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

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.