How to create and use a custom IFormatProvider for DateTime?

Question

I was trying to create an IFormatProvider implementation that would recognize custom format strings for DateTime objects. Here is my implementation:

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Answer 1

Use extension method :)

public static class FormatProviderExtension
    {
        public static string FormatIt(string format, object arg, IFormatProvider formatProvider)
        {
            if (arg == null) throw new ArgumentNullException("arg");
            if (arg.GetType() != typeof(DateTime)) return arg.ToString();
            DateTime date = (DateTime)arg;
            switch (format)
            {
                case "mycustomformat":
                    switch (CultureInfo.CurrentCulture.Name)
                    {
                        case "en-GB":
                            return date.ToString("ffffd dd MMM");
                        default:
                            return date.ToString("ffffd MMM dd");
                    }
                default:
                    throw new FormatException();
            }
        }

        public static string ToString(this DateTime d, IFormatProvider formatProvider, string format)
        {
            return FormatIt(format, d, formatProvider);
        }
    }

Answer 2

Checking the DateTime.ToString method with Reflector shows that the DateTime structure uses the DateTimeFormatInfo.GetInstance method to get the provider to be used for formatting. The DateTimeFormatInfo.GetInstance requests a formatter of type DateTimeFormatInfo from the provider passed in, never for ICustomFormmater, so it only returns an instance of a DateTimeFormatInfo or CultureInfo if no provider is found. It seems that the DateTime.ToString method does not honor the ICustomFormatter interface like the StringBuilder.Format method does, as your String.Format example shows.

I agree that the DateTime.ToString method should support the ICustomFormatter interface, but it does not seem to currently. This may all have changed or will change in .NET 4.0.

Answer 3

The short explanation is that while

DateTime.ToString(string format, IFormatProvider provider)

lets you pass anything implementing IFormatProvider as one of its parameters, it actually only supports 2 possible types implementing IFormatProvider inside its code:

DateTimeFormatInfo or CultureInfo

If your parameter cannot be casted (using as) as either or those, the method will default to CurrentCulture.

String.Format is not limited by such bounds.