Inheritance: IBuildEngine
		public void Remove (RemoteBuildEngine builder)
		{
			foreach (var p in builders) {
				if (p.Value.Remove (builder)) {
					if (p.Value.Count == 0)
						builders.Remove (p.Key);
					return;
                }
            }
        }
		internal static async Task<RemoteProjectBuilder> GetProjectBuilder (TargetRuntime runtime, string minToolsVersion, string file, string solutionFile, int customId, bool lockBuilder = false)
		{
			using (await buildersLock.EnterAsync ())
			{
				//attempt to use 14.0 builder first if available
				string toolsVersion = "14.0";
				string binDir = runtime.GetMSBuildBinPath ("14.0");
				if (binDir == null) {
					toolsVersion = "12.0";
					binDir = runtime.GetMSBuildBinPath ("12.0");
					if (binDir == null) {
						//fall back to 4.0, we know it's always available
						toolsVersion = "4.0";
					}
				}

				// Check the ToolsVersion we found can handle the project
				// The check is only done for the .NET framework since Mono doesn't really have the concept of ToolsVersion.
				// On Mono we'll just try to build with whatever is installed.
				Version tv, mtv;
				if (runtime is MsNetTargetRuntime && Version.TryParse (toolsVersion, out tv) && Version.TryParse (minToolsVersion, out mtv) && tv < mtv) {
					string error = null;
					if (minToolsVersion == "12.0")
						error = "MSBuild 2013 is not installed. Please download and install it from " +
						"http://www.microsoft.com/en-us/download/details.aspx?id=40760";
					throw new InvalidOperationException (error ?? string.Format (
						"Runtime '{0}' does not have MSBuild '{1}' ToolsVersion installed",
						runtime.Id, toolsVersion)
					);
				}

				//one builder per solution
				string builderKey = runtime.Id + " # " + solutionFile + " # " + customId;

				RemoteBuildEngine builder = null;

				if (lockBuilder) {
					foreach (var b in builders.GetBuilders (builderKey)) {
						if (b.Lock ()) {
							builder = b;
							break;
						}
						b.Unlock ();
					}
				} else
					builder = builders.GetBuilders (builderKey).FirstOrDefault ();
				
				if (builder != null) {
					builder.ReferenceCount++;
					return new RemoteProjectBuilder (file, builder);
				}

				return await Task.Run (() => {
					//always start the remote process explicitly, even if it's using the current runtime and fx
					//else it won't pick up the assembly redirects from the builder exe
					var exe = GetExeLocation (runtime, toolsVersion);

					MonoDevelop.Core.Execution.RemotingService.RegisterRemotingChannel ();
					var pinfo = new ProcessStartInfo (exe) {
						UseShellExecute = false,
						CreateNoWindow = true,
						RedirectStandardError = true,
						RedirectStandardInput = true,
					};
					runtime.GetToolsExecutionEnvironment ().MergeTo (pinfo);

					Process p = null;

					try {
						IBuildEngine engine;
						if (!runLocal) {
							p = runtime.ExecuteAssembly (pinfo);

							// The builder app will write the build engine reference
							// after reading the process id from the standard input
							ManualResetEvent ev = new ManualResetEvent (false);
							string responseKey = "[MonoDevelop]";
							string sref = null;
							p.ErrorDataReceived += (sender, e) => {
								if (e.Data == null) {
									if (string.IsNullOrEmpty (sref))
										LoggingService.LogError ("The MSBuild builder exited before initializing");
									return;
								}

								if (e.Data.StartsWith (responseKey, StringComparison.Ordinal)) {
									sref = e.Data.Substring (responseKey.Length);
									ev.Set ();
								} else
									Console.WriteLine (e.Data);
							};
							p.BeginErrorReadLine ();
							p.StandardInput.WriteLine (Process.GetCurrentProcess ().Id.ToString ());
							if (!ev.WaitOne (TimeSpan.FromSeconds (5)))
								throw new Exception ("MSBuild process could not be started");

							byte [] data = Convert.FromBase64String (sref);
							MemoryStream ms = new MemoryStream (data);
							BinaryFormatter bf = new BinaryFormatter ();
							engine = (IBuildEngine)bf.Deserialize (ms);
						} else {
							var asm = System.Reflection.Assembly.LoadFrom (exe);
							var t = asm.GetType ("MonoDevelop.Projects.MSBuild.BuildEngine");
							engine = (IBuildEngine)Activator.CreateInstance (t);
						}
						engine.SetCulture (GettextCatalog.UICulture);
						engine.SetGlobalProperties (GetCoreGlobalProperties (solutionFile));
						foreach (var gpp in globalPropertyProviders)
							engine.SetGlobalProperties (gpp.GetGlobalProperties ());
						builder = new RemoteBuildEngine (p, engine);
					} catch {
						if (p != null) {
							try {
								p.Kill ();
							} catch {
							}
						}
						throw;
					}

