DateOnly string interpolation causes OOM crash

[tgrunshaw]

Description

Using string interpolation with DateOnly causes the runtime to crash with invalid string formats, such as below:

var dateOnly = new DateOnly(2020, 05, 03);
var dateOnlyString = $"{dateOnly:yyyy:MM:dd}"; // We never proceed beyond this line

See an example: https://dotnetfiddle.net/jLN3uv

Reproduction Steps

var dateOnly = new DateOnly(2020, 05, 03);
var dateOnlyString = $"{dateOnly:yyyy:MM:dd}"; // We never proceed beyond this line

See an example: https://dotnetfiddle.net/jLN3uv

Expected behavior

Not have the runtime crash/stick/run out of memory.

Whether this should throw an exception like it does when .ToString() is used or whether it should behave like DateTime where it works correctly is a decision I'll leave to the .NET team.

Actual behavior

Out of memory failure or stuck with high GC when debugging.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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.

[jcouv]

The C# compiler emits:

        DateOnly dateOnly = new DateOnly(2020, 5, 3);
        string.Format("{0:yyyy:MM:dd}", dateOnly);

That seems correct (and the IL too). Here's the relevant section of the spec.

Using yyyy-MM-dd or yyyy\\:MM\\:dd work as expected. So the problem has to do with the colons : (not escaped with \) in the format specification.
I think it's a BCL issue.

[KeterSCP]

It falls into infinite loop here because TryFormat returns false from this check. So it will also go into an infinite loop with following format characters: t, f, F, h, H, m, s, z, K

[stephentoub]

The format string used here is incorrect for a DateOnly. You can see that if you change the repro to use ToString:

var dateOnly = new DateOnly(2020, 05, 03);
Console.WriteLine(dateOnly.ToString("yyyy:MM:dd"));

as that will result in:

Unhandled exception. System.FormatException: Input string was not in a correct format.
   at System.DateTimeFormat.IsValidCustomDateFormat(ReadOnlySpan`1 format, Boolean throwOnError)
   at System.DateOnly.ToString(String format, IFormatProvider provider)
   at Program.<Main>$(String[] args)

The problem is that DateOnly.TryFormat was erroneously returning false rather than throwing when passed an invalid format string. This was fixed for .NET 7 in #64398 and for the next servicing release of .NET 6.0.3 in #64460.

[tgrunshaw]

Perfect, thanks for the explanation.