title: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) fixed
body: <p>I'm trying to make a web browser application, but when I relaunch Visual Studio Community 2019, it shows me about 500 errors such as <code>The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?)</code>.</p>
  <p>I tried reloading Visual Studio Community 2019 a few times, but still doesn't work either.</p>
  <p>Some project infos:</p>
  <ul>
  <li>Target framework: .NET Framework 4.8</li>
  <li>NuGet Packages used: EasyTabs, CefSharp.WinForm</li>
  </ul>
  <pre class="lang-cs prettyprint-override"><code>using System;
  using System.Collections.Generic;
  using System.ComponentModel;
  using System.Data;
  using System.Drawing;
  using System.Linq;
  using System.Text;
  using System.Threading.Tasks;
  using System.Windows.Forms;
  using CefSharp;
  using CefSharp.WinForms;
  using EasyTabs;
  </code></pre>