C# 0 The Complete Reference

The Query Methods

The query syntax described by the preceding sections is the way you will probably write 

most queries in C#. It is convenient, powerful, and compact. It is, however, not the only way 

to write a query. The other way is to use the 

query methods.

 These methods can be called on 

any enumerable object, such as an array.

The Basic Query Methods

The query methods are defined by 

System.Linq.Enumerable

 and are implemented as 

extension methods

 that extend the functionality of 

IEnumerable

. (Query methods are 

also defined by 

System.Linq.Queryable

, which extends the functionality of 

IQueryable

,

but this interface is not used in this chapter.) An extension method adds functionality to 

another class, but without the use of inheritance. Support for extension methods was added 

by C# 3.0, and we will look more closely at them later in this chapter. For now, it is sufficient 

to understand that query methods can be called only on an object that implements 

IEnumerable

.

The

Enumerable

 class provides many query methods, but at the core are those that 

correspond to the query keywords described earlier. These methods are shown here, along 

with the keywords to which they relate. Understand that these methods have overloaded 

forms and only their simplest form is shown. However, this is also the form that you will 

often use.

Query Keyword

Equivalent Query Method

select

Select(

arg

)

where

Where(

arg

)

orderby

OrderBy(

arg

) or OrderByDescending(

arg

)

join

Join(

seq2

,

key1

,

key2

,

result

)

group

GroupBy(

arg

)

Except for 

Join( )

, the other methods take one argument, 

arg,

 which is an object of type 

FuncTResult>

, as a parameter. This is a delegate type defined by LINQ. It is declared 

like this:

delegate TResult Func(T 

arg

)

Here, 

TResult

 specifies the result of the delegate and 

T

 specifies the parameter type. In the 

query methods, 

arg

 determines what action the query method takes. For example, in the 

case of 

Where( )

,

arg

 determines how the query filters the data. Each of these query methods 

returns an enumerable object. Thus, the result of one can be used to execute a call on 

another, allowing the methods to be chained together.

The

Join( ) 

method takes four arguments. The first is a reference to the second sequence 

to be joined. The first sequence is the one on which 

Join( )

 is called. The key selector for the 

first sequence is passed via 

key1,

 and the key selector for the second sequence is passed via 

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