Add expiration to C# windows application

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C#

Hello everyone,

i am trying to make make app expire after some days using the registry option, i have successfully written and read from registry, my issue is to check for expiration, from my code i assumed the user's first run is the 04/16/2017, i also assumed i want the user to run the application for one day, which supposed to expire on the 04/17/2017, meaning if the user tries to start the application after 04/17/2017 the if part should execute, but i am not really getting it right, the else part always execute, i will appreciate a better way of doing it. Thanks

What I have tried:

<pre>regKey = Registry.CurrentUser.OpenSubKey("Systemfiles");//subkeyname
            if (regKey == null)
            {
                regKey = Registry.CurrentUser.CreateSubKey("Systemfiles");
                regKey.SetValue("tere", Encrypt("4/16/2017"));
            }
            else
            {
                regKey = Registry.CurrentUser.OpenSubKey("Systemfiles");//subkeyname
                string decryptDateValue = Decrypt(regKey.GetValue("tere").ToString());  //Keyname
                DateTime mainDate = Convert.ToDateTime(decryptDateValue);

                DateTime expiringDate = mainDate.AddDays(1);
                if (mainDate > expiringDate)
                {
                    expire = true;
                }
                else
                {
                    //Continue execution
                }
            }