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

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?

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.

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

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:

The documentation is quite clear on this:
The particular date and time symbols and strings used in input are defined by the formatProvider parameter...

Convert to DateTime from a string '10-April-2020'

Looking for assistance in converting a date string i receive from a web form, where the format will be something like "10-April-2020". I need to save this into the database in the US date format "yyyy-mm-dd" so that the example date provided would go in as '2020-04-10'.
This is what I have so far, which complains that it is not a valid datetime.
string LicenseExpiry = LicenseExpiry.Text;
IFormatProvider culture = new CultureInfo("en-US", true);
DateTime dateExpires = DateTime.ParseExact(LicenseExpiry, "yyyy-MM-dd", culture);
I have also tried the following which also fails.
DateTime dateExpires;
string LicenseExpiry = LicenseExpiry.Text;
IFormatProvider culture = new CultureInfo("en-US", true);
if (DateTime.TryParseExact(LicenseExpiry, "yyyy-MM-dd", culture, DateTimeStyles.None, out dateExpires))
{
// Do something
}
Can anyone help with either of the attempts to see what went wrong? I am not allowed to change the Ui/Form to do any client side date manipulation either, and so my solution needs to be done in the C# code behind file.

MM means the month number (from 01 through 12)
To parse 10-April-2020, you
need MMMM, see
Custom date and time format strings
The "MMMM" custom format specifier represents the full name of the month

Particular DateTime exception

I need to convert to DateTime some strings with this format "MMM-yy". I'm working with the culture "{es-ES}".
It works fine with all month except with March (In spanish Marzo).
This throws me this exception:
'Convert.ToDateTime("Mar-13")' threw an exception of type
'System.FormatException' System.DateTime {System.FormatException}
I've tried:
string format = "yyyyMM";
DateTime result;
CultureInfo provider = CultureInfo.InvariantCulture;
result = DateTime.ParseExact("Mar-13", format, provider);
and this:
DateTime date = Convert.ToDateTime("Mar-13");
This works fine for example with:
"Jun-13"
"Feb-13"
"Nov-13"
...
EDIT
The real problem is with:
DateTime date = Convert.ToDateTime("Ene-13"); -> ok
DateTime date = Convert.ToDateTime("Feb-13"); -> ok
DateTime date = Convert.ToDateTime("Mar-13"); -> crash
DateTime date = Convert.ToDateTime("Abr-13"); -> ok
....

Your date string "Mar-13" doesn't match your format "yyyyMM". Your format should be MMM-yy.
You should see: Custom Date and Time Format Strings
In your format
"MMM" - The abbreviated name of the month.
"yy" - The year, from 00 to 99.
EDIT:
For your question why Convert.ToDateTime("Mar-13"); is failing. You need to look at the following lines of code:
var currentCulture = new CultureInfo("es-ES");
var monthNames = currentCulture.DateTimeFormat.AbbreviatedMonthNames;
var dayOfWeeks = currentCulture.DateTimeFormat.AbbreviatedDayNames;
If you watch the returned values in debugger, You will see that for the culture es-ES there is a match between Month Name and Day name and that is on mar.
Marzo/March as Month
Martes/Tuesday as day
Both of these uses the same abbreviation i.e. Mar. Since Convert.ToDateTime would try to use possible formats for the string it fails to recognize Mar as Month or Day Name. That is why you get exception.
It is always a good idea to use DateTime.ParseExact and specify a single or multiple possible formats.

Your format and value don't match. Try this instead:
string format = "MMM-yy";
DateTime result;
CultureInfo provider = CultureInfo.InvariantCulture;
result = DateTime.ParseExact("Mar-13", format, provider);
EDIT
For the es-ES format provider, my guess is that Mar is ambiguous between Marzo (March) and Martes (Tuesday). You should be fine if you use ParseExact with the proper format:
string format = "MMM-yy";
DateTime result;
CultureInfo provider = CultureInfo.GetCultureInfo("es-ES");
// fails
result = DateTime.Parse("Mar-13", provider);
// works
result = DateTime.ParseExact("Mar-13", format, provider);
result = DateTime.ParseExact("Abr-13", format, provider);
EDIT 2
This appears to be a known bug. Their workaround is to use a similar culture, however if your date format is known, I'd recommend using ParseExact to explicitly define the format rather than letting the framework try and infer the format.

Reference to my answer about logic behind automatic DateTime parsing: How Convert.ToDateTime() parses a given string when the given culture does not know the format
You can automatically parse Abr-13 in es-ES culture, because Abr can be matched only as MonthToken. But in case of Mar-13 - Mar can be matched as MonthToken and also it can be matched as DayOfWeekToken (Tuesday), so DateTime.Parse/Convert.ToDateTime methods are confused and throw exception.
If you execute the code from referenced answer against es-ES culture, you would see the following in the output:
mar DayOfWeekToken 2
Mar MonthToken 3
There are no multiple matches for full months names though, so you can safely parse 'marzo', 'abril' values.
Since DateTime.Parse/Convert.ToDateTime methods are confused by this duality of Mar value, we need to provide a hint to it using DateTime.ParseExact method:
DateTime output = DateTime.ParseExact("Mar-13", "MMM-yy", CultureInfo.GetCultureInfo("es-ES"));

I think you need to use the correct CultureInfo and fix the format string
CultureInfo provider = new CultureInfo("ES-es");
DateTime result = DateTime.ParseExact("Abr-13", "MMM-yy", provider);

You need to use the Spanish culture info in association with the "MMM-yy" format :
string format = "MMM-yy";
DateTime result;
CultureInfo provider = new CultureInfo("es-ES");
result = DateTime.ParseExact("mar-13", format, provider);
result = DateTime.ParseExact("abr-13", format, provider);

On my system the current culture is en-US the following code will fail when trying to convert abril (April):
string format = "MMM-yy";
DateTime result;
CultureInfo provider = CultureInfo.InvariantCulture;
result = DateTime.ParseExact("abr-13", format, provider);
Therefore on my system I have to create a Spanish culture for the code to work:
string format = "MMM-yy";
DateTime result;
CultureInfo provider = CultureInfo.CreateSpecificCulture("es-ES");
result = DateTime.ParseExact("abr-13", format, provider);
So you need to be aware what culture your code is running on and take the proper action.

