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In the First overload of ParseExact method 

public static DateTime ParseExact (string s, string format, IFormatProvider provider);

according to Microsoft:

If format is a custom format pattern that does not include date or time separators (such as "yyyyMMddHHmm"), use the invariant culture for the provider parameter and the widest form of each custom format specifier. For example, if you want to specify hours in the format pattern, specify the wider form, "HH", instead of the narrower form, "H".

In particular if we use stander format pattern we could use any other cultures

what is the really purpose of using invariant culture and widest custom specifier if we use custom format pattern that does not include date or time separators if we use?

Ammar
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  • For starters, format patterns like `MM` are still culture-specific. Per [the docs](https://docs.microsoft.com/dotnet/standard/base-types/custom-date-and-time-format-strings#the-mm-custom-format-specifier-1): "The "MM" custom format specifier represents the month as a number from 01 through 12 (or from 1 through 13 for calendars that have 13 months)." To exclude all such variations, being explicit about the invariant culture helps. (Also, several code paths in the framework use optimized variants if the invariant culture is used, though I don't know if `DateTime.ParseExact` is one of them.) – Jeroen Mostert Aug 13 '18 at 10:56

2 Answers2

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The purpose of InvariantCulture is to have a well-known way to format dates and numbers, that does not depend on the system or user locale.

You should use it every time you format something that is not meant to be parsed by humans. For example in a JSON or XML file you want to store the date in ISO format so there is no ambiguity. On the other hand if you display the date on the screen, you generally respect the user's choice of culture and display it in the preferred way.

If you use a custom format, then it would surely be good if the resulting string can be parsed back into the exact same date and time. In order to do that without separators you have to use fixed length strings for each component.

// omitting CultureInfo.InvariantCulture for brevity
var dt = new DateTime(2018,1,2,3,45,6);
dt.ToString("yyyyMMddHHmmss") // returns "20180102034506"
dt.ToString("yyyyMdHms")      // returns "2018123456"

You can easily see that the second one is not unique, i.e. there are other dates which will return the same string. In order to avoid that you use the wider form of each component.

Dirk
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the real purpose of invariant culture is taken taken from a an answer in this question, as it describes it's purpose best:

Not all cultures use the same format for dates and decimal / currency values.

This will matter for you when you are converting input values (read) that are stored as strings to DateTime, float, double or decimal. It will also matter if you try to format the aforementioned data types to strings (write) for display or storage.

If you know what specific culture that your dates and decimal / currency values will be in ahead of time, you can use that specific CultureInfo property (i.e. CultureInfo("en-GB")). For example if you expect a user input.

The CultureInfo.InvariantCulture property is used if you are formatting or parsing a string that should be parseable by a piece of software independent of the user's local settings.

To sum this up, the invariant culture will help with conversions be a string stored to float, decimal or asDateTime, it is here also to help when you are trying to format or parse a string that should be parseable by a piece of software independent of the user's local settings, as the quote said and that is what

widest custom specifier

means by that.

Barr J
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