LINQ Query Operators

LINQ provides more than 50 query operators for different functionalities. Every query operator is an extension method.

These operators can be categorized into following one:

Operator Category	LINQ Query Operators Names
Filtering	Where, OfType
Sorting	OrderBy, OrderByDescending, ThenBy, ThenByDescending
Set	Except, Intersect, Union, Distinct
Quantifier	All, Any, Contains
Projection	Select, SelectMany
Partitioning	Skip, SkipWhile, Take, TakeWhile
Join	Join, GroupJoin
Grouping	GroupBy, ToLookup
Sequencing	DefaultIfEmpty, Empty, Range, Repeat
Equality	SequenceEqual
Element	ElementAt, ElementAtOrDefault, First, FirstOrDefault, Last, LastOrDefault, Single, SingleOrDefault
Conversion	AsEnumerable, AsQueryable, Cast, OfType, ToArray, ToDictionary, ToList, ToLookup
Concatenation	Concat
Aggregation	Aggregate, Average, Count, LongCount, Max, Min, Sum

Below is basic explanation of each LINQ operator. I shall cover all operator with examples in next blogs.

Filtering Operators

Filter operators are used to select only those elements from sequence that satisfy a condition. For example, suppose we have ten names in a collection sequence and we have to filter out those names that start with “K”.

Filtering Operators	Description
Where	Filter elements based on the condition.
OfType	This operator takes a Type name and select only those elements in the collection that have matching type.For example, we have a collection of base class (List<BaseClass>) and we have added some items of derived class in the collection. Now we have to find out those item that have of type derived type OfType<DerivedClass>.

Sorting Operators

These operators are used to order the elements in a sequence either ascending or in descending order. We can apply sort elements multiple fields.

Sorting Operators	Description
OrderBy	Sort elements in ascending order
OrderByDescending	Sort elements in descending order
ThenBy	Again sort elements in ascending order. Must use after OrderBy or OrderByDescending operator. This operator is useful when we have to apply sorting based on multiple fields. For example, we want to sort first based on name and then age.
ThenByDescending	Again sort elements in descending order. Must use after OrderBy or OrderByDescending operator. This operator is useful when we have to apply sorting based on multiple fields. For example, we want to sort descending first based on age and then name.

Set Operators

These LINQ query operators are used to find common elements, unique elements, missing elements between two collection sequences.

Set Operators	Description
Distinct	Removes duplicates elements from single sequence
Except	Returns elements which are not present in second sequence
Intersect	Returns common elements between two sequences
Union	Returns unique elements between two sequence

Quantifier Operators

These LINQ query operators are used in conditional statements like if and switch to test whether any or all elements in a sequence satisfy a condition.

Quantifier Operators	Description
All	Returns true when all elements in a sequence satisfy a condition else returns false.
Any	Returns true when any single element in a sequence satisfy a condition else returns false
Contains	Returns true when a sequence contains a matching element else returns false.

Projection Operators

These LINQ query operators are used in create a new type by choosing only those objects or properties that we need in our result set. Projection operators provides cross joins and non equi joins like functionality.

For example, suppose we use two objects in a LINQ query and in the result set we only need two properties of first object and one property of second object so we create a new object by using projection operator that has three properties and return that object from query.

Projection Operators	Description
Select	Transform each element by using lambda expression. For example, students.Select(w => w.Name)
SelectMany	We use this operator when each element is a collection and SelectMany transform each collection in a single collection and returns as a result set. For example, we have two elements {“First”, “Second”} and {“Third”,”Fourth”} and SelectMany returns {“First”, “Second”, “Third”, “Fourth”}

Partitioning Operators

These LINQ query operators are used to filter out elements based on the indexes or by a condition in a sequence. These operators are also used for implementing paging.

