gistfile1.cs

namespace Utility
{
  public static class ExtensionMethod
	{
		public static int DaysOfThisMonth(this DateTime datetime)
		{
			return (datetime.AddMonths(1) - datetime).Days;
		}

		public static bool IsImageFile(this IEnumerable<string> list)
		{
			string[] imageFormats = {".png", ".jpg", ".gif"};
			var result = list.Where(x => Array.IndexOf(imageFormats, GetExt(x)) >= 0);
			return result != null && result.Count() == list.Count();
		}

		private static string GetExt(string value)
		{
			var index = value.LastIndexOf(".");
			return index >= 0 ? value.Substring(index) : string.Empty;
		}
	}
}