Unable to find namespace for FileSystemEntry

using System;
using System.IO;
using System.Security;
namespace DosAPI1
{
	class MyExplorer
	{
		public static void Main(string[] args)
		{
			MyExplorer oExplorer=new MyExplorer();
			string Command;
			string[] CommandLineArgs;
			int ArgumentLength;
			bool CanExit=false;
			Console.WriteLine("Welcome to the MyExplorer Dos Utility");
			do
			{
				Console.Write("MyExplorer>");
				Command=Console.ReadLine ();
				Command=ToSingleSpace(Command.Trim().ToLower());
				CommandLineArgs=oExplorer.ParseCommand(Command.ToLower(),out ArgumentLength);
				switch(CommandLineArgs[0])
				{
					case"cpy":
						if(ArgumentLength!=2)
						{
							ShowError("Usage cpy<source><dest>");
						}
						else
						{
							oExplorer.Copy(CommandLineArgs[1],CommandLineArgs[2]);
						}
						break;
					case"rem":
						if(ArgumentLength!=1)
						{
							ShowError("Usage rem<filename>");
						}
						else
						{
							oExplorer.Remove(CommandLineArgs[1]);
						}
						break;
					case"dir":
					case"ls":
					      switch(ArgumentLength)
					         {
							  case 0:
								  oExplorer.ListFiles(".");
								  break;
							  case 1:
								  oExplorer.ListFiles(CommandLineArgs[1]);
							      break;
							  default:
								  ShowError("Usage dir[<path>]");
								  break;
					         }
						break;
					case "help":
					case "h":
						oExplorer.ShowHelp();
						break;
					case "quit":
					case "exit":
					case "bye":
						CanExit = true;
					  break;
					default:
						 ShowError("Invalid Command");
						break;
				}
			}while(!CanExit);
		}
		public static void ShowError(string StrError)
		{
			Console.WriteLine(StrError);
		}
		public static string ToSingleSpace(string StrToReplace)
		{
			int Position;
			Position=StrToReplace.IndexOf(" ");
			if(Position==-1)
			  return StrToReplace;
			else 
			  return ToSingleSpace(StrToReplace.Substring(0,Position)+StrToReplace.Substring(Position+1));
        }
		public string[] ParseCommand(string Command,out int ArgumentLength)
		{
			string[] CommandLineArgs=Command.Split(new char[]{' '});
			ArgumentLength=CommandLineArgs.Length-1;
            return CommandLineArgs;
		}
		/* Function to the Usage of MyExplorer*/
		public void ShowHelp()
		{
			Console.WriteLine("MyExplorer Help Contents");
			Console.WriteLine("-------------------------");
			Console.WriteLine("cpy<source><dest>-Copies the content of source to dest");
            Console.WriteLine("rem<filename>    -Filename to be deleted");
			Console.WriteLine("dir/ls[<path>]   -Directory Listing");
			Console.WriteLine("h/help           -Help for using MyExplorer");
			Console.WriteLine("quit/exit/bye    -Exit MyExplorer");
		}
		/* Method to perform File Copy operation*/
		public void Copy(string SourceFile,string DestinationFile)
		{
			try
			{
				File.Copy(SourceFile,DestinationFile,true);
				Console.WriteLine("File Copied Successfully");
			}
			catch(FileNotFoundException)
			{
				Console.WriteLine("Source File does not Exists");
			}
			catch(ArgumentException)
			{
				Console.WriteLine("Invalid Argument for cpy");
			}
			catch(SecurityException)
			{
				Console.WriteLine("Write Access is Denied");
			}
			catch(Exception)
			{
				Console.WriteLine("UnKnown Exception");
			}
		}
		/* Function to Delete the Specified File*/
		public void Remove(string FileToRemove)
		{
			try
			{
				File.Delete(FileToRemove);
				Console.WriteLine("File Removed Successfully");
			}
			catch(DirectoryNotFoundException)
			{
				Console.WriteLine("File does not exists");
			}
			catch(SecurityException)
			{
				Console.WriteLine("Delete Permission Denied");
			}
			catch(AccessException)
			{
				Console.WriteLine("Directory name is specified where file is required");
			}
			catch(Exception)
			{
				Console.WriteLine("UnKnown Exception");
			}
		}
        /* Directory Listing*/
		public void ListFiles(string StrDirectoryToList)
		{
            Directory oDir;
			FileSystemEntry[] oFiles;
			try
			{   
				oDir=new Directory(StrDirectoryToList);
				oFiles=oDir.GetFileSystemEntries();
				bool IsFileFound = true;
				foreach(FileSystemEntry f in oFiles)
				{
					if(IsFileFound)
					{
						Console.WriteLine("{0,-32}{1,8}{2}","Name","Size","Directory/File");
						Console.WriteLine(new string('=',80));
							IsFileFound=false;
					}
					if(f.IsFile)
					{
						Console.WriteLine("{0,-32}{1,8}{2}",f.Name,((File)f).Length,"F");
					}
					else 
					{
						Console.WriteLine("{0,-32}{1,8}{2}",f.Name,"0","F");
					}
				}
				
				
			}
			catch(DirectoryNotFoundException)
			{
				Console.WriteLine("Directory Not Found");
			}
			catch(Exception)
			{
			}
		}
	}
}