How to fix error The type or namespace name 'Linq' does not exist in the namespace 'System'

Question

ASP.NET MVC4 application contains Helper.chtml file in App_Code folder containing Razor helpers. After upgrading to Visual Studio Communitity edition compile error

Error   CS0234  The type or namespace name 'Linq' does not exist in the namespace 'System' (are you missing an assembly reference?) C:\Users\user\AppData\Local\Temp\Temporary ASP.NET Files\admin\e209c0e0\c74032ba\Sources_App_Code\helper.cshtml.72cecc2a.cs 16

Starts to occur. Generated code contains:

namespace ASP {
    using System;
    using System.Collections.Generic;
    using System.IO;
    using System.Linq;  // Error occurs in this line
    using System.Net;
    using System.Web;
    using System.Web.Helpers;
    using System.Web.Security;
    using System.Web.UI;
    using System.Web.WebPages;
    using System.Web.WebPages.Html;

    #line 1 "C:\Web\Erp\App_Code\Helper.cshtml"
    using System.Web.Mvc;

    #line default
    #line hidden

    #line 2 "C:\Web\Erp\App_Code\Helper.cshtml"
    using System.Threading;

    #line default
    #line hidden

    #line 3 "C:\Web\Erp\App_Code\Helper.cshtml"
    using MyApp.Business;

    #line default
    #line hidden


    public class Helper : System.Web.WebPages.HelperPage {

Error points to using System.Linq; line.

How to fix this ? Using ASP.NET4

I tried to add reference to System.Core to project but got error that it is already referenced. web.confing contains

<compilation debug="true">
</compilation>

I changed it to

<compilation debug="true" targetFramework="4.0">
</compilation>

according The type or namespace name 'Linq' does not exist in the namespace 'System' and https://social.msdn.microsoft.com/Forums/en-US/c970ea3d-5238-4ec7-8859-e16b40dccd3d/the-type-or-namespace-name-linq-does-not-exist-in-the-namespace-systemare-you-missing-an?forum=linqprojectgeneral

but after that compile error changes to

Error   CS0234  The type or namespace name 'global_asax' does not exist in the namespace 'ASP' (are you missing an assembly reference?)

which points to code

    protected static ASP.global_asax ApplicationInstance {
        get {
            return ((ASP.global_asax)(Context.ApplicationInstance));
        }
    }

in generated helper.cshtml.72cecc2a.cs file.

How to fix this error ?

Update

Relevant parts of web.config are:

<?xml version="1.0" encoding="utf-8"?>

<configuration>
  <system.data>
    <DbProviderFactories>
      <clear />
      <add name="Npgsql Data Provider" invariant="Npgsql" support="FF" description=".Net Framework Data Provider for Postgresql Server" type="Npgsql.NpgsqlFactory, Npgsql" />
    </DbProviderFactories>
  </system.data>

  <system.web>
    <httpModules>
      <remove name="RoleManager" />
    </httpModules>

    <compilation debug="true">
    </compilation>

    <customErrors mode="Off" />
    <globalization uiCulture="auto" culture="auto" enableClientBasedCulture="true" />
    <pages>
      <namespaces>
        <add namespace="MYApp.Business" />
        <add namespace="System.Web.Helpers" />
        <add namespace="System.Web.Mvc" />
        <add namespace="System.Web.Mvc.Ajax" />
        <add namespace="System.Web.Mvc.Html" />
        <add namespace="System.Web.Routing" />
        <add namespace="System.Web.Optimization" />
        <add namespace="System.Web.WebPages" />
      </namespaces>
    </pages>
  </system.web>

  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="WebGrease" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-1.6.5135.21930" newVersion="1.6.5135.21930" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="Antlr3.Runtime" publicKeyToken="eb42632606e9261f" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.5.0.2" newVersion="3.5.0.2" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
</configuration>

Answer 1

Try add System.Linq namespace in your default controller.