Query operations on LINQ topics

In the previous chapter we focused on LINQ query expressions, and LINQ provided some basic query expressions, as well as dozens of query operations. such as filtering operations, aggregation operations, projection operations and so on. These query operations make it easier to work with data sources.

LINQ provides dozens of query operations, most of which are for sequences that implement the iqueryable<t> and Ienumerbale<t> interfaces.

Serial number	Query operations	The corresponding query expression	description
1	Aggregate		Custom Aggregation Operations
2	All		Detects whether all elements in a sequence meet the specified criteria
3	Any		Detects if there are elements in the sequence that meet the specified criteria
4	Average		Calculates the average of all elements in a sequence
5	Cast		Converts an element type in a sequence to a specified type
6	Contact		Appends all elements of a sequence to another sequence and forms a new sequence
7	Contains		Detects if the specified element exists in the sequence
8	Count		Calculates the number of all elements in a sequence or calculates the number of elements that meet certain criteria
9	DefaultIfEmpty		Returns a sequence of type ienumerable<t> that returns a sequence containing only one element if the sequence is empty (the value is the default value or the specified value)
10	Distinct		Duplicate elements in the data source can be stripped back to a new sequence, and a comparer can be specified to compare whether two elements are equal
11	Element		Returns the element at the specified index in the collection
12	Elementatordefault		Returns the element at the specified index in the collection, returning the default value if the index is out of range
13	Empty		Returns an empty sequence of type ienumerable<t>
14	Equalall/sequenceequal		Determine if two sequences are equal
15	Except		Computes the difference set of two sets (a collection of elements that belong to one collection and not to another collection)
16	First		Returns the first element of a collection or returns the first element that satisfies a condition
17	FirstOrDefault		Returns the first element of the collection, or returns the first element that satisfies the condition, or returns a default value if there are no elements that satisfy the condition.
18	GroupBy	Group clause	Group elements in a sequence
19	GroupJoin	Join clause	Generates a hierarchical data structure that matches all elements in the first sequence with the related elements in the second sequence, and in the query results, the elements in the first collection appear, if the elements in the first sequence are related to the elements in the second sequence Match, then the found element is used otherwise null
20	Intersect		You can calculate the intersection of two sets, that is, the elements in the new collection exist with the first collection and the second collection
21st	Join	Join clause	Similar to inner join in SQL Server, requires that the join relationship of an element must satisfy both the two data sources being joined
22	Last		Returns the last element of the collection, or returns the last element that satisfies the specified condition
23	LastOrDefault		Returns the last element of the collection, or returns the last element that satisfies the specified condition, or returns a default value if the element does not exist
24	LongCount		Calculates the number of elements in a collection, or the number of elements that meet a specified condition, typically used to calculate the number of elements in a large collection
25	Max		Calculates the maximum value of an element in a sequence
26	Min		Calculates the minimum value of an element in a sequence
27	OfType		Filters the elements of a specified type from a sequence and constructs it as a sequence
28	By	ORDER BY clause	Sort the elements in a sequence in ascending order based on the keyword
29	OrderByDescending	ORDER BY clause	Sort the elements in a sequence in descending order based on the keyword
30	Range		Returns a sequence of numbers for a specified range
31	Repeat		Returns a sequence containing duplicate values of type ienum<t>
32	Reverse		Invert elements in a sequence
33	Select	SELECT clause	Projects the elements in the data source into a new sequence and specifies the type and representation of the element,
34	SelectMany	Multiple from clauses	Casts an element from the data source into a new sequence and specifies the type and representation of the element, but SelectMany can apply a function over multiple sequences and merge the results into a sequence
35	Single		Returns the only element in the collection or returns the only element that satisfies the specified condition
36	Singleordefault		Returns the only element in the collection, or returns the only element that satisfies the specified condition, and returns the default value if the element does not exist
37	Skip		Skips a specified number of elements in a sequence, and then returns a sequence of the remaining elements
38	SkipWhile		Skips a specified number of elements in a sequence, and then returns a sequence of the remaining elements
39	Sum		Computes the and of all the elements in the sequence
40	Take		Gets the specified number of elements from the beginning of the data source, and then returns a new sequence of these elements
41	TakeWhile		Gets the specified number of elements from the beginning of the data source, and then returns a new sequence of these elements
42	ThenBy	ORDER BY clause	Sort sequences in ascending order based on secondary keywords
43	ThenByDescending	ORDER BY clause	Sort sequences in ascending order based on secondary keywords
44	ToArray		Convert a sequence of type ienumerable<t> to an array of type t[]
45	ToDictionary		Converts a sequence of type ienumerable<t> to an array of type dictionary<tkey,tvalue>
46	ToList		Converts a sequence of type ienumerable<t> to an array of type list<t>
47	ToLookup		Puts the elements of a sequence into a one-to-many dictionary sequence by key value lookup<tkey,tvalue>
48	Union		Computes the set of two sets, consisting of a sequence of elements that belong to the first collection or to a second set
49	Where	Where clause	Processing logical operators to filter data from the data source

Query operations on LINQ topics