C# 0 The Complete Reference

n

 in 

nums

 when the query executes. Only those values that satisfy this 

condition will be obtained. In other words, a 

where

 clause acts as a filter on the data source, 

allowing only certain items through.

All queries end with either a 

select

 clause or a 

group

 clause. This example employs the 

select

 clause. It specifies precisely what is obtained by the query. For simple queries, such as 

the one in this example, the range value is selected. Therefore, it returns those integers from 

nums

 that satisfy the 

where

 clause. In more sophisticated situations, it is possible to finely 

tune what is selected. For example, when querying a mailing list, you might return just the 

last name of each recipient, rather than the entire address. Notice that the 

select

 clause ends 

with a semicolon. Because 

select

 ends a query, it ends the statement and requires a semicolon. 

Notice, however, that the other clauses in the query do not end with a semicolon.

At this point, a query variable called 

posNums

 has been created, but no results have 

been obtained. It is important to understand that a query simply defines a set of rules. It is 

not until the query is executed that results are obtained. Furthermore, the same query can 

be executed two or more times, with the possibility of differing results if the underlying 

data source changes between executions. Therefore, simply declaring the query 

posNums

does not mean that it contains the results of the query.

To execute the query, the program uses the 

foreach

 loop shown here:

foreach(int i in posNums) Console.WriteLine(i + " ");

Notice that 

posNums

 is specified as the collection being iterated over. When the 

foreach

executes, the rules defined by the query specified by 

posNums

 are executed. With each pass 

through the loop, the next element returned by the query is obtained. The process ends when 

there are no more elements to retrieve. In this case, the type of the iteration variable 

i

 is 

explicitly specified as 

int

 because this is the type of the elements retrieved by the query. 

Explicitly specifying the type of the iteration variable is fine in this situation, since it is easy 

to know the type of the value selected by the query. However, in more complicated situations, 

it will be easier (or in some cases, necessary) to implicitly specify the type of the iteration 

variable by using