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