limited trial period in C #

using System;

using System.IO;

using System.IO.IsolatedStorage;

static class Program

{

   static void Main(string[] args)

   {

      int period = 30; // trial period

      using (IsolatedStorageFile isf  = IsolatedStorageFile.GetMachineStoreForAssembly())

      {

         if(!isf.FileExists("expiry.txt"))

         {

            using(var iss = new IsolatedStorageFileStream("expiry.txt", FileMode.Create, isf))

            {

               using (var sw = new StreamWriter(iss))

               { 

                  string s = DateTime.Today.AddDays(period).ToString();                

                  sw.WriteLine(s);

               }

            } 

          }

          else

          {  

            using(var iss = new IsolatedStorageFileStream("expiry.txt", FileMode.Open, isf))

            {

               using (var sr = new StreamReader(iss))

               {

                  string s = sr.ReadToEnd();

                  DateTime expiry = DateTime.Parse(s);

                  if (DateTime.Today > expiry)

                  {

                     Console.WriteLine("Your free trial has expired. Please register to continue using the application");

                     return;

                  }

               }

             }         

         }

      }

      // code for rest of application

      Console.WriteLine("Rest of application continuing ...");

      Console.ReadKey();

   }

} 

It will be more secure if you give your assembly a strong name and store it in the GAC. Otherwise, it will be identified by its file path and the user will then be able to defeat the trial period by either renaming or copying it to a different location which will cause the isolated storage to be reset. 

However, all limited trial applications which depend on the system clock can be defeated by the user temporarily changing the system date. You can prevent this by obtaining the date from the internet, using a web service or otherwise, but this obviously requires that the user has an internet connection which can cause a number of problems in itself.

A better idea IMO is to restrict the number of times the application can be run without any time restriction. You can then simply update the count in the file on each running.