C# Starter L05-LINQ

Mohammad Shakermohammadshaker.com

C# Programming Course@ZGTRShaker

2011, 2012, 2013, 2014

C# StarterL05 – LINQ and Lambda Expressions

LINQOne of the most awesome things ever done!

LINQWhere’s Java?

2013 with some tool (not as good as .NET)

LINQstatic void Main(string[] args)

{string[] names = { "Alonso", "Zheng", "Smith", "Jones", "Smythe",

"Small", "Ruiz", "Hsieh", "Jorgenson", "Ilyich", "Singh", "Samba"};var queryResults = from n in names

where n.StartsWith("S")select n;

Console.WriteLine("Names beginning with S:");foreach (var item in queryResults) {

Console.WriteLine(item);}Console.Write("Program finished, press Enter/Return to continue:");Console.ReadLine();

List<string> queryResults = new List<string>();foreach (var item in names){

if (n.StartsWith(“S”)){

queryResults.Add(item);}

var queryResults = from n in nameswhere n.StartsWith("S")select n;

The SAME!

Names beginning with S:SmithSmytheSmallSinghSambaProgram finished, press Enter/Return to continue:

select n;Console.WriteLine("Names beginning with S:");foreach (var item in queryResults) {

Names beginning with S:AlonsoZhengSmithJonesSmytheSmallRuizHsiehJorgensonIlyichSinghSambaProgram finished, press Enter/Return to continue:

Lambda Expressions

Tons of code in a line

Lambda Expressions

• The operator => is called the lambda operator.

n => n < 1000

“ Lambda ” comes from the lambda calculus, the mathematical formalism underlying

functional programming languages, which are the kind of programming on which LINQ is

based. If you are interested, check out sources such as the Wikipedia article on lambda

calculus. You don ’ t need to understand the mathematics to use lambda functions, although

understanding functional programming is helpful for advanced LINQ programming.

Lambda Expressions

n => n.StartsWith(“S”)

Lambda Expressions

n => n.StartsWith(“S”)

Lambda Expressions

static void Main(string[] args){

string[] names = { "Alonso", "Zheng", "Smith", "Jones", "Smythe","Small", "Ruiz", "Hsieh", "Jorgenson", "Ilyich", "Singh", "Samba"};

var queryResults = names.Where(n => n.StartsWith("S"));Console.WriteLine("Names beginning with S:");

foreach (var item in queryResults) {

Console.WriteLine(item);}

Console.Write("Program finished, press Enter/Return to continue:");Console.ReadLine();

Lambda Expressions

Names beginning with S:SmithSmytheSmallSinghSambaProgram finished, press Enter/Return to continue:

Query Techniques

Ordering Query Results

var queryResults = from n in nameswhere n.StartsWith("S")orderby nselect n;

Console.WriteLine("Names beginning with S:");

var queryResults = from n in nameswhere n.StartsWith("S")orderby nselect n;

Names beginning with S:SambaSinghSmallSmithSmytheProgram finished, press Enter/Return to continue:

• Descending order

• This orders the example results as follows:

orderby n descending

SmytheSmithSmallSinghSamba

• Ordering Using Method Syntax

var queryResults = names.OrderBy(n => n).Where(n => n.StartsWith(“S”));

var queryResults = names.OrderBy(n => n).Where(n => n.StartsWith("S"));

IOrderedEnumerable < string > and IEnumerable < string >

• Descending order

var queryResults = names.OrderByDescending(n => n).Where(n => n.StartsWith(“S”));

• What does this do?

var queryResults =names.OrderBy(n => n.Substring(n.Length-1)).Where(n => n.StartsWith(“S”));

Querying Complex Objects

Querying Complex Objectsclass Customer{

public string ID { get; set; }public string City { get; set; }public string Country { get; set; }public string Region { get; set; }public decimal Sales { get; set; }

Querying Complex Objectsstatic void Main(string[] args){

List < Customer > customers = new List < Customer > {

new Customer { ID=”A”, City=”New York”, Country=”USA”,Region=”North America”, Sales=9999},new Customer { ID=”B”, City=”Mumbai”, Country=”India”,Region=”Asia”, Sales=8888 },new Customer { ID=”C”, City=”Karachi”, Country=”Pakistan”,Region=”Asia”, Sales=7777 },new Customer { ID=”D”, City=”Delhi”, Country=”India”,Region=”Asia”, Sales=6666 },new Customer { ID=”E”, City=”S ã o Paulo”, Country=”Brazil”,Region=”South America”, Sales=5555 },new Customer { ID=”F”, City=”Moscow”, Country=”Russia”,Region=”Europe”, Sales=4444 },…

var queryResults =from c in customerswhere c.Region == “Asia”select c;

Console.WriteLine(“Customers in Asia:”);foreach (Customer c in queryResults){

Console.WriteLine(c);}

Console.Write(“Program finished, press Enter/Return to continue:”);Console.ReadLine();

• The output:

• Why?!

