"Missing method System.Web.Security.FormsAuthentication::get_IsEnabled() in assembly System.Web.dll

Compiler Error Message: CS1703: An assembly with the same identity `mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' 
has already been imported. Consider removing one of the references

 git clone git://github.com/mono/xsp.git
cd xsp
./autogen.sh --prefix=/opt
make
sudo make install

 System.IO.FileNotFoundException: Could not load file or assembly XSP, Version=3.0.0.0

 cd xsp/src/Mono.WebServer.XSP/bin/Debug
sudo gacutil -i Mono.WebServer.XSP.exe
sudo gacutil -i Mono.WebServer.dll

 cd myMvcAppPath
sudo mono Mono.WebServer.XSP.exe

 CS0234: The type or namespace name `Helpers' does not exist in the namespace `System.Web'.
Are you missing an assembly reference?

  <system.web> 
    <compilation debug="true" targetFramework="4.0">
      <assemblies>
        <add assembly="System.Web.Helpers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />        
        <add assembly="System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />        
      </assemblies>
    </compilation>

 $ gacutil -l Microsoft.Web.Infrastructure
The following assemblies are installed into the GAC:
Microsoft.Web.Infrastructure, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35
Number of items = 1

using Microsoft.AspNetCore.Mvc;

namespace Core3xWebApi.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class HomeController : ControllerBase
    {
        [HttpGet]
        public ActionResult<string> Get()
        {
            return null;
        }
    }
}

namespace Core3xWebApi
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers(options =>
            {
                // remove formatter that turns nulls into 204 - No Content responses
                // this formatter breaks SPA's Http response JSON parsing
                options.OutputFormatters.RemoveType<HttpNoContentOutputFormatter>();
                options.OutputFormatters.Insert(0, new HttpNoContentOutputFormatter
                {
                    TreatNullValueAsNoContent = false
                });
            });
        }

Install-Package Microsoft.AspNet.WebApi.HelpPage

@Html.ActionLink("API", "Index", "Help", new { area = "" }, null)

protected void Application_Start()
{
    // Add this code, if not present.
    AreaRegistration.RegisterAllAreas();

    // ...
}

config.SetDocumentationProvider(new XmlDocumentationProvider(
    HttpContext.Current.Server.MapPath("~/App_Data/XmlDocument.xml")));

/// <summary>
/// Gets some very important data from the server.
/// </summary>
public IEnumerable<string> Get()
{
    return new string[] { "value1", "value2" };
}

/// <summary>
/// Looks up some data by ID.
/// </summary>
/// <param name="id">The ID of the data.</param>
public string Get(int id)
{
    return "value";
}

[ApiExplorerSettings(IgnoreApi=true)]
public HttpResponseMessage Get(int id) {  }

 sudo gedit  /etc/apache2/mods-available/mod_mono.conf

 AddType application/x-asp-net .svc .aspx .ashx .asmx .ascx .asax .config .ascx

 sudo gedit /etc/apache2/sites-available/default

 AddHandler mono .svc .aspx .ascx .asax .ashx .config .cs .asmx .axd

  <system.serviceModel>
    <behaviors>
      <serviceBehaviors>
        <behavior>
          <serviceMetadata httpGetEnabled="true" httpGetUrl="wsdl"/>
        </behavior>
      </serviceBehaviors>

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