The term deferred loading, sometimes also described as lazy loading. is used for the deferred loading of child objects. Suppose you are loading a customer, and the customer has a navigational property Orders. LINQ to SQL applies deferred loading by default, which means that the orders are not retrieved immediately when the customer is retrieved, but rather when you actually request the orders by enumerating over them (or bind them to a grid). LINQ to SQL will then automatically fire of a query to load the orders.
However, in most cases this is not what you want because you of the excessive round-tripping this can cause to and from the database server. In most cases, it would be far more efficient to execute a single query and eager-load the parent object and it’s child objects with the results or that one query. As in all OR/M frameworks, LINQ to SQL offers you the possibility to specify how to load the child objects. This is done by specifying the dataload options.
First of all, you can set the default lazy loading mechanism off by specifying
ctx.DeferredLoadingEnabled = false;
where ctx is the instance of your DataContext class. At this point, no loading of child objects takes place at all. Next, you can specify which child objects you want to eager load
DataLoadOptions dlo = new DataLoadOptions();
dlo.LoadWith<Customer>(c => c.Orders);
ctx.LoadOptions = dlo;
This means that the customers and orders will be loaded by a single query, where the customers and orders table are queried using an inner join.
Note that this works well with the first level of child objects, but people have pointed out that LINQ to SQL will not join a third level of child objects in the same single query. Serious limitation, don’t you think?
Ofcourse, you can also execute the loading per object. Suppose you want to pass the object through a service, then it would make sense to make sure the child objects are loaded. If you have not set eager loading on the DataLoadOptions of the context, you can use the Customer.Orders.Load() method.
On a final note, you can also specify for specific attributes that you want to defer loading until such a time that you really use the value. Suppose for example you have a blob column for the customer with a picture of that customer. You will want to avoid retrieving the field unless it is really used in your UI. In the LINQ to SQL designer, specify ‘False’ for the ‘Delay Loaded’ property of that field. LINQ to SQL will then automatically fire off a separate query to retrieve the content at the last possible moment.
