LINQ Tutorial

LINQ is of different types based on the source of data.For example for querying in-memory data sources or collections LINQ to objects is used.

LINQ to objects is used for querying collections such as List<T>.Any collection which implements IEnumerable<T> or IEnumerable interfaces can be queried using LINQ.

Arrays can be querid using LINQ queries since it implements IEnumerable.

Below is a sample LINQ to Objects query.

string[] users = { "Marc","John","Robin","Prashant" };
var lstUsers = from user in users
where user="Marc"
select user;

This is called query expression.

We can also write the above query using query operators:

var lstUsers =
users.Where(user => user=="Marc")
.Select(user);

In the  above example we have used the following query operators:

• Where   Where operator is used to filter the collection based on the given condition.In this example we have specified the condition that user should be equal to Marc
• Select  Select operator is used to project or return values as specified.Here we are returning the user.The return values of the operator is also IEnumerable<T>.

Following are some of the main LINQ query operators:

Use	Query operator
Filtering	Where
Projection	Select
Partitioning	Skip, SkipWhile, Take, TakeWhile
Ordering	OrderBy, OrderByDescending,ThenBy
Grouping	GroupBy
Set	Intersect

 

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Intersect() method in C# is an extension method.It applies the set theory of mathematics.In set theory the Intersect method returns the common elements of two sets.
Similarly the Intersect method returns the common elements in two collections.It is defined in the namespace System.Linq

For example if we define two lists as:

IList<int> intList1 = new List<int>() { 100, 200, 300, 400 };
IList<int> intList2 = new List<int>() { 100, 500, 600 };

then we can find the common elements in two lists as:

var commonElements = intList1.Intersect(intList2);

foreach (var item in commonElements)
 Console.WriteLine(item);

the above program will output the value 100.Since 100 is common element in the two lists it is returned by the Intersect method.
Intersect method accepts a single argument which is IEnumerable<T>.

Important point is that IEnumerable<T> argument should be of same type as the collection on which Intersect method is called.
So if you try to invoke the Intersect method as:

IList<string> strList = new List<string>() { "" };
var commonElements = intList1.Intersect(strList);

then you will get compilation error since the types of intList1 and strList are different.

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Here we will see how we can sort rows in a group using LINQ.Also we will select only the top n rows in a group.
This is useful in scenario where we are working on data which consists of different groups.For Sorting and selecting top n rows in a group using in LINQ in C# we will use OrderBy,ThenBy and Where operators of LINQ.

For example there could be group of students in different grades in a school.If we want to find the top 3 students who have scored the maximum marks in a grade then we need to group ,sort and select the top three students:

We have defined the student class as:

public class Student
{
public string Grade
{
get;
set;
}

public string Name
{
get;
set;
}

public int TotalMarks
{
get;
set;
}
}

We have added a sample list of students as:

List<Student> students = new List<Student>()
{

new Student() {Grade = "1", Name = "Amit", TotalMarks = 200}, 
new Student() {Grade = "2", Name = "Greg", TotalMarks = 150}, 
new Student() {Grade = "1", Name = "Marc", TotalMarks = 100}, 
new Student() {Grade = "2", Name = "Rohit", TotalMarks = 125}, 
new Student(){Grade = "1", Name = "John", TotalMarks = 98}, 
new Student() {Grade = "1", Name = "Priya", TotalMarks = 95}, 
new Student() {Grade = "2", Name = "Anthony", TotalMarks = 120}
};

To perform the sorting and grouping we need to follow the below steps:

We perform sorting in LINQ using the orderby operator as:

students.OrderBy(x => x.Grade)

this will order the students based on grade.So we will have a list of grade 1 students followed by a list of grade 2 students.

To sort by multiple columns we use the ThenBy operator as:

students.OrderBy(x => x.Grade).ThenByDescending(x => x.TotalMarks)

this will order the students in a particular grade based on total marks.So we will have list of students in grade 1 sorted by total marks followed by grade 2 students.

To select only the first n rows we will use the where operator:

students.OrderBy(x => x.Grade).ThenByDescending(x => x.TotalMarks)
.Where(x =>
{
if (initialGrade == x.Grade) { count++; } else { count = 1; } initialGrade = x.Grade; if (count <=3) return true;
else
return false;
;} );

Here we are ensuring that we select only the top 3 students in a grade.

