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:

 public class MyDateFormatProvider : IFormatProvider, ICustomFormatter  {   public object GetFormat(Type formatType)   {    if (formatType == typeof(ICustomFormatter))    {     return this;    }    return null;   }    public string Format(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("ddd dd MMM");       default:        return date.ToString("ddd MMM dd");      }     default:      throw new FormatException();    }   }  

I was expecting to be able to use it in the DateTime.ToString(string format, IFormatProvider provider) method like so, but :

DateTime d = new DateTime(2000, 1, 2); string s = d.ToString("mycustomformat", new MyDateFormatProvider()); 

In that example, running in the US Culture, the result is "00cu0Ao00or0aA", apparently because the standard DateTime format strings are being interpreted.

However, when I use the same class in the following way:

DateTime d = new DateTime(2000, 1, 2); string s = String.Format(new MyDateFormatProvider(), "{0:mycustomformat}", d); 

I get what I expect, namely "Sun Jan 02"

I don't understand the different results. Could someone explain?

Thanks!

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.