Enterprise Library 2.0 Hands On Lab 翻譯（10）：快取應用程式區塊（二）

練習 2：持久快取

示範如何實作持久快取。

步驟 1：開啟專案

開啟 EmployeeBrowser.sln，預設路徑 C:\Program Files\Microsoft Enterprise Library January 2006\labs\cs\Caching\exercises\ex02\begin，編譯。

步驟 2：實作離線快取

1. 開啟 EmployeeServices.cs，加入命名空間：using Microsoft.Practices.EnterpriseLibrary.Caching.Expirations;
2. 修改 GetContactDetails 方法：

public static EmployeesDataSet GetContactDetails()
{
    EmployeesDataSet dsEmployees = null;
    CacheManager cache = CacheFactory.GetCacheManager();
    dsEmployees = (EmployeesDataSet)cache[CACHE_KEY];
    if (dsEmployees == null && ConnectionManager.IsOnline)
    {
        EmployeeDataProvider dataProvider = new EmployeeDataProvider();
        dsEmployees = dataProvider.GetEmployees();
        AbsoluteTime expiry = new AbsoluteTime(new TimeSpan(2, 0, 0, 0)); // 2天過期
        cache.Add(CACHE_KEY, dsEmployees, CacheItemPriority.High, null, new ICacheItemExpiration[] { expiry });
    }
    return dsEmployees;
}

3. 修改 GetEmployeePhoto，離線時不嘗試取得資料：

public static Bitmap GetEmployeePhoto(Guid employeeId)
{
    byte[] photoData = null;
    CacheManager cache = CacheFactory.GetCacheManager();
    photoData = (byte[])cache[employeeId.ToString()];
    if (photoData == null && ConnectionManager.IsOnline)
    {
        EmployeeDataProvider dataProvider = new EmployeeDataProvider();
        photoData = dataProvider.GetEmployeePhotoData(employeeId);
        cache.Add(employeeId.ToString(), photoData);
    }
    if (photoData == null) return null;
    using (MemoryStream ms = new MemoryStream(photoData))
        return new Bitmap(ms);
}

持久快取支援離線存取，設定過期時間自動清理。