String Parsing Performance

The original version of this package used the simplified string parsing approach - it just reuses the default System.DateTime parsing logic as is. It works well in most use cases but parsing behavior was not matched 100% to the .NET 6 System.DateOnly and System.TimeOnly data types. Their raise the System.FormatException exception in case if the input string contains the time part for the System.DateOnly or the date part in the case of System.TimeOnly types.

Unfortunately, this behavior in .NET 6 implementation depends heavily on an internal parser that provides a lot more information about parsing results than a public version of parsing API. Porting this parser required porting a lot of culture-specific code and looks unpractical for such a simple package. So another approach selected for solving this issue - double parsing using System.DateTime.TryParseExact method with proper formatting strings.

This approach, of course, has an obvious disadvantage - in some cases, we have to parse the same string twice. If performance is critical for you and you are OK with the incorrect parsing results in some rare cases you can restore the original behavior using the application context switch named Portable.System.DateTimeOnly.UseFastParsingLogic. Check the FastStrictParsingLogicTests for more examples for this issue.