Part of answer for http://answers.unity3d.com/questions/364580/scripting-works-in-editor-try-it-but-not-build.html?sort=oldest A modified version of https://github.com/mosa/Mono-Class-Libraries/blob/master/mcs/class/System/Microsoft.CSharp/CSharpCodeCompiler.cs for UnityEngine, i just changed the mono and mcs paths to suit my needs. Also i found…

CSharpCodeCompiler for UnityEngine.cs

//
// Mono.CSharp CSharpCodeCompiler Class implementation
//
// Authors:
//	Sean Kasun (seank@users.sf.net)
//	Gonzalo Paniagua Javier (gonzalo@ximian.com)
//
// Copyright (c) Novell, Inc. (http://www.novell.com)
//

//
// Permission is hereby granted, free of charge, to any person obtaining
// a copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
// 
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
// LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
// OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
// WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//

namespace Modified.Mono.CSharp
{
    
    using System;
    using System.CodeDom;
    using System.CodeDom.Compiler;
    using System.ComponentModel;
    using System.IO;
    using System.Text;
    using System.Reflection;
    using System.Collections;
    using System.Collections.Specialized;
    using System.Diagnostics;
    using System.Text.RegularExpressions;
    using System.Linq;

#if NET_2_0
	using System.Threading;
	using System.Collections.Generic;
#endif

    internal class CSharpCodeCompiler : ICodeCompiler
    {
        static string windowsMcsPath;
        static string windowsMonoPath;

#if NET_2_0
		Mutex mcsOutMutex;
		StringCollection mcsOutput;
#endif

        static CSharpCodeCompiler()
        {
            if (Path.DirectorySeparatorChar == '\\')
            {

                // default mono install directory
                string monoFolderPath = "C:\\Program Files (x86)\\Mono";

                if (UnityEngine.Application.isEditor == false)
                {
                    monoFolderPath = Path.Combine(Path.GetDirectoryName(UnityEngine.Application.dataPath), "Mono");
                }


                windowsMonoPath = Path.Combine(monoFolderPath, "bin\\mono.bat");
                if (!File.Exists(windowsMonoPath)) windowsMonoPath = Path.Combine(monoFolderPath, "bin\\mono.exe");

                if (!File.Exists(windowsMonoPath))
                    throw new FileNotFoundException("Windows mono path not found: " + windowsMonoPath);
/*#if NET_4_0
				windowsMcsPath =
					Path.Combine (p, "4.0\\dmcs.exe");
#elif NET_2_0
				windowsMcsPath =
					Path.Combine (p, "2.0\\gmcs.exe");
#else*/
                windowsMcsPath =
                    Path.Combine(monoFolderPath, "lib\\mono\\4.5\\mcs.exe");
//#endif


                if (!File.Exists(windowsMcsPath))
                    throw new FileNotFoundException("Windows mcs path not found: " + windowsMcsPath);
            }
        }

        //
        // Constructors
        //
        public CSharpCodeCompiler()
        {
        }

#if NET_2_0
		public CSharpCodeCompiler (IDictionary <string, string> providerOptions) :
			base (providerOptions)
		{
		}
#endif

        //
        // Methods
        //
        public CompilerResults CompileAssemblyFromDom(CompilerParameters options, CodeCompileUnit e)
        {
            return CompileAssemblyFromDomBatch(options, new CodeCompileUnit[] { e });
        }

        public CompilerResults CompileAssemblyFromDomBatch(CompilerParameters options, CodeCompileUnit[] ea)
        {
            if (options == null)
            {
                throw new ArgumentNullException("options");
            }

            try
            {
                return CompileFromDomBatch(options, ea);
            }
            finally
            {
                options.TempFiles.Delete();
            }
        }

        public CompilerResults CompileAssemblyFromFile(CompilerParameters options, string fileName)
        {
            return CompileAssemblyFromFileBatch(options, new string[] { fileName });
        }

        public CompilerResults CompileAssemblyFromFileBatch(CompilerParameters options, string[] fileNames)
        {
            if (options == null)
            {
                throw new ArgumentNullException("options");
            }

            try
            {
                return CompileFromFileBatch(options, fileNames);
            }
            finally
            {
                options.TempFiles.Delete();
            }
        }

