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

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.