LINQ - Query Operators

A set of extension methods forming a query pattern is known as LINQ Standard Query Operators. As building blocks of LINQ query expressions, these operators offer a range of query capabilities like filtering, sorting, projection, aggregation, etc.

LINQ standard query operators can be categorized into the following ones on the basis of their functionality.

• Filtering Operators
• Join Operators
• Projection Operations
• Sorting Operators
• Grouping Operators
• Conversions
• Concatenation
• Aggregation
• Quantifier Operations
• Partition Operations
• Generation Operations
• Set Operations
• Equality
• Element Operators

Filtering Operators

Filtering is an operation to restrict the result set such that it has only selected elements satisfying a particular condition.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
where	Filter values based on a predicate function	where	Where
OfType	Filter values based on their ability to be as a specified type	Not Applicable	Not Applicable

Join Operators

Joining refers to an operation in which data sources with difficult to follow relationships with each other in a direct way are targeted.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
Join	The operator join two sequences on basis of matching keys	join … in … on … equals …	From x In …, y In … Where x.a = y.a
GroupJoin	Join two sequences and group the matching elements	join … in … on … equals … into …	Group Join … In … On …

Projection Operations

Projection is an operation in which an object is transformed into an altogether new form with only specific properties.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
Select	The operator projects values on basis of a transform function	select	Select
SelectMany	The operator project the sequences of values which are based on a transform function as well as flattens them into a single sequence	Use multiple from clauses	Use multiple From clauses

Sorting Operators

A sorting operation allows ordering the elements of a sequence on basis of a single or more attributes.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
OrderBy	The operator sort values in an ascending order	orderby	Order By
OrderByDescending	The operator sort values in a descending order	orderby ... descending	Order By ... Descending
ThenBy	Executes a secondary sorting in an ascending order	orderby …, …	Order By …, …
ThenByDescending	Executes a secondary sorting in a descending order	orderby …, … descending	Order By …, … Descending
Reverse	Performs a reversal of the order of the elements in a collection	Not Applicable	Not Applicable

Grouping Operators

The operators put data into some groups based on a common shared attribute.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
GroupBy	Organize a sequence of items in groups and return them as an IEnumerable collection of type IGrouping	group … by -or- group … by … into …	Group … By … Into …
ToLookup	Execute a grouping operation in which a sequence of key pairs are returned	Not Applicable	Not Applicable

Conversions

The operators change the type of input objects and are used in a diverse range of applications.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
AsEnumerable	Returns the input typed as IEnumerable	Not Applicable	Not Applicable
AsQueryable	A (generic) IEnumerable is converted to a (generic) IQueryable	Not Applicable	Not Applicable
Cast	Performs casting of elements of a collection to a specified type	Use an explicitly typed range variable. Eg:from string str in words	From … As …
OfType	Filters values on basis of their , depending on their capability to be cast to a particular type	Not Applicable	Not Applicable
ToArray	Forces query execution and does conversion of a collection to an array	Not Applicable	Not Applicable
ToDictionary	On basis of a key selector function set elements into a Dictionary and forces execution of a LINQ query	Not Applicable	Not Applicable
ToList	Forces execution of a query by converting a collection to a List	Not Applicable	Not Applicable
ToLookup	Forces execution of a query and put elements into a Lookup on basis of a key selector function	Not Applicable	Not Applicable

Concatenation

Performs concatenation of two sequences and is quite similar to the Union operator in terms of its operation except of the fact that this does not remove duplicates.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
Concat	Two sequences are concatenated for the formation of a single one sequence.	Not Applicable	Not Applicable