– Because ToString() is called for each Customer object. ToString() Is inherited directly

from the object class.

• Solution: Override the ToString() method.

Customers in Asia:CSharpCourse2011.CustomerCSharpCourse2011.CustomerCSharpCourse2011.CustomerCSharpCourse2011.CustomerCSharpCourse2011.CustomerCSharpCourse2011.CustomerCSharpCourse2011.CustomerCSharpCourse2011.CustomerCSharpCourse2011.CustomerCSharpCourse2011.CustomerProgram finished, press Enter/Return to continue:

• From this:

• To this:class Customer{

public override string ToString(){

return “ID: “ + ID + “ City: “ + City + “ Country: “ + Country +“ Region: “ + Region + “ Sales: “ + Sales;

class Customer{

• The output now is:

Customers in Asia:ID: B City: Mumbai Country: India Region: Asia Sales: 8888ID: C City: Karachi Country: Pakistan Region: Asia Sales: 7777ID: D City: Delhi Country: India Region: Asia Sales: 6666ID: G City: Seoul Country: Korea Region: Asia Sales: 3333ID: H City: Istanbul Country: Turkey Region: Asia Sales: 2222ID: I City: Shanghai Country: China Region: Asia Sales: 1111ID: L City: Jakarta Country: Indonesia Region: Asia Sales: 3000ID: M City: Tokyo Country: Japan Region: Asia Sales: 4000ID: P City: Tehran Country: Iran Region: Asia Sales: 7000ID: R City: Beijing Country: China Region: Asia Sales: 9000Program finished, press Enter/Return to continue:

var queryResults =from c in customerswhere c.Region == “Asia”select c.City;

MumbaiKarachiDelhiSeoulIstanbulShanghaiJakartaTokyoTehranBeijing

Selecting Multiple Properties

• Let’s see this:

select c.City, c.Country, c.Sales

• It’s an error. Unlike in SQL, LINQ does not allow multiple fields in a select clause!

– So, what’s the solution?

• Just creating a new object on-the-fly in the select clause to hold the results you want for your query!

var queryResults =from c in customerswhere c.Region == “North America”select new { c.City, c.Country, c.Sales };

foreach (var item in queryResults){

{ City = New York, Country = USA, Sales = 9999 }{ City = Mexico City, Country = Mexico, Sales = 2000 }{ City = Los Angeles, Country = USA, Sales = 5000 }Program finished, press Enter/Return to continue:

• Now, instead of all this:

• Let’s try this:

var queryResults = customers.Where(c => c.Region == “North America”).Select(c => new { c.City, c.Country, c.Sales });

• Now, instead of all this:

{ City = New York, Country = USA, Sales = 9999 }{ City = Mexico City, Country = Mexico, Sales = 2000 }{ City = Los Angeles, Country = USA, Sales = 5000 }Program finished, press Enter/Return to continue:

• Now, instead of this:

var queryResults = customers.Select(c => new { c.City, c.Country, c.Sales }).Where(c => c.Region == “North America”);

• Now, instead of this:

• It’s a Compiler Error! Because: The Region property is not included in the

anonymous type.

{c.City, c.Country, c.Sales } created by the Select() projection. The

compiler doesn’t know it yet!

var queryResults = customers.Select(c => new { c.City, c.Country, c.Sales }).Where(c => c.Region == “North America”);

• OK, now, instead of this:

var queryResults = customers.Select(c => new { c.City, c.Country, c.Sales }).Where(c => c.City == “New York”);

• OK, now, instead of this:

• Works beautifully!

var queryResults = customers.Select(c => new { c.City, c.Country, c.Sales }).Where(c => c.City == “New York”);

• Distinct

var queryResults = customers.Select(c => c.Region).Distinct();

var queryResults = (from c in customers select c.Region).Distinct();

Ordering By Multiple Levels

var queryResults =from c in customersorderby c.Region, c.Country, c.Cityselect new { c.ID, c.Region, c.Country, c.City };

Ordering By Multiple Levels

var queryResults =from c in customersorderby c.Region, c.Country, c.Cityselect new { c.ID, c.Region, c.Country, c.City };

{ ID = O, Region = Africa, Country = Egypt, City = Cairo }{ ID = J, Region = Africa, Country = Nigeria, City = Lagos }{ ID = R, Region = Asia, Country = China, City = Beijing }{ ID = I, Region = Asia, Country = China, City = Shanghai }{ ID = D, Region = Asia, Country = India, City = Delhi }{ ID = B, Region = Asia, Country = India, City = Mumbai }{ ID = L, Region = Asia, Country = Indonesia, City = Jakarta }{ ID = P, Region = Asia, Country = Iran, City = Tehran }{ ID = M, Region = Asia, Country = Japan, City = Tokyo }{ ID = G, Region = Asia, Country = Korea, City = Seoul }….

Drilling further

• Many other techniques

• Grouping

• Joining

• Take

• Skip

• ….

C# Starter L05-LINQ

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