练习2：持久缓存
该练习将示范如何持久缓存。

第一步
打开EmployeeBrowser.sln 项目，默认的安装路径应该为C:\Program Files\Microsoft Enterprise Library January 2006\labs\cs\Caching\exercises\ex02\begin，并编译。

第二步 实现离线缓存
1．在解决方案管理器中选择EmployeeServices.cs文件，选择View | Code菜单命令并添加如下命名空间

using Microsoft.Practices.EnterpriseLibrary.Caching.Expirations;

2．定位到GetContactDetails方法，并添加如下代码。

public static EmployeesDataSet GetContactDetails(){
EmployeesDataSet dsEmployees = null;
// TODO: Add persistent caching with time-out
// Attempt to retrieve from cache
    CacheManager cache = CacheFactory.GetCacheManager();
dsEmployees = (EmployeesDataSet)cache[CACHE_KEY];
// Retrieve from dataProvider if not in Cache and Online
    if (dsEmployees == null && ConnectionManager.IsOnline){
EmployeeDataProvider dataProvider = new EmployeeDataProvider();
dsEmployees = dataProvider.GetEmployees();
// Expire in 2 days
        AbsoluteTime expiry = new AbsoluteTime(new TimeSpan(2, 0, 0, 0));
cache.Add(CACHE_KEY, dsEmployees,
CacheItemPriority.High, null,
new ICacheItemExpiration[] { expiry });
}
return dsEmployees;
}

3．修改方法GetEmployeePhoto为如下代码，即离线时不尝试去获取信息。

public static Bitmap GetEmployeePhoto(Guid employeeId){
byte[] photoData = null;
// Attempt to retrieve from cache
    CacheManager cache = CacheFactory.GetCacheManager();
photoData = (byte[])cache[employeeId.ToString()];
// TODO: Retrieve from dataProvider if not in Cache and Online
    if (photoData == null && ConnectionManager.IsOnline){
EmployeeDataProvider dataProvider = new EmployeeDataProvider();
photoData = dataProvider.GetEmployeePhotoData(employeeId);
cache.Add(employeeId.ToString(), photoData);
}
// No data found.
    if (photoData == null)
return null;
// Convert bytes to Bitmap
    using (MemoryStream ms = new MemoryStream(photoData){
return new Bitmap(ms);
}
}

static public bool IsOnline{
get { return false; }
}