了解System.Data.Linq.Table <T>

Question

I'm looking at this blog :

If you scroll down a bit there's a section called , where it says

Inside DataClasses1DataContext is property called Customers

public System.Data.Linq.Table<Customer> Customers
{
    get
    {
        return this.GetTable<Customer>();
    }
}

and then right below that:

If you write a LINQ query that retrieves customer records from the database, then you can access that data via this property. It is of type Table, where the internal Table class becomes in this case a collection of Customer records. One would typically access this property by writing code that looks like this:

DataClasses1DataContext db = new DataClasses1DataContext(ConnectionString);

var query = from c in db.Customers
            select c;

Does this mean that at the point the property is referenced the entire Customer table is queried and returned?

if so, isn't this a bit inefficient if there's a lot of records in the table, especially if you just want to retrieve one customer?

Thanks

Answer 1

Does this mean that at the point the property is referenced the entire Customer table is queried and returned?

No - Table<T> implements IQueryable<T> which means that queries are deferred until the collection is enumerated.

Where , OrderBy , Select , and other clauses are "attached" to the query and converted to SQL when the query is either enumerated or converted to a linq-to-objects query via AsEnumerable() .

Note that the article you reference is discussing Linq-to-SQL, but the principle is the same in Linq-to-Enitites (Entity Framework). Queries are build up using where , orderby , group by , select , etc. and converted to SQL when the query is executed.