Basic introduction of LINQ

Introduction

One of the most exciting features of .NET 3.5 would be LINQ(Language Integrated Query), a set of language extensions that allow you to perform queries without leaving the comfort of the C# language.
These query expressions can select, filter, sort, group and transform data. Different LINQ extensions allow you to use the same query expressions with different data sources.

LINQ is a deeply integrated part of .NET 3.5, the C# 2008 language and Visual Basic 2008. However, it isn't an ASP.NET specific feature and can be used equally well in any type of .NET application;

The most common use of LINQ is related to data retrieval such as LINQ to SQL which allows you to query SQL Server database without writing data access code.LINQ to XML allows you to read an XML File without using .NET specialized XML classes. 

LINQ Basics

The easiest way to learn LINQ is by example. Below is the sample code of how you normally retrieve and manipulate the data using ordinary .NET code

Sample code on how to retrieve the list of the employees that have First Name start with H.

List<EmployeeDetails> lstEmployee = db.GetEmployees();
List<EmployeeDetails> matches = new List<EmployeeDetails>();

foreach(EmployeeDetails oEmployee in lstEmployee) 
{
    if(oEmployee.FirstName.StartsWith("H")) 
    {
        matches.Add(oEmployee);
    }
}
gridEmployees.DataSource = matches;
gridEmployees.DataBind();

Sample code on doing the same thing by using LINQ

List<EmployeeDetails> lstEmployee = db.GetEmployees();
IEnumerable<EmployeeDetails> matches;
matches = from oEmployee in lstEmployee
          where oEmployee.FirstName.StartsWith("H")
          select oEmployee;
gridEmployees.DataSource = matches;
gridEmployees.DataBind();

LINQ Expressions

Below, I would explain the basic syntax of LINQ. As you have noticed above, LINQ syntax is similar to SQL queries. LINQ expressions must have a from clause that indicates the datasource and a select clause that indicates the data you want to retrieve.

Projection using LINQ

IEnumerable<STRING> matches;
matches = from oEmployee in employees
          select employee.FirstName;

IEnumerable <EmployeeDetails> matches;
matches = from oEmployee in lstEmployees
          orderby oEmployee.LastName,oEmployee.FirstName
          select oEmployee;

Another Example for Filtering using LINQ

IEnumerable<PRODUCT> matches;
matches = from product in products
          where product.Price > 400 && product.Name.StartsWith("D")
          select product;

private bool IsGoodEmployee(Employee oEmployee) {
    return oEmployee.FirstName.StartsWith("D");
}

IEnumerable<EMPLOYEEDETAILS> matches;
matches = from oEmployee in employees
          where IsGoodEmployee(oEmployee)
          select oEmployee;

IEnumerable <EMPLOYEEDETAILS> matches;
matches = from oEmployee in employees
          orderby oEmployee.LastName
          select oEmployee;

var matches = from oEmployee in employees
              group oEmployee by oEmployee.FamilyName into g
              select g.Key;

var matches = from oEmployee in employees
              group oEmployee by oEmployee.FamilyName into g
              select new {FamilyName= g.Key, Employees = g.Count()};

    var categories = from p in products
        group p by p.Category into g
        select new { Category = g.Key,
                     MaximumPrice = g.Max(p => p.UnitPrice),
                     MinimumPrice = g.Min(p => p.UnitPrice),
                     AveragePrice = g.Average(p => p.UnitPrice) };

LINQ to DataSet

     var matches = from oEmployee in ds.Tables["Employees"].AsEnumerable()
                   where oEmployee.Field<STRING>("LastName").StartsWith("D")
                   select new {First = oEmployee.Field<STRING>("FirstName"),
                               Last = oEmployee.Field<STRING>("LastName")};
     datagrid1.DataSource = matches;
     datagrid1.DataBind();

    var matches = from oEmployee in ds.Employees
                  where oEmployee.LastName.StartsWith("D")
                  select new {First = oEmployee.FirstName, 
                  Last = oEmployee.LastName};

LINQ to SQL

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