Following is the complete code of the application:

public class Student
{
public string Grade
{
get;
set;
}

public string Name
{
get;
set;
}

public int TotalMarks
{
get;
set;
}
}

List<Student> students = new List<Student>()
{

new Student() {Grade = "1", Name = "Amit", TotalMarks = 200}, 
new Student() {Grade = "2", Name = "Greg", TotalMarks = 150}, 
new Student() {Grade = "2", Name = "Rohit", TotalMarks = 125}, 
new Student() {Grade = "1", Name = "Marc", TotalMarks = 100}, 
new Student(){Grade = "1", Name = "John", TotalMarks = 98}, 
new Student() {Grade = "1", Name = "Priya", TotalMarks = 95}, 
new Student() {Grade = "2", Name = "Anthony", TotalMarks = 120}
};

string initialGrade=students[0].Grade;
int count=0;

var topSellingItems = students.OrderBy(x => x.Grade).ThenByDescending(x => x.TotalMarks)
.Where(x =>
{
if (initialGrade == x.Grade) { count++; } else { count = 1; } initialGrade = x.Grade; if (count <=3) return true;
else
return false;
;} );

//GroupBy<Brand, string>(x => x.Name).SelectMany(x => x).OrderBy(x=>x.Qty);
foreach(var item in topSellingItems)
{
Console.WriteLine("name {0} ,grade {1} marks{2}",item.Name, item.Grade, item.TotalMarks);
}

On executing above program we get the following output:

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What is LINQ?

LINQ stands for Language Integrated Query.Before LINQ developers have to use different technologies when working with different data sources.
For example to query databases developers have to use SQL.To query XML they need to use XPath.LINQ enables us to use the same language which we use to develop our application ,to query data sources.
We can use C# or VB.NET to query different data sources such as in memory collections ,databases,XML or any other data source.

What are data providers?

Data providers allows us to query different data sources using LINQ.There are different data providers for different data sources such as:

• LINQ to Objects
• LINQ to SQL
• LINQ to XML
• LINQ to ADO.NET

LINQ providers converts the LINQ queries to the queries which specific data source understands.

What are query expressions?

Query expressions are used to write LINQ queries in similar way we write SQL queries.Below is a simple query expression:

IEnumerable<int> result = from item in listItmes
where item.name > "TV"
select item.price;

The above query finds the price of the product “TV”.

What are the advantages of LINQ?

• It allows to write query using the same language we use to develop application
• It allows to find errors at compile time
• LINQ can be used to query any data source for which LINQ provider exists.

What is Lambda expression?

Lambda expression is like anonymous function.It does not not have a body and return type.It can assigned to a delegate.We use lambda expression in LINQ to assign it to a delegate.

It has the following form:

input arguments=>body

we can assign a lambda expression to a delegate which returns int as:

delegateMult del= x => x * x;

we can call above delegate as:
del(10);

LINQ components?

What are some LINQ providers?

• LINQ to Objects
• LINQ to SQL
• LINQ to XML
• LINQ to JSON
• LINQ to Sharepoint

LINQ components?

LINQ providers
Query operators
Language extensions

Which namespace defines LINQ extension methods?

System.Linq

What are extension methods?

Extension methods allows us to add new methods to an existing type.When we use extension methods we add new methods without modifying the existing type.

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LINQ provides us operator for different tasks such as selection,filtering,ordering.Usually we filter and select the same type as the original type when working with LINQ.

In many scenarios we want to filter and map items to a different list type.For example we are performing LINQ operations on a list of type Product and we want to add the filtered products to a list of string.

In the following example we have a list of Product.We are filtering the items in Product list.We are filtering the items whose price is more than 50.We are using the x => x.Price > 50 lambda expression for this.

Where(x => x.Price > 50)

After we filter the items we are adding the string elements to a list of string type

string.Format("Id: {0} Name: {1} Price: {2}", x.Id, x.Name, x.Price)

Below is the complete example

List<Product> products = new List<Product>();

products.AddRange(new List<Product>{
new Product { Id = 1, Name = "Headset", Price = 100 },
new Product { Id = 2, Name = "Mic", Price = 50 },
new Product { Id = 3, Name = "TV", Price = 1000 }
}
);

List<string> lstProducts = new List<string>();
products.Where(x => x.Price > 50).Select(x => x).ToList().ForEach(x => { lstProducts.Add(string.Format("Id: {0} Name: {1} Price: {2}", x.Id, x.Name, x.Price)); });

foreach(var product in lstProducts)
{
Console.WriteLine(product);
}

Console.ReadLine();

Output:

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