        public CompilerResults CompileAssemblyFromSource(CompilerParameters options, string source)
        {
            return CompileAssemblyFromSourceBatch(options, new string[] { source });
        }

        public CompilerResults CompileAssemblyFromSourceBatch(CompilerParameters options, string[] sources)
        {
            if (options == null)
            {
                throw new ArgumentNullException("options");
            }

            try
            {
                return CompileFromSourceBatch(options, sources);
            }
            finally
            {
                options.TempFiles.Delete();
            }
        }

        private CompilerResults CompileFromFileBatch(CompilerParameters options, string[] fileNames)
        {
            if (null == options)
                throw new ArgumentNullException("options");
            if (null == fileNames)
                throw new ArgumentNullException("fileNames");

            CompilerResults results = new CompilerResults(options.TempFiles);
            Process mcs = new Process();

#if !NET_2_0
            string mcs_output;
            string mcs_stdout;
            string[] mcsOutput;
#endif

            // FIXME: these lines had better be platform independent.
            if (Path.DirectorySeparatorChar == '\\')
            {
                mcs.StartInfo.FileName = windowsMonoPath;
                mcs.StartInfo.Arguments = "\"" + windowsMcsPath + "\" " +
#if NET_2_0
					BuildArgs (options, fileNames, ProviderOptions);
#else
                    BuildArgs(options, fileNames);
#endif
            }
            else
            {
#if NET_2_0
				// FIXME: This is a temporary hack to make code genaration work in 2.0+
#if NET_4_0
				mcs.StartInfo.FileName="dmcs";
#else
				mcs.StartInfo.FileName="gmcs";
#endif
				mcs.StartInfo.Arguments=BuildArgs(options, fileNames, ProviderOptions);
#else
                mcs.StartInfo.FileName = "mcs";
                mcs.StartInfo.Arguments = BuildArgs(options, fileNames);
#endif
            }

#if NET_2_0
			mcsOutput = new StringCollection ();
			mcsOutMutex = new Mutex ();
#endif

            string monoPath = Environment.GetEnvironmentVariable("MONO_PATH");
            if (monoPath == null)
                monoPath = String.Empty;

            string privateBinPath = AppDomain.CurrentDomain.SetupInformation.PrivateBinPath;
            if (privateBinPath != null && privateBinPath.Length > 0)
                monoPath = String.Format("{0}:{1}", privateBinPath, monoPath);

            if (monoPath.Length > 0)
            {
                StringDictionary dict = mcs.StartInfo.EnvironmentVariables;
                if (dict.ContainsKey("MONO_PATH"))
                    dict["MONO_PATH"] = monoPath;
                else
                    dict.Add("MONO_PATH", monoPath);
            }

            mcs.StartInfo.CreateNoWindow = true;
            mcs.StartInfo.UseShellExecute = false;
            mcs.StartInfo.RedirectStandardOutput = true;
            mcs.StartInfo.RedirectStandardError = true;
#if NET_2_0
			mcs.ErrorDataReceived += new DataReceivedEventHandler (McsStderrDataReceived);
#endif

            try
            {
                mcs.Start();
            }
            catch (Exception e)
            {
                Win32Exception exc = e as Win32Exception;
                if (exc != null)
                {
                    /*throw new SystemException(String.Format("Error running {0}: {1}", mcs.StartInfo.FileName,
                                    Win32Exception.W32ErrorMessage(exc.NativeErrorCode)));*/
                }
                throw;
            }

            try
            {
#if NET_2_0
				mcs.BeginOutputReadLine ();
				mcs.BeginErrorReadLine ();
#else
                // If there are a few kB in stdout, we might lock
                mcs_output = mcs.StandardError.ReadToEnd();
                mcs_stdout = mcs.StandardOutput.ReadToEnd();
#endif
                mcs.WaitForExit();

                results.NativeCompilerReturnValue = mcs.ExitCode;
            }
            finally
            {
#if NET_2_0
				mcs.CancelErrorRead ();
				mcs.CancelOutputRead ();
#endif

                mcs.Close();
            }

#if NET_2_0
			StringCollection sc = mcsOutput;
#else
            mcsOutput = mcs_output.Split(System.Environment.NewLine.ToCharArray());
            StringCollection sc = new StringCollection();
#endif

            bool loadIt = true;
            foreach (string error_line in mcsOutput)
            {
#if !NET_2_0
                sc.Add(error_line);
#endif
                CompilerError error = CreateErrorFromString(error_line);
                if (error != null)
                {
                    results.Errors.Add(error);
                    if (!error.IsWarning)
                        loadIt = false;
                }
            }

            if (sc.Count > 0)
            {
                sc.Insert(0, mcs.StartInfo.FileName + " " + mcs.StartInfo.Arguments + Environment.NewLine);

