can't import Linq using “using System.Linq;” in C#

Question

I have imported external website into visual studio 2015. I am using .net 4.8. I am having trouble to import system.linq. It is saying "the type or namespace does not exist in the namespace system". I dont know Why does it not exist even I am using .net 4.8.

I have also tried to update to visual studio 2019 but same error occurs

using DAL;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;   /// here error is showing
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Text.RegularExpressions;
using CsvHelper;
using System.Text;

the type or namespace does not exist in the namespace system

Answer 1

The System.Linq namespace is in the System.Core assembly (in System.Core.dll). So, it is needed to put system.core.dll in references.

Answer 2

you have to Add System.Linq dll to your project

do this:

1. right click on References in your project from solution explorer
2. click on Manage NuGet Packages
3. search System.Linq then click on Install