LINQ Tutorial

To Compare two generic lists in c# using linq ,we need to compare some of the properties of the generic lists.

So if we have two generic lists declared as:

List<string> lstA = new List<string>{ "A", "B", "C" };
List<string> lstB = new List<string> { "A", "B", "C" };

then we can compare the lists for equality by using the following logic:

1.Compare the types of generic lists

2.Compare the number of elements in the list

3.Compare if both the lists consists of common items

We can implement the above logic as:

//get the type of elements in the list
var a = lstA.GetType();
var b = lstB.GetType();
//check if the types are same
if (a == b)
{
//check if the count is same
   if (lstA.Count() == lstB.Count())
     {
          //check if both the lists contains the common items
          var filteredSequence = lstA.Where(x => lstB.Contains(x));
          if(filteredSequence.Count()==lstA.Count())
            {
             //lists are same
             }
         else
           {
           //lists are not same
            }
}
}

We can move the above logic in a method as:

private static bool CompareGenericLists(List<string> lstA, List<string> lstB)
{
var a = lstA.GetType();
var b = lstB.GetType();

if (a == b)
{
if (lstA.Count() == lstB.Count())
{
var filteredSequence = lstA.Where(x => lstB.Contains(x));
if (filteredSequence.Count() == lstA.Count())
{
return true;
}
else
{
return false;
}
}
}
else
{
return false;
}
return false;
}

 

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LINQ is used for writing queries using C# for querying in memory collections as well as remote data sources.Any data source which implements IEnumerable<T> can be queried using LINQ.It could be any data source which can be queried using LINQ as long as it implements IEnumerable<T> interface.Data source implementing IEnumerable<T> is also called a sequence.
There are different query operators in LINQ which transforms sequence in some way.Some of the query operators are:

• Select
• Where
• OrderBy

select clause is used to project or select the values fromt the given sequence.So in the simplest form you use the range variable in the select clause:

var lst= from rangeVariable in collection
where rangeVariable.ID ==1
select rangeVariable;

But many times we want to select only some of the values in the range variable object.We can project only some of the properties in the range variable as:

var lst= from rangeVariable in collection
where rangeVariable.ID ==1
select rangeVariable.ID,rangeVariable.Name;

Suppose you want to select custom properties which are not there in the range variable.In such cases you can use anonynous type as:

var lst= from rangeVariable in collection
where rangeVariable.ID ==1
select new {ID=rangeVariable.ID+1,rangeVariable.Name+" ."};

In the following example we have Employee object.We want to select only the last two digits of the employee ID since starting five digits are common for all the employees.Also we want to select only the first name from the full name.

Since the required values in the result set are different from the original collection ,we can select only the last two digits and project the values in a new anonymous type.

public class Employee
{
public string FullName { get; set; }
public string Id { get; set; }
}

var emps = new List<Employee>
{
new Employee{Id="4000028",FullName="Marc Roberts"}
};

//select employees from the list
var intermediate = from e in emps
select new
{
Firstname = e.FullName.Split(new char[] { ' ' })[0],
ShortCode=e.Id.Substring(4,2)
};

foreach(var i in intermediate)
{
Console.WriteLine(i);
}

Instead of the query expression you can project anonymous type using Lambda expression as:

var x= emps.Select(e=> new { Firstname = e.FullName.Split(new char[] { ' ' })[0], ShortCode = e.Id.Substring(4, 2)});

 

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LINQ is used for querying collections in .NET.There are different LINQ providers for querying different data sources.For example if you want to query SQL then you use LINQ to SQL.When working with JSON you can use LINQ to JSON.It is not a LINQ provider but an API which allows working with JSON objects in C#.It is part of JSON.NET framework which provides JSON support in .NET.

Following steps could be used for querying JSON :

1. Install Netwonsoft.Json using nuget package manager or package manager console.

If you are using package manager console then
you can use the following command:
Install-Package Newtonsoft.Json

import the JSON.NET namespaces
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

 

  2. Parse the JSON string using the Parse method of JObject class.

This method will return an object representing the JSON string
JObject.Parse(“JSON String”)

3. Now you can query the JSON object using the linq query

In the following example Employee class is declared.

public class Employee
{
public int ID { get; set; }
public string Name { get; set; }
}

then we are fetching the Employee list using the JSON source in DBHelper class as:

public static class DBHelper
{
   public static JObject GetEmployeesJSON()
        {
            JObject o = JObject.Parse(@"{
                      'Organization': 'MyOrganization',
                      'Address':'Address',
                      'Employees': [
                        {'Name':'1'},
                        {'Name':'2'}
                      ]
                    }");

            return o;
        }
    }

Now you can get the Employee list as:

 var employees= DBHelper.GetEmployeesJSON()["Employees"].Select(emp =>emp).ToList();

LINQ to JSON provides the following useful classes

• JObject
• JArray
• JProperty

Children() method returns the children as an IEnumerable<JToken>.It can be used with JObject or JArray

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In these LINQ Tutorials you will understand what is LINQ and how to use LINQ.

LINQ stands for Language Integrated Query.It is used for writing queries using .NET language such as C#.Data consumed by application comes from different data sources so there is some complexity involved when accessing data from different data sources.When accessing data from XML data source you need to write XML access logic in your application.So you need to mix C# or VB.NET code with XML access logic.We can use the same query syntax to work with different data types such as SQL,Text.

In LINQ we use same query syntax to work with different data sources such as SQL,Text.

Use of LINQ    Language Integrated Query enables us to write queries using C# or VB.NET.

LINQ to Objects   LINQ flavor used to Query in memory collections.

LINQ Queries  Main LINQ Query Operators

references:

https://en.wikipedia.org/wiki/Language_Integrated_Query

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Use of LINQ

Every application needs to work with data.Whether it is a small application or enterprise application ,all the application needs to store details such as the users,data to display,data related to problem domain.

Most of the languages provides different ways to manage the different types of data.For example in C# you can retrieve and update data from different sources:

• Databases such as SQL Server,Oracle
• XML files
• Text Files
• JSON data

.NET provides different technologies for working with these different data sources.For example for working with databases you can write SQL queries and execute them using ADO.NET.For XML data you can using XML APIs such as XPath.Though these different APIs makes working with different data sources convient ,it has one big disadvantage.We need to learn different APIs to work with these different data sources.
This is problem is solved by LINQ.LINQ stands for Lnaguage Integrated Query and allows us to use the same language syntax to query different data sources.

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