C# 0 The Complete Reference

n

 to be an 

int

:

var posNums = from int n in nums

  // ...

Of course, the explicit type specification is not needed here because all arrays are implicitly 

convertible to 

IEnumerable

, which enables the type of the range variable to be inferred.

The type of object returned by a query is an instance of 

IEnumerable

, where 

T

 is 

the type of the elements. Thus, the type of the query variable must be an instance of 

IEnumerable

. The value of 

T

 is determined by the type of the value specified by the 

select

 clause. In the case of the preceding examples, 

T

 is 

int

 because 

n

 is an 

int

. (As explained, 

n

 is an 

int

 because 

int

 is the type of elements stored in 

nums

.) Therefore, the query could 

have been written like this, with the type explicitly specified as 

IEnumerable

:

IEnumerable posNums = from n in nums

                           where n > 0

                           select n;

The key point is that the type of the item selected by 

select

 must agree with the type 

argument passed to 

IEnumerable

 used to declare the query variable. Often query 

variables use 

var

 rather than explicitly specifying the type because this lets the compiler 

infer the proper type from the 

select

 clause. As you will see, this approach is particularly 

useful when 

select

 returns something other than an individual element from the data source.

When a query is executed by the 

foreach

 loop, the type of the iteration variable must be 

the same as the type of the range variable. In the preceding examples, this type was explicitly 

specified as 

int

, but you can let the compiler infer the type by specifying this variable as 

var

.

As you will see, there are also some cases in which 

var

 must be used because the data type 

has no name.

The General Form of a Query

All queries share a general form, which is based on a set of contextual keywords, shown here:

ascending

descending

equals

from

group

in

into

join

let

on

orderby

select

where

Of these, the following begin query clauses:

from

group

join

let

orderby

select

where

A query must begin with the keyword 

from

 and end with either a 

select

 or 

group

 clause. 

The

select

 clause determines what type of value is enumerated by the query. The 

group

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