Bug in DateTime format handling for ".F", optional fractional seconds.

[MarkPflug]

Description

There is a bug in the handling of the DateTime custom format handler for the "F" (optional fractional seconds) format specifier. It only occurs when a single "F" is specified.

Reproduction Steps

using System.Globalization;

var dateTime = new DateTime(2025, 1, 2, 3, 4, 5);

var c = CultureInfo.InvariantCulture;
var f = "yyyy-MM-ddTHH:mm:ss.F";
var s = dateTime.ToString(f, c);//"2025-01-02T03:04:05"

// throws FormatException: 'String '2025-01-02T03:04:05' was not recognized as a valid DateTime.'
var r = DateTime.ParseExact(s, f, c, DateTimeStyles.None); 

Expected behavior

In the minimal repro, the ParseExact should "roundtrip" the input value.

Actual behavior

It throws a FormatException with the message 'String '2025-01-02T03:04:05' was not recognized as a valid DateTime.'

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

The bug appears to be in this block of code:

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

Lines 4458 to 4471 in 252034f

	if (!str.Match(ch))
	{
	if (format.GetNext())
	{
	// If we encounter the pattern ".F", and the dot is not present, it is an optional
	// second fraction and we can skip this format.
	if (format.Match('F'))
	{
	format.GetRepeatCount();
	break;
	}
	}
	result.SetBadDateTimeFailure();
	return false;

Line 4460 consumes the first F after the ., then line 4464 attempts to match a second F, which if not present results in the format exception from line 4470.

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[tarekgh]

@MarkPflug your analysis is correct. I have opened the PR #120518 to fix the issue. Thanks for your report.