csc_net46.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace csc_net46
{
    class Program
    {
        // to install: copy to "packages\Microsoft.Net.Compilers.x.y.z\tools" and run
        static int Main(string[] args)
        {
            var cd = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);

            if (!args.Any())
            {
                // install
                if (!File.Exists(Path.Combine(cd, "csc-orig.exe")))
                {
                    Console.WriteLine("Renaming csc.exe to csc-orig.exe ...");
                    File.Move(Path.Combine(cd, "csc.exe"), Path.Combine(cd, "csc-orig.exe"));
                }
                if (!File.Exists(Path.Combine(cd, "csc-orig.exe.config")))
                {
                    Console.WriteLine("Renaming csc.exe.config to csc-orig.exe.config ...");
                    File.Move(Path.Combine(cd, "csc.exe.config"), Path.Combine(cd, "csc-orig.exe.config"));
                }

                Console.WriteLine($"Replacing csc.exe with {Path.GetFileName(Assembly.GetExecutingAssembly().Location)} ...");
                File.Copy(Assembly.GetExecutingAssembly().Location, Path.Combine(cd, "csc.exe"), overwrite: true);

                if (!File.Exists(Path.Combine(cd, "mscorlib-46.dll")))
                    Console.WriteLine("Warning: mscorlib-46.dll not found in directory");

                Console.WriteLine("Installed.");
                Console.ReadKey(true);
                return 0;
            }
            else
            {
                // find '@C:\foo\blah.cmdline' file
                var cmdlinefilearg = args.FirstOrDefault(a => a.StartsWith("@"));
                if (cmdlinefilearg != null)
                {
                    var cmdline = File.ReadAllText(cmdlinefilearg.Substring(1));
                    // cmdline looks like:
                    //      /t:library /utf8output /nostdlib+ /R:"C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscorlib.dll" /R:"C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\System.Runtime\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Runtime.dll" 
                    //      ... /D:DEBUG /debug+ /optimize- /warnaserror- /w:4 /nowarn:1659;1699;1701;612;618 /langversion:7 /nowarn:1659;1699;1701 
                    //      ... "C:\Users\duncan\AppData\Local\Temp\Temporary ASP.NET Files\vs\e3b3eac8\d85b4372\App_global.asax.rpag7uhc.0.cs" "C:\Users\duncan\AppData\Local\Temp\Temporary ASP.NET Files\vs\e3b3eac8\d85b4372\App_global.asax.rpag7uhc.1.cs"

                    // Replace reference to mscorlib with our 4.6 copy
                    cmdline = Regex.Replace(cmdline,
                        @"/R:""[^""]+mscorlib\.dll""",
                        @"/R:""" + cd + @"\mscorlib-46.dll""",
                        RegexOptions.IgnoreCase);

                    File.WriteAllText(cmdlinefilearg.Substring(1), cmdline);
                }

                // run orig
                var p = Process.Start(new ProcessStartInfo
                {
                    FileName = "csc-orig.exe",
                    Arguments = string.Join(" ", args.Select(a => a.Contains(' ') ? '"' + a + '"' : a)),
                    UseShellExecute = false,
                });
                p.WaitForExit();

                return p.ExitCode;
            }
        }
    }
}