					builders.Add (builderKey, builder);
					builder.ReferenceCount = 1;
					builder.Disconnected += async delegate {
						using (await buildersLock.EnterAsync ())
							builders.Remove (builder);
					};
					if (lockBuilder)
						builder.Lock ();
					return new RemoteProjectBuilder (file, builder);
				});
			}
		}
		internal static async void ReleaseProjectBuilder (RemoteBuildEngine engine)
		{
			using (await buildersLock.EnterAsync ()) {
				if (--engine.ReferenceCount != 0)
					return;
				builders.Remove (engine);
			}
			engine.Dispose ();
		}
		public void Add (string key, RemoteBuildEngine builder)
		{
			List<RemoteBuildEngine> list;
			if (!builders.TryGetValue (key, out list))
				builders [key] = list = new List<RemoteBuildEngine> ();
			list.Add (builder);
        }
		internal static async Task<RemoteProjectBuilder> GetProjectBuilder (TargetRuntime runtime, string minToolsVersion, string file, string solutionFile, int customId, bool requiresMicrosoftBuild, bool lockBuilder = false)
		{
			Version mtv = Version.Parse (minToolsVersion);
			if (mtv >= new Version (15,0))
				requiresMicrosoftBuild = true;

			using (await buildersLock.EnterAsync ())
			{
				string binDir;
				var toolsVersion = GetNewestInstalledToolsVersion (runtime, requiresMicrosoftBuild, out binDir);

				Version tv;
				if (Version.TryParse (toolsVersion, out tv) && Version.TryParse (minToolsVersion, out mtv) && tv < mtv) {
					throw new InvalidOperationException (string.Format (
						"Project requires MSBuild ToolsVersion '{0}' which is not supported by runtime '{1}'",
						toolsVersion, runtime.Id)
					);
				}

				//one builder per solution
				string builderKey = runtime.Id + " # " + solutionFile + " # " + customId + " # " + requiresMicrosoftBuild;

				RemoteBuildEngine builder = null;

				if (lockBuilder) {
					foreach (var b in builders.GetBuilders (builderKey)) {
						if (b.Lock ()) {
							builder = b;
							break;
						}
						b.Unlock ();
					}
				} else
					builder = builders.GetBuilders (builderKey).FirstOrDefault ();
				
				if (builder != null) {
					builder.ReferenceCount++;
					return new RemoteProjectBuilder (file, builder);
				}

				return await Task.Run (async () => {
					//always start the remote process explicitly, even if it's using the current runtime and fx
					//else it won't pick up the assembly redirects from the builder exe
					var exe = GetExeLocation (runtime, toolsVersion, requiresMicrosoftBuild);

					MonoDevelop.Core.Execution.RemotingService.RegisterRemotingChannel ();
					var pinfo = new ProcessStartInfo (exe) {
						WorkingDirectory = binDir,
						UseShellExecute = false,
						CreateNoWindow = true,
						RedirectStandardError = true,
						RedirectStandardInput = true,
					};
					runtime.GetToolsExecutionEnvironment ().MergeTo (pinfo);

					Process p = null;

					try {
						IBuildEngine engine;
						if (!runLocal) {
							p = runtime.ExecuteAssembly (pinfo);

							// The builder app will write the build engine reference
							// after reading the process id from the standard input
							var processStartedSignal = new TaskCompletionSource<bool> ();
							string responseKey = "[MonoDevelop]";
							string sref = null;
							p.ErrorDataReceived += (sender, e) => {
								if (e.Data == null) {
									if (string.IsNullOrEmpty (sref))
										LoggingService.LogError ("The MSBuild builder exited before initializing");
									return;
								}

								if (e.Data.StartsWith (responseKey, StringComparison.Ordinal)) {
									sref = e.Data.Substring (responseKey.Length);
									processStartedSignal.SetResult (true);
								} else
									Console.WriteLine (e.Data);
							};
							p.BeginErrorReadLine ();

							p.StandardInput.WriteLine (binDir);

							p.StandardInput.WriteLine (Process.GetCurrentProcess ().Id.ToString ());
							if (await Task.WhenAny (processStartedSignal.Task, Task.Delay (5000)) != processStartedSignal.Task)
								throw new Exception ("MSBuild process could not be started");

							byte [] data = Convert.FromBase64String (sref);
							MemoryStream ms = new MemoryStream (data);
							BinaryFormatter bf = new BinaryFormatter ();
							engine = (IBuildEngine)bf.Deserialize (ms);
						} else {
							var asm = System.Reflection.Assembly.LoadFrom (exe);
							var t = asm.GetType ("MonoDevelop.Projects.MSBuild.BuildEngine");
							engine = (IBuildEngine)Activator.CreateInstance (t);
						}
						engine.SetCulture (GettextCatalog.UICulture);
						engine.SetGlobalProperties (GetCoreGlobalProperties (solutionFile));
						foreach (var gpp in globalPropertyProviders)
							engine.SetGlobalProperties (gpp.GetGlobalProperties ());
						builder = new RemoteBuildEngine (p, engine);
					} catch {
						if (p != null) {
							try {
								p.Kill ();
							} catch {
							}
						}
						throw;
					}

