C# 0 The Complete Reference

LINQ Fundamentals

At LINQ’s core is the 

query

. A query specifies what data will be obtained from a data source. 

For example, a query on a customer mailing list might request the addresses of all customers 

that reside in a specific city, such as Chicago or Tokyo. A query on an inventory database 

might request a list of out-of-stock items. A query on a log of Internet usage could ask for a 

list of the websites with the highest hit counts. Although these queries differ in their specifics, 

all can be expressed using the same LINQ syntactic elements.

After a query has been created, it can be executed. One way this is done is by using the 

query in a 

foreach

 loop. Executing a query causes its results to be obtained. Thus, using a 

query involves two key steps. First, the form of the query is created. Second, the query is 

executed. Therefore, the query defines 

what

 to retrieve from a data source. Executing the 

query actually 

obtains the results.

In order for a source of data to be used by LINQ, it must implement the 

IEnumerable

interface. There are two forms of this interface: one generic, one not. In general, it is easier if 

the data source implements the generic version,