Essential LINQ Operators

By John Paul Mueller

Part of LINQ For Dummies Cheat Sheet

Language INtegrated Query (LINQ) uses an amazing array of operators. An operator is a special word that tells LINQ what kind of query to perform, such as checking whether two values are equal. Operators can run into the hundreds, but in most cases you’ll perform the majority of tasks using only a few standard operators. Below is a list of essential LINQ operators.

Operator Description
from Defines the temporary object used to perform a query. Think of
this operator as defining the individual element object from which
you derive all other data for the query. For example, you choose
what data will appear in the output and how that data is ordered
based on the temporary variable. The from
operator is the only one that must appear in every query you
create.
in Defines the data source used to obtain the data. A data source
can be any object supported by a data provider. The provider
creates a connection between the data source and the LINQ query.
For example, a data source can include an object such as a list
box, an XML file, an SQL Server table, Active Directory, a Web
service, or just about anything else you want to use to collect
information. It’s even possible to work directly with files on your
hard drive using a LINQ query. A query may contain as many matching
pairs of from and in operators as is needed to fully describe the
data sources used in the query.
select Chooses the output information for the query. The simplest
output is the same object you define using the from operator. However, a select operator can include calculated values,
arrays of data, or individual object properties. The limiting
factor in most cases is the functionality offered by the LINQ
provider. All four of the default providers include the complete
range of select operator behaviors. A
query contains only one select operator
and this operator must appear as the last operator in the
query.
where Describes the conditions under which an element appears as part
of the output. The where operator always
requires a Boolean argument to define the output conditions. You
can include as many where operators as
needed to fully describe the output criteria. LINQ interprets the
where operators in the order in which
they appear in the query, so order is important. When a query
doesn’t work as anticipated, try changing the order of the where operators to better describe the output
requirements. Use the debugger to help locate errors in the logical
flow of where operators.
orderby Defines the order in which the output elements appear. The
simplest orderby operator relies on the
object you define using the from
operator. However, an orderby operator
can accept any statement that defines a logical ordering argument
as input — normally numeric, alphabetical, symbol, or binary.
You can use as many orderby operators as
needed to fully describe the output order of the data. As with the
where operator, LINQ interprets the orderby operator in the order in which it
appears in the query. Consequently, the order of the orderby operators is important.
join and on Combines two or more data sources to create a composite view of
the data. The data sources need not rely on the same providers. You
can use as many join operators as
required to fully describe all of the data sources used for a
query. The on operator defines the
criterion used to combine two data sources. Pair the join and on operators
to create a complete data source.
let Creates a calculated value for use within the query. Calculated
values make complex queries easier to formulate. In addition, you
can pass a calculated value onto the caller to provide additional
output information. In many cases, the let operator provides the means for passing
data that LINQ won’t ordinarily pass due to a naming conflict. You
can use as many let operators as required
to provide full query functionality, but you should limit the use
of let to situations where a calculated
value enhances query speed.