LINQ means "Language Integrated Query". LINQ is a new feature  which is used to query the data. By using LINQ, one can query any type of data like XML, Relational, and Objects.  Generally, In Object Oriented Programming, Everything that deal with the class as well as the object. As a programmer, it is compulsory to learn different kinds of Query Languages. Query Languages will differ from database to database as shown in the below figure.

Above figure shows the different kinds of databases.

LINQ is a Data Access Technology like ADO.Net. But Why should one use LINQ?  The answer for this question is "To avoid the Impedance Mismatch". Impedance Mismatch means the gap between the database and the objects as shown in the above figure. Here, we need OR Mapping Tools to fill the gap between Object Oriented Programming(C#.Net) and database. See the below figure, to get an idea.

There are different types of OR Mapping Tools such as

• LINQ
• Hibernate
• NHibernate

let's concentrate on LINQ only. LINQ was introduced in .Net framework 3.0 but, the complete implementations with all the features came in the .Net Framework3.5.

The complete architecture of LINQ is shown in the below figure.

The below are some of the advantages of LINQ.

• Programmer burden will decrease.
• No need to learn different Query Languages.
• Rapid Application Development(RAD) facilities are provided.
• Execution of the queries will be fastest. Because Queries are executed in the front end application only.
• Built in  Security will be provided.

There are two types of data types in LINQ

• Named Types
• Anonymous Types

Named Types are built-in types or user-defined types. In Named types, the user will decide the type of the variable.

int a=10; double b=20.5; String s="john";

Var a=10; Var b=20.5; var s="john";

Anonymous Types can not be used as data fields. Anonymous Types can be used as local variables only.

[csharp] class c1 { var a = 10; //Not Allowed, Raises Compile time error public void f1() { var a = 10;//Allowed } } [/csharp]

An Anonymous Typed Variable should be initialized.

[csharp] var a; //Wrong var a=10; // Right [/csharp]

[csharp] namespace AnonymousFunctionExample { class Example { delegate void sampledelegate(string s);//Anonymous Function with in the delegate static void main() { sampledelegate obj=delegate(string s) { Console.WriteLine("Value is:"+s); }; obj("James"); Console.ReadLine(); } } } [/csharp] See the above example. Above example shows the Anonymous Function with in the delegate.

A  Lambda Expression is an anonymous function or method which returns a value. Lambda Expression derived from a mathematical term 'Lambda'.  Lambda Expression decrease the burden on the programmer. By using Lambda Expression, one can write an anonymous function without a delegate.  

Arguments=>Body of the Function or Expression. Here,'=>' indicates the Lambda Expression.

X=>X*X The above Expression implies that X goes to X*X Here, the first value i.e X means the arguments in the function and the second value i.e X*X means the Expression.

See the below examples to know the difference between An anonymous function and a lambda expression.

[csharp] //An anonymous function delegate int mul(int i); mul obj= delegate(int value) { return (value*2); }; int i = obj(10); [/csharp]

[csharp] //Lambda Expression <span class="code-keyword">delegate</span> <span class="code-keyword">int mul</span> <span class="code-keyword">int(i) mul obj</span> <span class="code-keyword">value=></span> <span class="code-keyword">value*</span> <span class="code-digit">2</span> <span class="code-keyword">int i = obj</span> <span class="code-digit">20</span>[/csharp] Observe the difference between above two examples. Lambda Expression reduces the code and decreases the burden on the programmer.

There are two categories under the Lambda Expression.

• Expression Lambdas
• Statement Lambdas

See the syntax of the Lambda Expression. The right-hand side of the syntax i.e expression is called as Expression Lambdas. Expression Lambdas are used in the Expression Trees. Mainly the Expression Lambda returns the result of the expression.

[csharp] delegate int cube(int x); static void main() { cube obj = a =<a * a * a;//calculates the cube value of given value int Result = obj(2); // pass the value to lambda expression Console.WriteLine("Cube Value is" + Result); //displays the result } [/csharp]

Statement Lambdas is also in the right-hand side of the Lambda Operator. But, Statement Lambdas contains two or more statements which are enclosed in the braces.

