Projection, filtering, and sorting of LINQ learning in C #

Select

A select statement is used to project a set sequence based on a given condition. A select statement can return the filtering results of a combination, return the anonymous type, operate on the returned results, and return the combined subquery results.
The select method is defined as follows:
Public static ienumerable <tresult> select <tsource, tresult> (this ienumerable <tsource> source, func <tsource, tresult> selector)

This extension method is defined in the enumerable type.

// Data source type attributes
VaR result = from student in datasource. Students
Where student. Name. length> 3
Select student. Name;

// Result of data source type filtering
VaR result = from student in datasource. Students
Where student. Name. length> 3
Select student;

// New Type
VaR result = from student in datasource. Students
Where student. Name. length> 3
Select New Student {name = student. Name, studentid = student. studentid };

// Anonymous type
VaR result = from student in datasource. Students
Where student. Name. length> 3
Select New {name = student. Name, studentid = student. studentid };

// Perform operations on the returned results
VaR result = from student in datasource. Students
Where student. Name. length> 3
Select student. tostring ();

The returned result is ienumerable <t> type.

Selectworkflow

Selectworkflow is defined as follows:
Public static ienumerable <tresult> selectult <tsource, tresult> (this ienumerable <tsource> source, func <tsource, ienumerable <tresult> selector)

The prototype shows that the ienumerable <t> interface should be implemented for each element type of the filtering result.
String implements the ienumerable <t> interface, which can be used to construct a scenario: Query all character sequences that comprise the Student name.
VaR result = datasource. Students. selectmany (STR => Str. Name );

The equivalent select clause is:
VaR result = from student in datasource. Students
From Ch in student. Name
Select ch;

It can be considered that selectsequence projects each element of the sequence to ienumerable <(of <(T>)> and merges the result sequence into a sequence.

Distinct

Prototype:
Public static ienumerable <tsource> distinct <tsource> (this ienumerable <tsource> source)
Remove duplicate elements from the projection result set. This operation can only be performed by calling methods.
In the same scenario: query the non-Repeated sequences of all characters that constitute the Student name.
VaR result = datasource. Students. selectiterator (STR => Str. Name). Distinct ();

First, last, Skip, take, single

First select the first element of the set.
VaR result = datasource. Students. Select (student => Student). First (); // student ID: 1, Student name: Andy
The last element of the last selection set.
VaR result = datasource. Students. Select (student => Student). Last (); // student ID: 4, Student name: dark
Skip skips n elements.
VaR result = datasource. Students. Select (student => Student). Skip (2). Count (); // 2
Take: select the first n elements of the set.
VaR result = datasource. Students. Select (student => Student). Skip (2). Take (1). Count (); // 1
Single returns the unique element of the sequence. If the sequence does not exactly contain one element, an exception is thrown.
VaR result = datasource. Students. Select (student => Student). Single (); // exception: sequence contains more than one element

Orderby [Descending]

Sort the set.
Public static iorderedenumerable <tsource> orderby [Descending] <tsource, tkey> (this ienumerable <tsource> source, func <tsource, tkey> keyselector [, icomparer <tkey> comparer])

Tkey must have implemented the icomparer <t> interface.

VaR result = from student in datasource. Students
Orderby student. Name descending
Select student. Name;
// Equivalent:
VaR result = datasource. Students. orderbydescending (student => Student. Name). Select (student => Student. Name );
// Result:
// Dark
// Cindy
// Bill
// Andy