Aggregation

Performs any type of desired aggregation and allows creating custom aggregations in LINQ.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
Aggregate	Operates on the values of a collection to perform custom aggregation operation	Not Applicable	Not Applicable
Average	Average value of a collection of values is calculated	Not Applicable	Aggregate … In … Into Average()
Count	Counts the elements satisfying a predicate function within collection	Not Applicable	Aggregate … In … Into Count()
LonCount	Counts the elements satisfying a predicate function within a huge collection	Not Applicable	Aggregate … In … Into LongCount()
Max	Find out the maximum value within a collection	Not Applicable	Aggregate … In … Into Max()
Min	Find out the minimum value existing within a collection	Not Applicable	Aggregate … In … Into Min()
Sum	Find out the sum of a values within a collection	Not Applicable	Aggregate … In … Into Sum()

Quantifier Operations

These operators return a Boolean value i.e. True or False when some or all elements within a sequence satisfy a specific condition.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
All	Returns a value ‘True’ if all elements of a sequence satisfy a predicate condition	Not Applicable	Aggregate … In … Into All(…)
Any	Determines by searching a sequence that whether any element of the same satisfy a specified condition	Not Applicable	Aggregate … In … Into Any()
Contains	Returns a ‘True’ value if finds that a specific element is there in a sequence if the sequence doe not contains that specific element , ‘false’ value is returned	Not Applicable	Not Applicable

Partition Operators

Divide an input sequence into two separate sections without rearranging the elements of the sequence and then returning one of them.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
Skip	Skips some specified number of elements within a sequence and returns the remaining ones	Not Applicable	Skip
SkipWhile	Same as that of Skip with the only exception that number of elements to skip are specified by a Boolean condition	Not Applicable	Skip While
Take	Take a specified number of elements from a sequence and skip the remaining ones	Not Applicable	Take
TakeWhile	Same as that of Take except the fact that number of elements to take are specified by a Boolean condition	Not Applicable	Take While

Generation Operations

A new sequence of values is created by generational operators.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
DefaultIfEmpty	When applied to an empty sequence, generate a default element within a sequence	Not Applicable	Not Applicable
Empty	Returns an empty sequence of values and is the most simplest generational operator	Not Applicable	Not Applicable
Range	Generates a collection having a sequence of integers or numbers	Not Applicable	Not Applicable
Repeat	Generates a sequence containing repeated values of a specific length	Not Applicable	Not Applicable

Set Operations

There are four operators for the set operations, each yielding a result based on different criteria.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
Distinct	Results a list of unique values from a collection by filtering duplicate data if any	Not Applicable	Distinct
Except	Compares the values of two collections and return the ones from one collection who are not in the other collection	Not Applicable	Not Applicable
Intersect	Returns the set of values found t be identical in two separate collections	Not Applicable	Not Applicable
Union	Combines content of two different collections into a single list that too without any duplicate content	Not Applicable	Not Applicable

Equality

Compares two sentences (enumerable ) and determine are they an exact match or not.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
SequenceEqual	Results a Boolean value if two sequences are found to be identical to each other	Not Applicable	Not Applicable

Element Operators

Except the DefaultIfEmpty, all the rest eight standard query element operators return a single element from a collection.

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Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
ElementAt	Returns an element present within a specific index in a collection	Not Applicable	Not Applicable
ElementAtOrDefault	Same as ElementAt except of the fact that it also returns a default value in case the specific index is out of range	Not Applicable	Not Applicable
First	Retrieves the first element within a collection or the first element satisfying a specific condition	Not Applicable	Not Applicable
FirstOrDefault	Same as First except the fact that it also returns a default value in case there is no existence of such elements	Not Applicable	Not Applicable
Last	Retrieves the last element present in a collection or the last element satisfying a specific condition	Not Applicable	Not Applicable
LastOrDefault	Same as Last except the fact that it also returns a default value in case there is no existence of any such element	Not Applicable	Not Applicable
Single	Returns the lone element of a collection or the lone element that satisfy a certain condition	Not Applicable	Not Applicable
SingleOrDefault	Same as Single except that it also returns a default value if there is no existence of any such lone element	Not Applicable	Not Applicable
DefaultIfEmpty	Returns a default value if the collection or list is empty or null	Not Applicable	Not Applicable