C# Starter L05-LINQ

Mohammad Shaker
mohammadshaker.com
C# Programming Course
@ZGTRShaker
2011, 2012, 2013, 2014
C# Starter
L05 – LINQ and Lambda Expressions

LINQ
One of the most awesome things ever done!

LINQ
Where’s Java?
2013 with some tool (not as good as .NET)

LINQ
static 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();
}

LINQ
{
select n;
{
}
Console.ReadLine();
}
List<string> queryResults = new List<string>();
foreach (var item in names)
{
if (n.StartsWith(“S”))
{
queryResults.Add(item);
}
}
select n;

LINQ
{
select n;
{
}
Console.ReadLine();
}
List<string> queryResults = new List<string>();
foreach (var item in names)
{
if (n.StartsWith(“S”))
{
queryResults.Add(item);
}
}
select n;
The SAME!

LINQ
{
select n;
{
}
Console.ReadLine();
}
Names beginning with S:
Smith
Smythe
Small
Singh
Samba
Program finished, press Enter/Return to continue:

LINQ
{
select n;
{
}
Console.ReadLine();
}

LINQ
{
select n;
{
}
Console.ReadLine();
}
Alonso
Zheng
Smith
Jones
Smythe
Small
Ruiz
Hsieh
Jorgenson
Ilyich
Singh
Samba

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
select n;

Lambda Expressions
n => n.StartsWith(“S”)
select n;

Lambda Expressions
{
var queryResults = names.Where(n => n.StartsWith("S"));
{
}
Console.ReadLine();
}

Lambda Expressions
{
var queryResults = names.Where(n => n.StartsWith("S"));
{
}
Console.ReadLine();
}
Smith
Smythe
Small
Singh
Samba

Ordering Query Results
{
orderby n
select n;
{
}
Console.ReadLine();
}

{
orderby n
select n;
{
}
Console.ReadLine();
}
Samba
Singh
Small
Smith
Smythe

• Descending order
• This orders the example results as follows:
orderby n descending
Smythe
Smith
Small
Singh
Samba

• 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"));
{
}
Console.ReadLine();
}

{
{
}
Console.ReadLine();
}
Samba
Singh
Small
Smith
Smythe

{
{
}
Console.ReadLine();
}
Samba
Singh
Small
Smith
Smythe
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
class 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; }
}

{
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 customers
where 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.Customer

• 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: 8888
ID: C City: Karachi Country: Pakistan Region: Asia Sales: 7777
ID: D City: Delhi Country: India Region: Asia Sales: 6666
ID: G City: Seoul Country: Korea Region: Asia Sales: 3333
ID: H City: Istanbul Country: Turkey Region: Asia Sales: 2222
ID: I City: Shanghai Country: China Region: Asia Sales: 1111
ID: L City: Jakarta Country: Indonesia Region: Asia Sales: 3000
ID: M City: Tokyo Country: Japan Region: Asia Sales: 4000
ID: P City: Tehran Country: Iran Region: Asia Sales: 7000
ID: R City: Beijing Country: China Region: Asia Sales: 9000

var queryResults =
from c in customers
where c.Region == “Asia”
select c.City;
Mumbai
Karachi
Delhi
Seoul
Istanbul
Shanghai
Jakarta
Tokyo
Tehran
Beijing

• 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 customers
where c.Region == “North America”
select new { c.City, c.Country, c.Sales };
{
}

var queryResults =
from c in customers
{
}
{ City = New York, Country = USA, Sales = 9999 }
{ City = Mexico City, Country = Mexico, Sales = 2000 }
{ City = Los Angeles, Country = USA, Sales = 5000 }

• 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 });
var queryResults =
from c in customers
{
}

• 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 }
var queryResults =
from c in customers
{
}

• 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!
.Where(c => c.Region == “North America”);

• OK, now, instead of this:
.Where(c => c.City == “New York”);

• OK, now, instead of this:
• Works beautifully!
.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 customers
orderby c.Region, c.Country, c.City
select new { c.ID, c.Region, c.Country, c.City };

Ordering By Multiple Levels
var queryResults =
from c in customers
orderby c.Region, c.Country, c.City
select 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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