format.cs

using System;

public class CustomFormatter : IFormatProvider, ICustomFormatter
{
   public object GetFormat(Type formatType)
   {
      return this;
   }

   public string Format(string format, object arg, IFormatProvider formatProvider)
   {
      string s = null;
      
      // <Snippet1>
      if (arg is IFormattable) 
         s = ((IFormattable)arg).ToString(format, formatProvider);
      else if (arg != null)    
         s = arg.ToString();
      // </Snippet1>
      return s;
   }
}