FileSizeFormat.cs

using System;

namespace SpecialFNs
{
	public class FileSizeFormatProvider : IFormatProvider, ICustomFormatter
	{
		public object GetFormat(Type formatType)
		{
			if (formatType == typeof(ICustomFormatter)) return this;
			return null;
		}

		private const string fileSizeFormat = "fs";
		private const Decimal OneKiloByte = 1024M;
		private const Decimal OneMegaByte = OneKiloByte * 1024M;
		private const Decimal OneGigaByte = OneMegaByte * 1024M;

		public string Format(string format, object arg, IFormatProvider formatProvider)
		{
			if (format == null || !format.StartsWith(fileSizeFormat))
			{
				return defaultFormat(format, arg, formatProvider);
			}

			if (arg is string)
			{
				return defaultFormat(format, arg, formatProvider);
			}

			Decimal size;

			try
			{
				size = Convert.ToDecimal(arg);
			}
			catch (InvalidCastException)
			{
				return defaultFormat(format, arg, formatProvider);
			}

			string suffix;
			if (size > OneGigaByte)
			{
				size /= OneGigaByte;
				suffix = "GB";
			}
			else if (size > OneMegaByte)
			{
				size /= OneMegaByte;
				suffix = "MB";
			}
			else if (size > OneKiloByte)
			{
				size /= OneKiloByte;
				suffix = "kB";
			}
			else
			{
				suffix = " B";
			}

			string precision = format.Substring(2);
			if (String.IsNullOrEmpty(precision)) precision = "2";
			return String.Format("{0:N" + precision + "}{1}", size, suffix);

		}

		private static string defaultFormat(string format, object arg, IFormatProvider formatProvider)
		{
			IFormattable formattableArg = arg as IFormattable;
			if (formattableArg != null)
			{
				return formattableArg.ToString(format, formatProvider);
			}
			return arg.ToString();
		}
	}
}