12-3. 数据库连接日志
问题
你想为每次与数据库的连接和断开记录日志
解决方案
EF为DbContext的连接公开了一个StateChange 事件.我们需要处理这个事件, 为每次与数据库的连接和断开记录日志.
假设我们的模型如Figure 12-3所示. 在 Listing 12-3代码里, 我们创建一些Donation 实例,然后把它们保存到数据库. 这里的代码实现override SaveChanges() 方法为我们的StateChange 事件提供切入点.
Figure 12-3. The model with the Donation entity
Listing 12-3. Code to Implement Logging of Open and Close of a Database Connection
class Program
{
static void Main(string[] args)
{
RunExample();
}
static void RunExample()
{
using (var context = new EFRecipesEntities())
{
context.Donations.Add(new Donation
{
DonorName = "Robert Byrd",
Amount = 350M
});
context.Donations.Add(new Donation
{
DonorName = "Nancy McVoid",
Amount = 250M
});
context.Donations.Add(new Donation
{
DonorName = "Kim Kerns",
Amount = 750M
});
Console.WriteLine("About to SaveChanges()");
context.SaveChanges();
}
using (var context = new EFRecipesEntities())
{
var list = context.Donations.Where(o => o.Amount > 300M);
Console.WriteLine("Donations over $300");
foreach (var donor in list)
{
Console.WriteLine("{0} gave {1}", donor.DonorName,
donor.Amount.ToString("C"));
}
}
Console.WriteLine("Press any key to close...");
Console.ReadLine();
}
}
public partial class EFRecipesEntities
{
public override int SaveChanges()
{
this.Database.Connection.StateChange += (s, e) =>
{
var conn = (DbConnection)s;
Console.WriteLine("{0}: Database: {1}, State: {2}, was: {3}",
DateTime.Now.ToShortTimeString(), conn.Database,
e.CurrentState, e.OriginalState);
};
return base.SaveChanges();
}
}
上述Listing 12-3代码输出如下:
About to SaveChanges()
09:56 : Database: EFRecipes, State: Open, was: Closed
09:56: Database: EFRecipes, State: Closed, was: Open
Donations over $300
Robert Byrd gave $350.00
Kim Kerns gave $750.00
原理
我们实现override SaveChanges() 方法,为我们的StateChange事件提供切入点,该事件处理程序接收两个参数:事件的发送者(sender)和StateChangeEventArgs.第二个参数提供了访问连接的当前状态和原始状态, 我们为这两个与数据库相关的状态做日志记录.如果你特点注意日志内容的顺序,你会注意到在第二个using块里,与数据库的连接在foreach执行要查询时重新启动,而不是在写查询语句时. 这个演示说明了一个重要的概念:查询只在必要时才进行.在我们的例子中,查询在遍历期间才执行