(Arguments)=>{Statements;} Statement Lambdas are not used for constructing Expression Trees.

Object initializer is a new feature of C# 3.0 version. Object initializer is used to simplify the object construction. See the below example which shows a normal object declaration.

[csharp] class person { //Properties public int Name { get; set; } public string Age { get; set; } } //To create an object for the above class, we can write the following piece of code in C# 2.0 Person obj = new Person(); obj.Name="James"; obj.Age=30; [/csharp] But In C# 3.0, it is provided with the object initializer which reduces a little burden. See the below code to know Object initializer. [csharp] class person { //Properties public int Name { get; set; } public string Age { get; set; } } Person obj=new Person{Name="James",Age=30}; [/csharp] One can also use the nested concept here. That means, one can declare the object with in the object by using this object initializer.

Collection initializer is also a new feature which is introduced in the C# 3.0. Collection initializer is used to initialize the collection.

The C# supports 3 types of collections.

• Normal Collections
• Specialized Collections
• Generic Collections

Normal Collections are like Array list, Stack, Queue, and Hash Table. Specialized Collections are like List Dictionary, String Dictionary, and String Collection. Generic Collections are like following List() Stack() HashList()

CollectionType<datatype> CollectionName=new CollectionType<datatype>{Initializing values separeted with ','};

[csharp] class Employee { // Auto-implemented properties. public int EmpId { get; set; } public string EmpName { get; set; } public int EmpAge { get; set; } } class Example { static void main() { List<Employee> employee= new List<Employee>() { new Employee{EmpId=1,EmpName="James",EmpAge=23} new Employee{EmpId=2,EmpName="Jan",EmpAge=43} }; foreach(var x in employee) { Console.WriteLine(+x.EmpId+x.EmpName+x.EmpAge); } Console.ReadLine(); } } [/csharp]

A Query is a type of language which is used to communicate with the Database and returns some Output or Data. In LINQ, a Query is always will work on some data source which is inherited from the iEnumerable interface. LINQ Query contains 3 parts.

• From Clause
• Select Clause
• Where Clause

To execute the Query, there are two methods in LINQ.

1. Differed Execution
2. Forced Immediate Execution

Differed Execution In this method, Query will be executed when a foreach loop is iterated. As long as the loop is being  iterated query will be running. To print the elements, One have to use the for each loop. [csharp] foreach(var x in R) { Console.WriteLine(X+""); } [/csharp] Forced Immediate Execution In this method, Query will be run and it will return the result immediately. If one want to print the count of elements, Then write the code as shown below. [csharp] Console.WriteLne("Count of Elemts is"+R.Count()); [/csharp] Here, when using R.Count() immediately Query will be run & will return the result, and query will be run only once.

Obtain the Data Source. Example: List<int> L1=new List<int>{1,2,3,4,5}

Prepare Query. Example: var R= from Num in L1 where Num% 2==0 select Num;

Execute the Query.

OR Mapping Tool means Object Relational Mapping Tool. OR Mapping Tool is used to build a bridge between Database and the Object Oriented Programming(C# or VB).

The following diagram shows you the relation between them. OR Mapping Tool is used to map the database structure into the front end. Here, it will be converted completely into "Object Oriented Programming Format".

• In general. the database what we mapped will be converted into the class by adding "Data Context" word at the end.
• This class contains a method known as "Submit Changes". This method is used to save the modifications made in the data back to the database.
• A class will be created on the Table Name and the Fields of the table will become as Properties.
• As well as the class which created with the table name will become as a property in the Data Context class.

Data Context Class also contains two methods.

• Insert on Submit (Table Class Type)
• Delete on Submit (Table Class Type)

Insert on Submit Insert on Submit is used to insert a record into the respective table when the data context class sends the request to database. Delete on Submit Delete on submit is used to delete a record from the respective table when the data context class sends the request to database.

The following figure shows the structure when mapping takes place.