1. CS4540 Special Topics in Web Development LINQ to Objects
Chengyu Sun
California State University, Los Angeles

2. C# Language Features Used in LINQ
Implicit typing using var
Anonymous types and tuples
Extension methods
Delegates and lambda expressions

3. Example: Find Data in Array
List in the names that start with "J" in alphabetic order
var results = new List<string>();
foreach (var name in names)
if (name.StartsWith("J")) results.Add(name);
results.Sort();

4. Using LINQ (Language Integrated Query)
var results = from name in names
where name.StartsWith("J")
orderby name
select name; OR
var results = names
.Where(name => name.StartsWith("J"))
.OrderBy(name => name)
.Select(name => name);

5. About LINQ … Query and transform data using SQL-like syntax
Simplify coding
Standardize data access across objects, relational data and XML data
LINQ to Objects works on anything that implements IEnumerable, i.e. arrays and all collections

6. … About LINQ Query result is something that implements IEnumerable
Using var is usually necessary

7. LINQ vs SQL
LINQ is for objects while SQL is for relational data (i.e. rows and column)
Object
Collection
from name in names
where name.StartsWith("J")
orderby name
select name;
Object Method
OrderBy and projection
on the whole object
or some properties

8. Query Syntax vs. Method Syntax
They are equivalent (query expressions are eventually translated into extension methods)
Query syntax tends to be more concise and readable
Query syntax does not cover all extension methods (i.e. sometimes you have to use method syntax)

9. Query Execution
Deferred execution: LINQ queries not executed until the iteration of the results
Getting the latest result
Repeated execution
Immediate execution can be done by calling ToArray() or ToList()
Query operators that return a single value (e.g. Count()) also execute immediately

10. Basic LINQ Query Syntax
An arbitrary name
for an item in the
collection
The name of
the collection
from <item> in <collection>
where <conditions>
select <projection>
Conditions like in SQL
but expressed using
C# operators and
methods
The whole object or some of its
properties (using anonymous
type or tuple syntax)

11. Standard Query Operators
Implemented as extension methods in System.Linq.Enumerable
Only some of the extension methods have equivalent query expression

12. The Company Example
Employee
Project
Company

13. Simple Selection Q1. Find the employees whose last names are Doe
With order by
With projection
With duplicate removal

14. The Difference Between Objects and Relational Data
Q2. find the leader of the project Blue
SQL: JOIN
LINQ: ??

15. Joins Q3. Find Jane Doe's supervisor Query syntax:
from <item1> in <collection1>
join <item2> in <collection2>
on <key1> equals <key2>
Method syntax: ??

16. Set Operations
Q4. Find the employees who are on both project Firestone and project Blue
Set operations: intersect, except, union
Element operations

17. Aggregation Functions
5. Find the number of employees hired in 2015
Aggregation functions: count, sum, average, min, max

18. GroupBy in LINQ …
6. Group the employees by the year in which they were hired
Unlike in SQL, GroupBy in LINQ only creates groups without requiring aggregation
Each group is represented by an IGrouping<TKey, Telements>

19. … GroupBy in LINQ Query syntax:
from <item> in <collection>
group <element> by <key>
Method syntax: ??

20. Simple Query Builder by Chaining Where() …
Employee Search
First Name
Last Name
Year Hired
Search

21. … Simple Query Builder By Chaining Where()
IEnumerable<Employee> results = c.Employees;
if (!String.IsNullOrEmpty(firstName))
results = results.Where(
e => e.FirstName == firstName);
if (!String.IsNullOrEmpty(lastName))
e => e.LastName == lastName);
if (!String.IsNullOrEmpty(year))
e => e.DateHired.Year == Int32.Parse(year));
results = results.Select(e => e);

22. Readings
Pro C# 7: Chapter 12