What's the purpose of the format provider in ParseExact() of DateTime and DateTimeOffset?

Question

Let us consider DateTimeOffset.ParseExact

public static DateTime ParseExact (
    string s, 
    string format, 
    IFormatProvider? provider, 
    System.Globalization.DateTimeStyles style
);

The expected format is already described via the format parameter, so what's the purpose of the providerparameter? Can anyone please give an example how different provider values can lead to different results?

Answer 1

The meaning of various elements of a custom datetime format string depend on the format provider.

Example:

var aString = "12 avr. 2021";
var gbCulture = new CultureInfo("en-GB");
var frCulture = new CultureInfo("fr-FR");

var canParseGb = 
    DateTime.TryParseExact(aString, "dd MMM yyyy", gbCulture, DateTimeStyles.None, out var gbDateTime);
var canParseFr = 
    DateTime.TryParseExact(aString, "dd MMM yyyy", frCulture, DateTimeStyles.None, out var frDateTime);

Same input string s, same format string format, different providers => different results.

Answer 2

Different cultures will use different values for things such as month names and days of the week. Jul might be July to you, but in France it's not. For example:

var frenchDate = DateTimeOffset.ParseExact("1 févr. 2020", "d MMM yyyy", 
    CultureInfo.GetCultureInfo("fr-FR"));

var englishDate = DateTimeOffset.ParseExact("1 Feb 2020", "d MMM yyyy", 
    CultureInfo.GetCultureInfo("en-GB"));

All of these details can be seen in the CultureInfo object, for example fr-FR looks like this:

Answer 3

The documentation is quite clear on this:

The particular date and time symbols and strings used in input are defined by the formatProvider parameter...

Answer 4

For example CultureInfo implements IFormatProvider. So you can pass a culture which has it's own formatting rules apart from your provided format string.

Also note that the format string can contain specifiers that will behave differently with different cultures/format-providers like the "/" custom format specifier:

CultureInfo deCulture = new CultureInfo("de-DE"); // german
CultureInfo frCulture = new CultureInfo("fr-FR"); // french
// works because / is replaced with the passed culture's date-separator which is . in germany
DateTime dateDe = DateTime.ParseExact("25.02.2021", "dd/MM/yyyy", deCulture, DateTimeStyles.None); 
// works not since french use / as date-separator
DateTime dateFr = DateTime.ParseExact("25.02.2021", "dd/MM/yyyy", frCulture, DateTimeStyles.None); 

As Magnetron has commented the same applies to the ":" custom time format specifier