TimeOnly formats milliseconds wrong with format 'o'

[devsko]

It seems that with format 'o' TimeOnly.TryFormat() always fills the whole buffer after the decimal point with '0' and writes 7 decimal places right aligned at the end of the buffer. Regardless it always sets charsWritten = 16.

        static void Main(string[] args)
        {
            Span<char> buffer = new char[22];
            var time = new TimeOnly(1, 1, 1, 123);

            time.TryFormat(buffer, out int written, "O");
            
            Console.WriteLine('"' + buffer.Slice(0, written).ToString() + '"');
            // => "01:01:01.0000001" instead of "01:01:01.1230000"
            
            Console.WriteLine('"' + buffer.ToString() + '"');
            // => "01:01:01.0000001230000" TryFormat writes more than charsWritten characters

            Console.WriteLine('"' + time.ToString("O") + '"');
            // => "01:01:01.1230000" correct

            Console.WriteLine(string.Format("\"{0:O}\"", time)); 
            // => "01:01:01.0000000" no milliseconds at all (they are far to the right)
        }

I think the bug is here

runtime/src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeFormat.cs

Line 1268 in 94c13d0

WriteDigits((uint)fraction, destination.Slice(9));

It should be

 WriteDigits((uint)fraction, destination.Slice(9, 7));

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[Clockwork-Muse]

.... I think I'm more surprised that ToString() appears to have its own codepath here...

[ghost]

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Issue Details

---

It seems that with format 'o' TimeOnly.TryFormat() always fills the whole buffer after the decimal point with '0' and writes 7 decimal places right aligned at the end of the buffer. Regardless it always sets charsWritten = 16.

        static void Main(string[] args)
        {
            Span<char> buffer = new char[22];
            var time = new TimeOnly(1, 1, 1, 123);

            time.TryFormat(buffer, out int written, "O");
            
            Console.WriteLine('"' + buffer.Slice(0, written).ToString() + '"');
            // => "01:01:01.0000001" instead of "01:01:01.1230000"
            
            Console.WriteLine('"' + buffer.ToString() + '"');
            // => "01:01:01.0000001230000" TryFormat writes more than charsWritten characters

            Console.WriteLine('"' + time.ToString("O") + '"');
            // => "01:01:01.1230000" correct

            Console.WriteLine(string.Format("\"{0:O}\"", time)); 
            // => "01:01:01.0000000" no milliseconds at all (they are far to the right)
        }

I think the bug is here

runtime/src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeFormat.cs

Line 1268 in 94c13d0

WriteDigits((uint)fraction, destination.Slice(9));

It should be

 WriteDigits((uint)fraction, destination.Slice(9, 7));

Author:	devsko
Assignees:	-
Labels:	area-System.Runtime, in pr, untriaged
Milestone:	-