linq2sql中的简单Eager/Lazy加载示例

问题描述

有人在linq2sql中有一个简单的代码示例，该示例演示了渴望加载和延迟加载之间的区别吗?

Does anyone have a simple code example in linq2sql that demonstrates the difference between Eager Loading and Lazy Loading?

推荐答案

• 延迟加载:对于给定的实体，它的关联集合在初次加载时可能为空，但是当这些集合首次被迭代时，LINQ to SQL会触发要加载的查询这些集合在加载后即可用于将来使用，而无需其他查询:

  • Deferred Loading: for a given entity, it's associated collections may be empty when it is first loaded, but when these collections are first iterated, LINQ to SQL fires off a query to load these collections after it is loaded the collection is then available for future use without requiring another query:

    var query = from o in db.GetTable<Order>() //db is the datacontext
            select o;
    foreach(Order o in query)
    {
        foreach(OrderDetail d in o.OrderDetails)//Deferred loading
        {
           //Do something on these collections
        }
    }
    

  OrderDetails仅在被迭代时才加载，因此，如果从不迭代OrderDetatils，则永远不会执行相应的查询.

  The OrderDetails are loaded only if it is iterated, so If the OrderDetatils are never iterated the corresponding query is never executed.

  • 紧急加载:立即为所有引用的实体(例如LINQ to SQL)加载关联的馆藏，将自动为所有撤回的订单提供所有OrderDetails

  • Eager Loading: immediate loading the associated Collections for all referenced entities for example LINQ to SQL will automatically brings all the OrderDetails for all the retreived Orders

  DataLoadOptions op = new DataLoadOptions();
  op.LoadWith<Order>(o => o.OrderDetails);
  db.LoadOptions = op;
  var query = from o in db.GetTable<Order>()
           select o;
  foreach(Order o in query)
  {
      //Order details are eager loaded; additional queries are not needed
      foreach(OrderDetail d in o.OrderDetails)
      {
         //do something
      }
  }
  

  请注意:延迟执行是LINQ功能，但是延迟加载是LINQ to SQL功能

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