                //results.Output = sc;
            }

            if (loadIt)
            {
                if (!File.Exists(options.OutputAssembly))
                {
                    StringBuilder sb = new StringBuilder();
                    foreach (string s in sc)
                        sb.Append(s + Environment.NewLine);

                    throw new Exception("Compiler failed to produce the assembly. Output: '" + sb.ToString() + "'");
                }

                if (options.GenerateInMemory)
                {
                    using (FileStream fs = File.OpenRead(options.OutputAssembly))
                    {
                        byte[] buffer = new byte[fs.Length];
                        fs.Read(buffer, 0, buffer.Length);
                        results.CompiledAssembly = Assembly.Load(buffer, null, options.Evidence);
                        fs.Close();
                    }
                }
                else
                {
                    // Avoid setting CompiledAssembly right now since the output might be a netmodule
                    results.PathToAssembly = options.OutputAssembly;
                }
            }
            else
            {
                results.CompiledAssembly = null;
            }

            return results;
        }

#if NET_2_0
		void McsStderrDataReceived (object sender, DataReceivedEventArgs args)
		{
			if (args.Data != null) {
				mcsOutMutex.WaitOne ();
				mcsOutput.Add (args.Data);
				mcsOutMutex.ReleaseMutex ();
			}
		}		

		private static string BuildArgs(CompilerParameters options,string[] fileNames, IDictionary <string, string> providerOptions)
#else
        private static string BuildArgs(CompilerParameters options, string[] fileNames)
#endif
        {
            StringBuilder args = new StringBuilder();
            if (options.GenerateExecutable)
                args.Append("/target:exe ");
            else
                args.Append("/target:library ");

            string privateBinPath = AppDomain.CurrentDomain.SetupInformation.PrivateBinPath;
            if (privateBinPath != null && privateBinPath.Length > 0)
                args.AppendFormat("/lib:\"{0}\" ", privateBinPath);

            if (options.Win32Resource != null)
                args.AppendFormat("/win32res:\"{0}\" ",
                    options.Win32Resource);

            if (options.IncludeDebugInformation)
                args.Append("/debug+ /optimize- ");
            else
                args.Append("/debug- /optimize+ ");

            if (options.TreatWarningsAsErrors)
                args.Append("/warnaserror ");

            if (options.WarningLevel >= 0)
                args.AppendFormat("/warn:{0} ", options.WarningLevel);

            if (options.OutputAssembly == null || options.OutputAssembly.Length == 0)
            {
                string extension = (options.GenerateExecutable ? "exe" : "dll");
                options.OutputAssembly = GetTempFileNameWithExtension(options.TempFiles, extension,
                    !options.GenerateInMemory);
            }
            args.AppendFormat("/out:\"{0}\" ", options.OutputAssembly);

            string[] mcsDefaultReferencedAssemblies = { "mscorlib.dll", "System.dll", "System.Xml.dll", "System.Core.dll" };

            foreach (string import in options.ReferencedAssemblies)
            {
                if (mcsDefaultReferencedAssemblies.Contains(Path.GetFileName(import)))
                    continue;

                if (import == null || import.Length == 0)
                    continue;

                args.AppendFormat("/r:\"{0}\" ", import);
            }

            if (options.CompilerOptions != null)
            {
                args.Append(options.CompilerOptions);
                args.Append(" ");
            }

