Simple Eager / Lazy loading examples in linq2sql

Question

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

Answer 1

• 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
      }
  }
  

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

• 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
      }
  }
  

note that: Deferred execution is a LINQ Feature but Deferred Loading is a LINQ to SQL feature