Dynamic/Transparent Proxy using DynamicProxy.NET

using System;
using System.IO;
using System.Security.Principal;

namespace DynamicProxyTutorial {

	/// <summary>
	/// Hand made proxy for the IFileViewer.
	/// </summary>
	public class FileViewerProxy : IFileViewer {
		/// <summary>
		/// User ID that is allowed to read/write from files
		/// </summary>
		private string allowedUserID;
		/// <summary>
		/// The real fileviewer that we're proxying
		/// </summary>
		private IFileViewer realFileViewer;

		public FileViewerProxy(string filePath, string allowedUserID) {
			this.allowedUserID = allowedUserID;
			realFileViewer = new FileViewer(filePath);
		}

		/// <summary>
		/// Checks if the current user is the same as the allowed user. 
		/// If it isn't a SecurityException is thrown
		/// </summary>
		/// <exception cref="SecuritException">If the current user isn't allowed to 
		/// read/write from the file, this exception is thrown</exception>
		private void checkPermission() {
			if (WindowsIdentity.GetCurrent().Name != allowedUserID) {
				throw new SecurityException("From : " + this.GetType().Name + "\r\nUserID '" + 
							WindowsIdentity.GetCurrent().Name + 
							" isn't allowed to read/write from the file: '" + 
							realFileViewer.FilePath + "'. " + 
							"Allowed user: '" + allowedUserID + "'");
			}
		}

		// ---------------- Proxy methods  -------------
		// It's these method wrappers that DynamicProxy.NET creates 
		// for you in a generic way

		public void ReadFromFile() {
			checkPermission();
			realFileViewer.ReadFromFile();
		}

		public string Content {
			get {
				return realFileViewer.Content;
			}
		}

		public string FilePath {
			get {
				return realFileViewer.FilePath;
			}
		}
	}
}