What does IFormatProvider do?

In adition to Ian Boyd's answer:

Also CultureInfo implements this interface and can be used in your case. So you could parse a French date string for example; you could use

var ci = new CultureInfo("fr-FR");
DateTime dt = DateTime.ParseExact(yourDateInputString, yourFormatString, ci);

The IFormatProvider interface is normally implemented for you by a CultureInfo class, e.g.:

• CultureInfo.CurrentCulture
• CultureInfo.CurrentUICulture
• CultureInfo.InvariantCulture
• CultureInfo.CreateSpecificCulture("de-CA") //German (Canada)

The interface is a gateway for a function to get a set of culture-specific data from a culture. The two commonly available culture objects that an IFormatProvider can be queried for are:

• DateTimeFormatInfo: IFormatProvider.GetFormat(typeof(DateTimeFormatInfo));
• NumberFormatInfo: IFormatProvider.GetFormat(typeof(NumberFormatInfo));

The way it would normally work is you ask the IFormatProvider to give you a DateTimeFormatInfo object:

DateTimeFormatInfo? format;
format = (DateTimeFormatInfo)provider.GetFormat(typeof(DateTimeFormatInfo));
if (format != null)
   DoStuffWithDatesOrTimes(format);

There's also inside knowledge that any IFormatProvider interface is likely being implemented by a class that descends from CultureInfo, or descends from DateTimeFormatInfo, so you could cast the interface directly:

CultureInfo? info = provider as CultureInfo;
if (info != null)
   format = info.DateTimeInfo;
else
{
   DateTimeFormatInfo? dtfi = provider as DateTimeFormatInfo;
   if (dtfi != null)
       format = dtfi;
   else
       format = (DateTimeFormatInfo)provider.GetFormat(typeof(DateTimeFormatInfo));
}

if (format != null)
   DoStuffWithDatesOrTimes(format);

But don't do that

All that hard work has already been written for you:

To get a DateTimeFormatInfo from an IFormatProvider:

DateTimeFormatInfo format = DateTimeFormatInfo.GetInstance(provider);

To get a NumberFormatInfo from an IFormatProvider:

NumberFormatInfo format = NumberFormatInfo.GetInstance(provider);

The virtue of IFormatProvider is that you create your own culture objects. As long as they implement IFormatProvider, and return objects they're asked for, you can bypass the built-in cultures.

You can also use IFormatProvider for a way of passing arbitrary culture objects - through the IFormatProvider. E.g. the name of god in different cultures

• god
• God
• Jehova
• Yahwe
• יהוה
• אהיה אשר אהיה

This lets your custom LordsNameFormatInfo class ride along inside an IFormatProvider, and you can preserve the idiom.

In reality you will never need to call GetFormat method of IFormatProvider yourself.

Whenever you need an IFormatProvider you can pass a CultureInfo object:

DateTime.Now.ToString(CultureInfo.CurrentCulture);

endTime.ToString(CultureInfo.InvariantCulture);

transactionID.toString(CultureInfo.CreateSpecificCulture("qps-ploc"));

Note: Any code is released into the public domain. No attribution required.

Passing null as the IFormatProvider is not the correct way to do this. If the user has a custom date/time format on their PC you'll have issues in parsing and converting to string. I've just fixed a bug where somebody had passed null as the IFormatProvider when converting to string.

Instead you should be using CultureInfo.InvariantCulture

Wrong:

string output = theDate.ToString("dd/MM/yy HH:mm:ss.fff", null);

Correct:

string output = theDate.ToString("dd/MM/yy HH:mm:ss.fff", CultureInfo.InvariantCulture);