Partitioning Operators	Description
Take	Returns only those elements upto specify index. For example, returns only first first elements in a sequence.
TakeWhile	Returns first matching elements which satisfies a given condition.For example, we have numbers { 1, 2, 9, 10, 3, 4} in a sequence and we want to filter out elements which are smaller than 5 then TakeWhile returns only { 1, 2} in a result set as they are appearing first in a list and 9 element dissatisfy this condition.
Skip	Returns only those elements after specify index. For example, skip first five elements and returns all rest elements.
SkipWhile	Skip those elements that satisfies a given condition

Join Operators

These LINQ query operators are used to combine multiple sequence into one sequence just like joins in SQL queries. Join operators offers inner join and left outer joins like functionality.

Join Operators	Description
Join	Joins two sequences based on matching keys.
GroupJoin	Joins two sequence based on matching keys but returns hierarchical output.

Grouping Operators

These LINQ query operators are used to group the data based on specific keys just like groups in SQL.

Grouping Operators	Description
GroupBy	Group elements based on specific key.
ToLookup	Group elements and returns as (Key, Value) pair objects.

Sequencing Operators

These LINQ query operators are used to create a new sequence of values.

Sequencing Operators	Description
DefaultIfEmpty	Returns a default blank sequence. Mainly used for adding default element if source collection is empty.
Empty	Returns an empty sequence.
Range	Returns a range a numeric numbers. For example, we need a new sequence starting from 1 to 10.
Repeat	Returns a sequence of repeating same value at specific number of times. For example, we need five elements of value “Default” in a collection.

Equality Operators

There is only one operator in this category. This operator is used to compare sequences.

Equality Operators	Description
SequenceEqual	Compares two sequences and returns true if they are exact match. Also takes a second parameter of IEqualityComparer<T> if we want to use different comparer.

Element Operators

These LINQ query operators are used to find element at specific index in a sequence.

Element Operators	Description
ElementAt	Returns an element at specific index in a sequence. Throws ArgumentOutOfRangeException exception when index is outside length of sequence.
ElementAtOrDefault	Returns an element at specific index in a sequence. If element not found returns a blank value.
First	Returns first element in a sequence or first element that satisfy a condition. Throws InvalidOperationException exception when sequence is empty.
FirstOrDefault	Returns first element in a sequence or first element that satisfy a condition. If sequence is empty or no element matches the condition then returns default value.
Last	Returns last element in a sequence or last element that satisfy a condition. Throws InvalidOperationException exception when sequence is empty.
LastOrDefault	Returns last element in a sequence or last element that satisfy a condition. If sequence is empty or no element matches the condition then returns default value.
Single	Returns single element in a sequence or element that satisfy a condition. If a sequence has more than one elements then throws InvalidOperationException exception.
SingleOrDefault	Returns single element in a sequence or element that satisfy a condition. If sequence is empty or no element matches the condition then returns default value.

Conversion Operators

These LINQ query operators are used to convert element of sequence to another data types.

Conversion Operators	Description
AsEnumerable	Returns a new sequence of IEnumerable<T>.
AsQueryable	Returns a new sequnce of IQueryable<T>.
Cast	Cast the element to a specific type.
OfType	This operator takes a Type name and select only those elements in the collection that have matching type.For example, we have a collection of base class (List<BaseClass>) and we have added some items of derived class in the collection. Now we have to find out those item that have of type derived type OfType<DerivedClass>.
ToArray	Returns a new sequence of an array[] data type
ToDictionary	Returns a new sequence of generic Dictionary<Key,Value> type.
ToList	Return a new sequence of generic List type.
ToLookup	Group elements and returns as (Key, Value) pair objects.

Concatenation Operators

There is only one operator in this category. Used to create new collection based on two sequences.

Concatenation Operators	Description
Concat	Concatenate two sequences and create one new sequence joining all the elements of both sequences.

Aggregation Operators

These LINQ query operators are used to compute mathematical functions like sum, average, count, max and min operators on elements.

Aggregation Operators	Description
Average	Computer average values of all elements in a sequence.
Count	Counts the number of elements in a sequence.
LongCount	Count the number of elements in a huge sequence. Use LongCount where a sequence has more than int.MaxValue elements. Returns a long data type.
Max	Returns the maximum value in a sequence.
Min	Returns the minimum value in a sequence.
Sum	Returns the sum of all elements in a sequence.
Aggregate	Use for performing custom aggregation operation on a sequence.