ASP.NET 4 Unleashed

LINQ to Objects Versus LINQ to SQL

You can use standard LINQ (LINQ to Objects) with any object that implements the IEnumerable<T> interface interface>. You can use LINQ to SQL, on the other hand, with any object that implements the IQueryable<T> interface. Standard LINQ is implemented with the extension methods exposed by the System.Linq.Enumerable class. LINQ to SQL, on the other hand, uses the extension methods exposed by the System.Linq.Queryable class. Why the difference?

When you build a query using standard LINQ, the query executes immediately. When you build a query using LINQ to SQL, on the hand, the query does not execute until you start enumerating the results. In other words, the query doesn’t execute until you use a foreach loop to ...