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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...

Proper date formatting of string read from a SqlDataReader

I'm reading a Date field from a SqlDataReader and I want to write it to a label.
My code for this line looks like this:
lblTDate.Text = string.Format("{0:D}", (DT2(["TDate"].ToString()));
The label shows, for instance, "4/7/2016 12:00:00 AM" but I want it to show "Sunday, April 07, 2016".
I read on this site that using {0:D} should do it, but it's not working.
The field in the SQL Server table is a Date field, not a DateTime field, which makes it all the more baffling that the time is showing up. But, anyway, how can I get my label to show the date in the format I want to see?

Try this:
lblTDate.Text = Convert.ToDateTime(DT2["TDate"]).ToString("dddd, MMMM dd, yyyy", System.Globalization.CultureInfo.InvariantCulture);

You don't want to convert the object that is returned from the indexer to a string, before you hand it off to a String.Format. By leaving out the ToString call you would have got what you want.
lblTDate.Text = string.Format("{0:D}", DT2["TDate"]);
This enables the String.Format implementation to use IFormattable implementation of the DateTime type which makes that the format string you've found will work. Once it is a string there is no way for the String.Format implementation to know what to do.
Alternatively cast the value to the IFormattable interface, like so:
var fmt = DT2["creationdate"] as IFormattable;
if (fmt != null)
{
lblTDate.Text = fmt.ToString(
"dddd, MMMM dd, yyyy",
System.Globalization.CultureInfo.InvariantCulture);
}
but then you have to provide an IFormatProvider implementation, often in the form of an instance of a CultureInfo class.
Notice that in both of the above options, you don't have to to convert the value to a DateTime first.

CultureInfo culture = new CultureInfo("en-Us");
DateTimeFormatInfo dtfi = culture.DateTimeFormat;
var myDate = new DateTime(2016, 7, 4, 12, 0, 0);
string month = culture.TextInfo.ToTitleCase(dtfi.GetMonthName(myDate.Month));
string dayOfWeek = culture.TextInfo.ToTitleCase(dtfi.GetDayName(myDate.DayOfWeek));
string fullDate = $"{dayOfWeek}, {month} {myDate.Day}, {myDate.Year}";
Console.Write(fullDate);

Adding two dates to eachother

string final = Convert.ToString(DateTime.Parse(date, System.Globalization.CultureInfo.InvariantCulture) + TimeSpan.Parse(duration));
Hi, I use the above code to add two date's to eachother. It do work very well on Windows and returns the required format yyyy-MM-dd HH:mm:ss in a correct fashion. HOWEVER, when on Linux building with Mono it returns the following format dd/MM/yyyy HH:mm:ss which is not what I want.
How can I specify that I ONLY want the first formatting and nothing else? I tried playing around with ParseExact but it did not do very well. What I've heard ParseExact should not really be needed for this?
Here is a example of input:
string date = "2014-10-30 10:00:04"; // On windows
string duration = "05:02:10"; // duration to be added to date
Greetings.

Use ToString("yyyy-MM-dd HH:mm:ss") instead of Convert.ToString.
string date = "2014-10-30 10:00:04";
string duration = "05:02:10";
DateTime dt1 = DateTime.Parse(date, CultureInfo.InvariantCulture);
TimeSpan ts = TimeSpan.Parse(duration, CultureInfo.InvariantCulture);
DateTime dtFinal = dt1.Add(ts);
string final = dtFinal.ToString("yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture);
Convert.ToString uses your current culture's date separator, use CultureInfo.InvariantCulture.
Read: Custom Date and Time Format Strings

You can use the ToString() Method of the DateTime object.
var dt = DateTime.Now;
dt.ToString("yyyy-MM-dd HH:mm");

Using your code:
string _final = (DateTime.Parse(date, System.Globalization.CultureInfo.InvariantCulture) + TimeSpan.Parse(duration)).ToString("yyyy-MM-dd HH:mm:ss");

DateTime.ParseExact throws System.FormatException

Why this line of code sometimes throws System.FormatException?
DateTime d = DateTime.ParseExact("01.07.2014", "dd/MM/yyyy", CultureInfo.InvariantCulture);

Because your string and format doesn't match.
From documentation;
Converts the specified string representation of a date and time to its
DateTime equivalent using the specified format and culture-specific
format information. The format of the string representation must match
the specified format exactly.
Use dd.MM.yyyy format instead.
DateTime d = DateTime.ParseExact("01.07.2014",
"dd.MM.yyyy",
CultureInfo.InvariantCulture);
Here a demonstration.
Remember, "/" custom format specifier has a special meaning in custom date and time formats. It means as; replace me with the current culture date separator.
In your profile, it says you are from Azerbaijan. That means your CurrentCulture is probably az-Cyrl-AZ (Cyrillic, Azerbaijan) or az-Latn-AZ (Latin, Azerbaijan).
Actually, doesn't matter which culture you use on this case because both culture has . as a DateSeparator property.
That means your original code also works with your CurrentCulture.
DateTime d = DateTime.ParseExact("01.07.2014",
"dd/MM/yyyy",
CultureInfo.CurrentCulture);
// or you can use null
For more information, take a look;
Custom Date and Time Format Strings

