Hi

Onetime i was giving interview then i got this question.Then i told both is used for same purpose. But it was not the exact answer.

In simple word, FirstOrDefault() will handle the exception but First() will not handle the exception.

First() and FirstOrDefault() are two extension methods of the Enumerable class.

So lets test with some example in asp.net

 protected void Page_Load(object sender, EventArgs e)
    {
      
        int[] number = { 1, 5, 6 };
        var num = number.Where(n => n > 10).FirstOrDefault();;
        Response.Write(num);

    }

Note: This code will not throws the exception. So if we have to handle the exception then we can use FirstOrDefault();.

Now lets test with First()

 protected void Page_Load(object sender, EventArgs e)
    {
      
        int[] number = { 1, 5, 6 };
        var num = number.Where(n => n > 10).First();
        Response.Write(num);

    }

Note: This code will throw the exception.

LINQ supports the following clauses that you can use in a query:

1. from—Enables you to specify the data source and a variable for iterating over the

data source (a range variable).

2.where—Enables you to filter the results of a query.

3.select—Enables you to specify the items included in the results of the query.

4.group—Enables you to group related values by a common key.

5.into—Enables you to store the results of a group or join into a temporary variable.

6.orderby—Enables you to order query results in ascending or descending order.

7.join—Enables you to join two data sources using a common key.

8.let—Enables you to create a temporary variable to represent subquery results.

List of some of the more interesting and useful methods  in LINQ:

. Aggregate()—Enables you to apply a function to every item in a sequence.

. Average()—Returns the average value of every item in a sequence.

. Count()—Returns the count of items from a sequence.

. Distinct()—Returns distinct items from a sequence.

. Max()—Returns the maximum value from a sequence.

. Min()—Returns the minimum value from a sequence.

. Select()—Returns certain items or properties from a sequence.

. Single()—Returns a single value from a sequence.

. Skip()—Enables you to skip a certain number of items in a sequence and return the remaining elements.

. Take()—Enables you to return a certain number of elements from a sequence.

. Where()—Enables you to filter the elements in a sequence.

What is the LINQ  To XML ?

It is the type of LINQ. Using Linq we can perform all basic operation with XML file.

I will show one of the easiest example

Step1: Create one XML file in asp.net project  like this

<?xml version=”1.0″ encoding=”utf-8″ ?>

<movies>

<movie>

<Title>Ghost </Title>

<Director>ABC</Director>

</movie>

<movie>

<Title>Forest </Title>

<Director>CDE</Director>

</movie>

<movie>

<Title>Water </Title>

<Director>EEE</Director>

</movie>

<movie>

<Title>Salt</Title>

<Director>WWW</Director>

</movie>

</movies>

Step2: Go to Solution Explorer, Right click on it and add a class “Movie.cs” .

You declare the property like this

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

/// <summary>
/// Summary description for Movie
/// </summary>
public class Movie
{

public string Title { get; set; }
public string Director { get; set; }

}

Step3: Take one gridview Control  in design page

Step4: Add a namespace

using System.Xml.Linq;

Step5: Write the code in code behind page  like this

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Xml.Linq;

public partial class _Default : System.Web.UI.Page

{

protected void Page_Load(object sender, EventArgs e)

{

//var query = from a in

//                XElement.Load(MapPath(“Movie.xml”)).Elements(“movie”)

//            select a;

var query = from a in

XElement.Load(MapPath(“Movie.xml”)).Elements(“movie”)

select new Movie

{

Title = (string)a.Element(“Title”),

Director = (string)a.Element(“Director”)

};

GridView1.DataSource = query;

GridView1.DataBind();

}

}

Step6:You will get op like this