#if NET_2_0
			foreach (string embeddedResource in options.EmbeddedResources) {
				args.AppendFormat("/resource:\"{0}\" ", embeddedResource);
			}

			foreach (string linkedResource in options.LinkedResources) {
				args.AppendFormat("/linkresource:\"{0}\" ", linkedResource);
			}
			
			if (providerOptions != null && providerOptions.Count > 0) {
				string langver;

				if (!providerOptions.TryGetValue ("CompilerVersion", out langver))
#if NET_4_0
					langver = "3.5";
#else
					langver = "2.0";
#endif

				if (langver.Length >= 1 && langver [0] == 'v')
					langver = langver.Substring (1);

				switch (langver) {
					case "2.0":
						args.Append ("/langversion:ISO-2");
						break;

					case "3.5":
						// current default, omit the switch
						break;
				}
			}
#endif

            args.Append(" -- ");
            foreach (string source in fileNames)
                args.AppendFormat("\"{0}\" ", source);
            return args.ToString();
        }
        private static CompilerError CreateErrorFromString(string error_string)
        {
#if NET_2_0
			if (error_string.StartsWith ("BETA"))
				return null;
#endif
            if (error_string == null || error_string == "")
                return null;

            CompilerError error = new CompilerError();
            Regex reg = new Regex(@"^(\s*(?<file>.*)\((?<line>\d*)(,(?<column>\d*))?\)(:)?\s+)*(?<level>\w+)\s*(?<number>.*):\s(?<message>.*)",
                RegexOptions.Compiled | RegexOptions.ExplicitCapture);
            Match match = reg.Match(error_string);
            if (!match.Success)
            {
                // We had some sort of runtime crash
                error.ErrorText = error_string;
                error.IsWarning = false;
                error.ErrorNumber = "";
                return error;
            }
            if (String.Empty != match.Result("${file}"))
                error.FileName = match.Result("${file}");
            if (String.Empty != match.Result("${line}"))
                error.Line = Int32.Parse(match.Result("${line}"));
            if (String.Empty != match.Result("${column}"))
                error.Column = Int32.Parse(match.Result("${column}"));

            string level = match.Result("${level}");
            if (level == "warning")
                error.IsWarning = true;
            else if (level != "error")
                return null; // error CS8028 will confuse the regex.

            error.ErrorNumber = match.Result("${number}");
            error.ErrorText = match.Result("${message}");
            return error;
        }

        private static string GetTempFileNameWithExtension(TempFileCollection temp_files, string extension, bool keepFile)
        {
            return temp_files.AddExtension(extension, keepFile);
        }

        private static string GetTempFileNameWithExtension(TempFileCollection temp_files, string extension)
        {
            return temp_files.AddExtension(extension);
        }

        private CompilerResults CompileFromDomBatch(CompilerParameters options, CodeCompileUnit[] ea)
        {
            if (options == null)
            {
                throw new ArgumentNullException("options");
            }

            if (ea == null)
            {
                throw new ArgumentNullException("ea");
            }

            string[] fileNames = new string[ea.Length];
            StringCollection assemblies = options.ReferencedAssemblies;

            for (int i = 0; i < ea.Length; i++)
            {
                CodeCompileUnit compileUnit = ea[i];
                fileNames[i] = GetTempFileNameWithExtension(options.TempFiles, i + ".cs");
                FileStream f = new FileStream(fileNames[i], FileMode.OpenOrCreate);
                StreamWriter s = new StreamWriter(f, Encoding.UTF8);
                if (compileUnit.ReferencedAssemblies != null)
                {
                    foreach (string str in compileUnit.ReferencedAssemblies)
                    {
                        if (!assemblies.Contains(str))
                            assemblies.Add(str);
                    }
                }

                ((ICodeGenerator)this).GenerateCodeFromCompileUnit(compileUnit, s, new CodeGeneratorOptions());
                s.Close();
                f.Close();
            }
            return CompileAssemblyFromFileBatch(options, fileNames);
        }

        private CompilerResults CompileFromSourceBatch(CompilerParameters options, string[] sources)
        {
            if (options == null)
            {
                throw new ArgumentNullException("options");
            }

            if (sources == null)
            {
                throw new ArgumentNullException("sources");
            }

            string[] fileNames = new string[sources.Length];

            for (int i = 0; i < sources.Length; i++)
            {
                fileNames[i] = GetTempFileNameWithExtension(options.TempFiles, i + ".cs");
                FileStream f = new FileStream(fileNames[i], FileMode.OpenOrCreate);
                using (StreamWriter s = new StreamWriter(f, Encoding.UTF8))
                {
                    s.Write(sources[i]);
                    s.Close();
                }
                f.Close();
            }
            return CompileFromFileBatch(options, fileNames);
        }
    }
}

Thanks for posting this, saved me a lot of time figuring out what process options/args I'd need to invoke the mono compiler.

Thank you very much! It is work good!