, but ultimately it's your choice).

[TrayanZapryanov]

@tacosontitan check my question

You can use the same approach to build ToUpper for every object and to use it in interpolated strings.