internal static void CheckLicense()

        {

            if (!License.licenseChecked)

            {

                try

                {

                    using (IsolatedStorageFile isolatedStorageFile = IsolatedStorageFile.GetUserStoreForAssembly())

                    {

                        using (IsolatedStorageFileStream isolatedStorageFileStream = new IsolatedStorageFileStream("xxx.lic", FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.ReadWrite, isolatedStorageFile))

                        {

                            long storageLength = isolatedStorageFileStream.Length;

                            if (storageLength == (long))

                            {

                                Encoding uTF8 = Encoding.UTF8;

                                DateTime dateTime = DateTime.Now.AddDays();

                                byte[] bytes = uTF8.GetBytes(dateTime.Ticks.ToString());

                                isolatedStorageFileStream.Write(bytes, , (int)bytes.Length);

                            }

                            else

                            {

                                byte[] bytes = new byte[checked(storageLength)];

                                int bytesRead = isolatedStorageFileStream.Read(bytes, , (int)bytes.Length);

                                DateTime expiryDateTime = new DateTime(long.Parse(Encoding.UTF8.GetString(bytes, , bytesRead)));

                                if (DateTime.Now > expiryDateTime)

                                {

                                    License.licenseExpired = true;

                                }

                            }

                        }

                    }

                    License.licenseChecked = true;

                }

                catch (Exception exception)

                {

                }

            }

            if (License.licenseExpired)

            {

                throw new InvalidOperationException("The trial period has expired. Please contact us at xxx.com for further information.");

            }

        }

CheckLicense（）方法可以放在代码中任何地方使用。

基本原理就是在第一次运行时，检测是否生成了license，并把过期日期算好写进去，以后每次检测是否到期。
关键类是IsolatedStorageFile 。它创建的文件在文件系统中，不需要特殊路径，一般权限的托管代码是不能访问其他dll文件的IsolatedStorageFile 文件的，高级权限的托管代码可以访问其他dll的文件，非托管代码可以访问任何IsolatedStorageFile 文件。