What does IFormatProvider do?

I was playing around with the Datetime.ParseExact method, and it wants an IFormatProvider...

It works inputting null, but what exactly does it 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);

IFormatProvider provides culture info to the method in question. DateTimeFormatInfo implements IFormatProvider, and allows you to specify the format you want your date/time to be displayed in. Examples can be found on the relevant MSDN pages.

You can see here http://msdn.microsoft.com/en-us/library/system.iformatprovider.aspx

See the remarks and example section there.

Check http://msdn.microsoft.com/en-us/library/system.iformatprovider.aspx for the API.

By MSDN

The .NET Framework includes the following three predefined IFormatProvider implementations to provide culture-specific information that is used in formatting or parsing numeric and date and time values:

1. The NumberFormatInfo class, which provides information that is used to format numbers, such as the currency, thousands separator, and decimal separator symbols for a particular culture. For information about the predefined format strings recognized by a NumberFormatInfo object and used in numeric formatting operations, see Standard Numeric Format Strings and Custom Numeric Format Strings.
2. The DateTimeFormatInfo class, which provides information that is used to format dates and times, such as the date and time separator symbols for a particular culture or the order and format of a date's year, month, and day components. For information about the predefined format strings recognized by a DateTimeFormatInfo object and used in numeric formatting operations, see Standard Date and Time Format Strings and Custom Date and Time Format Strings.
3. The CultureInfo class, which represents a particular culture. Its GetFormat method returns a culture-specific NumberFormatInfo or DateTimeFormatInfo object, depending on whether the CultureInfo object is used in a formatting or parsing operation that involves numbers or dates and times.

The .NET Framework also supports custom formatting. This typically involves the creation of a formatting class that implements both IFormatProvider and ICustomFormatter. An instance of this class is then passed as a parameter to a method that performs a custom formatting operation, such as String.Format(IFormatProvider, String, Object[]).

The DateTimeFormatInfo class implements this interface, so it allows you to control the formatting of your DateTime strings.

The question asks about IFormatProvider and DateTime, but you can use IFormatProvider also in other contexts of .NET, such as for string.Format.

Then you pass in the IFormatProvider instance and you can by implementing IFormatProvider specify how the string is formatted for an object. So the usage of IFormatProvider and a related ICustomFormatter interface is more broad in .NET than just for dates.

E.g. consider this implementation :

using System;

namespace ConsoleApp
{
    class EmployeeProductivityFormatProvider : IFormatProvider, ICustomFormatter
    {
        public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            int rating = (int)arg;

            if (rating == 0)
            {
                return $"{rating} (new employee)";
            }

            if (rating > 0)
            {
                return $"{rating} (good employee)";
            }

            return $"{rating} (bad employee)";
        }

        public object GetFormat(Type formatType)
        {
            if (formatType == typeof(ICustomFormatter))
            {
                return this;
            }

            return null;
        }
    }
}

Now we can get a string representation of an employee by specifying an int value like this:

    string prod = string.Format(new EmployeeProductivityFormatProvider(),
                                "Productivity rating: {0}",
                                employee.ProductivityRating);

When it comes to DateTime, there are a lot of already created IFormatProvider implementation to choose from. This is a very flexible way of representating objects to strings and specifying their format in .NET and a very general concept.