You need a culture where "." is the DateSeparator, for example:
DateTime d = DateTime.ParseExact("01.07.2014", "dd/MM/yyyy",
CultureInfo.GetCultureInfo("az-Cyrl-AZ"));
if you are in Azerbaijan and use Azerbaijani language with the Cyrillic script.
You can use:
DateTime d = DateTime.ParseExact("01.07.2014", "dd/MM/yyyy",
null);
to just take the current culture.
Maybe you just need "d" instead of the verbose "dd/MM/yyyy", since the standard short date format in Azerbaijani is just like "01.07.2014".
The "invariant culture" uses "/" as its DateSeparator, so therefore you should not use it in your case.
Also, this works:
DateTime d = DateTime.ParseExact("01.07.2014", "dd/MM/yyyy",
new DateTimeFormatInfo { DateSeparator = ".", }
);
because new DateTimeFormatInfo() makes a read/write "invariant-culture" date/time info for which you can change the relevant property.

The / in the date format will match the date separator of the culture that you specify. If you use a culture that has period as date separator, the parsing will work.
Example:
DateTime d = DateTime.ParseExact("01.07.2014", "dd/MM/yyyy", CultureInfo.GetCultureInfo("de"));
You can also use a literal period instead of the date separator specificer, then it works with the invariant culture:
DateTime d = DateTime.ParseExact("01.07.2014", "dd.MM.yyyy", CultureInfo.InvariantCulture);
Ref: Custom Date and Time Format Strings

The format you have is different from the string provided:
Try either of the below, it will work :)
DateTime d1 = DateTime.ParseExact("01/07/2014", "dd/MM/yyyy", CultureInfo.InvariantCulture);
DateTime d2 = DateTime.ParseExact("01.07.2014", "dd.MM.yyyy", CultureInfo.InvariantCulture);

Problem:
Your separator in date is . while in string format it is /
Solution:
Your format should be "dd.MM.yyyy" or "MM.dd.yyyy" as your date is "01.07.2014". 01 and 07 exist both as date and month.
This date can be 01st July 2014 or 07 Jan 2014.
Your code should be
DateTime d = DateTime.ParseExact("01.07.2014",
"dd.MM.yyyy",
CultureInfo.InvariantCulture);
OR
DateTime d = DateTime.ParseExact("01.07.2014",
"MM.dd.yyyy",
CultureInfo.InvariantCulture);

Convert string to mm/dd/yyyy datetime

I'm trying to convert string which comes from textbox, for example in this format '03/24/2014' to DateTime. This is what I'm trying:
CultureInfo us = new CultureInfo("en-US");
dtAssemblyDate = DateTime.ParseExact(txtOperationSignatureDate.Value, "dd/MM/yyyy", us);
or
dtAssemblyDate = DateTime.ParseExact(txtOperationSignatureDate.Value, "dd/MM/yyyy", null);
But no luck and I'm getting exceptions that the value cannot be casted as DateTime. How can I fix this problem?

03/24/2014 isn't a valid date in dd/MM/yyyy format (there are only 12 months in a year1).
Either change your format string to MM/dd/yyyy or use a valid date in your chosen format.
1: Or 13 months in some types of Calendar, but "en-US" uses the 12-month Gregorian calendar.

DateTime myDate = DateTime.ParseExact("24/03/2014", "dd/MM/yyyy",
System.Globalization.CultureInfo.InvariantCulture);

03/24/2014 has the day of the month as the middle component. That might seem strange, but that's how it's done in some parts of the world (mostly Northern America).
Thus, when specifying the format for parsing, you also have to put the day of the month (dd) in the middle:
CultureInfo us = new CultureInfo("en-US");
dtAssemblyDate = DateTime.ParseExact(txtOperationSignatureDate.Value, "MM/dd/yyyy", us);
Obviously, it is not possible to parse a text field that accepts both middle-endian (MM/dd/yyyy) and small-endian (dd/MM/yyyy) dates, because ambiguities like 01/02/2014 cannot be resolved automatically.

If the string is expressed in the format MM/dd/yyyy then
CultureInfo us = new CultureInfo("en-US");
dtAssemblyDate = DateTime.ParseExact(txtOperationSignatureDate.Value, "MM/dd/yyyy", us);
but I prefer to use DateTime.TryParse to avoid surprises...
if(DateTime.TryParse(txtOperationSignatureDate.Value,
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out dtAssemblyDate))
Console.WriteLine(dtAssemblyDate.ToShortDateString());

CultureInfo us = new CultureInfo("en-US");
DateTime dt = Convert.ToDateTime(txtOperationSignatureDate.Value, us.DateTimeFormat);
Whatever DateTimeFormat you require, you just need to pass corresponding culture with it.

Try using
string date = textbox.Value;
DateTime dt = Convert.ToDateTime(date);