String was not recognized as a valid DateTime when parse exact

I get the following exception when converting to DateTime:
String was not recognized as a valid DateTime.
lbl_RequestDate.Text = "13/2/2013";
CultureInfo provider = CultureInfo.CurrentCulture;
string[] format = provider.DateTimeFormat.GetAllDateTimePatterns();
Follow.RequestDate = DateTime.ParseExact(lbl_RequestDate.Text, format, provider, DateTimeStyles.None);

You can use thje format d/M/yyyy, Notice the single M used for the month.
Follow.RequestDate = DateTime.ParseExact(lbl_RequestDate.Text, "d/M/yyyy", provider, DateTimeStyles.None);
The method: provider.DateTimeFormat.GetAllDateTimePatterns() returns almost 155 formats, but none of them (from your current culture seems to) supports format d/M/yyyy that is why you are getting the exception. If your date has Month as 13/02/2013 then the formats returned by the method would work since the closest format is dd/MM/yyyy in the formats array.

Maybe this will help :
DateTime.ParseExact("13/2/2013","d/M/yyyy",CultureInfo.GetCultureInfo("en-US"), DateTimeStyles.None );
notice :
d is for Day (01 is also acceptable)
M is for Month (11 is also acceptable)

Try it like this:
Follow.RequestDate = DateTime.ParseExact(lbl_RequestDate.Text, "d/M/yyyy", CultureInfo.InvariantCulture);

DateTimeFormatInfo.GetAllDateTimePatterns() method returns on my machine (tr-TR Culture) 29 format but none of these support d/M/yyyy date format, that's why you are getting FormatException.
But in my culture DateSeparator is . so I can't exactly solve this problem using CultureInfo.CurrentCulture but when I use Egypt cultureinfo (it's wrote on your profile) CultureInfo.GetCultureInfo("ar-EG") this code works without any error;
CultureInfo provider = CultureInfo.GetCultureInfo("ar-EG");
string[] format = provider.DateTimeFormat.GetAllDateTimePatterns();
DateTime d = DateTime.ParseExact("13/02/2013", format, provider, DateTimeStyles.None);
Unfortunatly your your all datetime pattern doesn't support d/M/yyyy format.
Unfortunatly, changing this string to 13/02/2013 doesn't solve this problem because as I said on before, my all formats (in tr-TR Culture) doesn't support dd/MM/yyyy format either.
My humble advice is here, list all your datetime patterns and check manually if your string is recognized format with this datetime pattern like;
string[] format = provider.DateTimeFormat.GetAllDateTimePatterns();
foreach (var f in format)
{
///
}

datetime.tostring month and day language

i have a list of email addresses of people that have different nationalities (for each person i have the iso code)
when i send the email to all these people, in the text of the mail i need to to convert a datetime field to a string formatted in their specific culture.
for this i'm doing
CultureInfo ci = new CultureInfo(ISO);
myStringDate = myDate.ToString(ci.DateTimeFormat.ShortDatePattern);
and work perfect, but if i use LongDatePattern instead short, for displaying date like "Monday, 13 June 2010" its work fine except the language of the day and month.
if the person culture is it-IT i need to display "Martedi" and "Giugno" not "monday" and "June"
how can i do that without change the current UI culture?

Those patterns describe year, month, day and other parameter locations in the output result of DateTime. But month and day names are taken from the CultureInfo object, not pattern. There's the DateTime.ToString() overload that supports passing the CultureInfo parameter along with the format.
CultureInfo culture = new CultureInfo(ISO);
DateTime.Now.ToString(culture.DateTimeFormat.LongDatePattern, culture);
This way, .ToString() will respect both pattern and names from specified culture

You have only specified the format pattern, so it takes the other settings from the default format. You should also specify the format provider:
myStringDate = myDate.ToString(ci.DateTimeFormat.LongDatePattern, ci);
or using the standard format string D for long date pattern (as it will take the actual pattern from the format provider that you specify):
myStringDate = myDate.ToString("D", ci);
Demo:
CultureInfo ci = new CultureInfo("it-IT");
Console.WriteLine(DateTime.Now.ToString("D", ci));
ci = new CultureInfo("sv-SE");
Console.WriteLine(DateTime.Now.ToString("D", ci));
Output:
giovedì 26 gennaio 2012
den 26 januari 2012

I just had a similar problem and solved it with:
DateTime.Now.ToString("dddd, dd MMMM yyyy", new System.Globalization.CultureInfo("it-IT"));
That was thanks to this link.

Use DateTimeFormatInfo class.
string code = "mk-MK";
DateTimeFormatInfo info =
DateTimeFormatInfo.GetInstance(CultureInfo.GetCultureInfo(code));
string longDate =
DateTime.Now.ToString(info.LongDatePattern, new System.Globalization.CultureInfo(code));

DateTime.ToString() has yet another overload with which you can specify both the pattern you want (as you are currently and the culture to use. Try:
myStringDate = myDate.ToString(ci.DateTimeFormat.LongDatePattern,
ci);