					builders.Add (builderKey, builder);
					builder.ReferenceCount = 1;
					builder.Disconnected += async delegate {
						using (await buildersLock.EnterAsync ())
							builders.Remove (builder);
					};
					if (lockBuilder)
						builder.Lock ();
					return new RemoteProjectBuilder (file, builder);
				});
			}
		}
		public void Dispose ()
		{
			if (!MSBuildProjectService.ShutDown && engine != null) {
				try {
					if (builder != null)
						engine.UnloadProject (builder);
					MSBuildProjectService.ReleaseProjectBuilder (engine);
				} catch {
					// Ignore
				}
				GC.SuppressFinalize (this);
				engine = null;
				builder = null;
			}
		}
		internal RemoteProjectBuilder (string file, RemoteBuildEngine engine)
		{
			this.file = file;
			this.engine = engine;
			builder = engine.LoadProject (file);
			referenceCache = new Dictionary<string, AssemblyReference[]> ();
		}
        /// <summary>
        /// Gets or creates a remote build engine
        /// </summary>
        async static Task <RemoteBuildEngine> GetBuildEngine(TargetRuntime runtime, string minToolsVersion, string solutionFile, string group, object buildSessionId, bool setBusy = false, bool allowBusy = true)
        {
            var binDir = MSBuildProjectService.GetMSBuildBinPath(runtime);

            Version tv = Version.Parse(MSBuildProjectService.ToolsVersion);

            if (Version.TryParse(minToolsVersion, out Version mtv) && tv < mtv)
            {
                throw new InvalidOperationException(string.Format(
                                                        "Project requires MSBuild ToolsVersion '{0}' which is not supported by runtime '{1}'",
                                                        minToolsVersion, runtime.Id)
                                                    );
            }

            // One builder per solution
            string builderKey = runtime.Id + " # " + solutionFile + " # " + group;

            RemoteBuildEngine builder = null;

            // Find builders which are not being shut down

            var candiateBuilders = builders.GetBuilders(builderKey).Where(b => !b.IsShuttingDown && (!b.IsBusy || allowBusy));

            if (buildSessionId != null)
            {
                // Look for a builder that already started the session.
                // If there isn't one, pick builders which don't have any session assigned, so a new one
                // can be started.

                var sessionBuilders = candiateBuilders.Where(b => b.BuildSessionId == buildSessionId);
                if (!sessionBuilders.Any())
                {
                    sessionBuilders = candiateBuilders.Where(b => b.BuildSessionId == null);
                }
                candiateBuilders = sessionBuilders;
            }
            else
            {
                // Pick builders which are not bound to any session
                candiateBuilders = candiateBuilders.Where(b => b.BuildSessionId == null);
            }

            // Prefer non-busy builders

            builder = candiateBuilders.FirstOrDefault(b => !b.IsBusy) ?? candiateBuilders.FirstOrDefault();

            if (builder != null)
            {
                if (setBusy)
                {
                    builder.SetBusy();
                }
                if (builder.BuildSessionId == null && buildSessionId != null)
                {
                    // If a new session is being assigned, signal the session start
                    builder.BuildSessionId = buildSessionId;
                    var si = (SessionInfo)buildSessionId;
                    await builder.BeginBuildOperation(si.Monitor, si.Logger, si.Verbosity, si.Configurations).ConfigureAwait(false);
                }
                builder.CancelScheduledDisposal();
                return(builder);
            }

            // No builder available with the required constraints. Start a new builder

            return(await Task.Run(async() => {
                // Always start the remote process explicitly, even if it's using the current runtime and fx
                // else it won't pick up the assembly redirects from the builder exe

                var exe = GetExeLocation(runtime);
                RemoteProcessConnection connection = null;

                try {
                    connection = new RemoteProcessConnection(exe, runtime.GetExecutionHandler());
                    await connection.Connect().ConfigureAwait(false);

                    var props = GetCoreGlobalProperties(solutionFile, binDir, MSBuildProjectService.ToolsVersion);
                    foreach (var gpp in MSBuildProjectService.GlobalPropertyProviders)
                    {
                        foreach (var e in gpp.GetGlobalProperties())
                        {
                            props [e.Key] = e.Value;
                        }
                    }

                    await connection.SendMessage(new InitializeRequest {
                        IdeProcessId = Process.GetCurrentProcess().Id,
                        BinDir = binDir,
                        CultureName = GettextCatalog.UICulture.Name,
                        GlobalProperties = props
                    }).ConfigureAwait(false);

                    builder = new RemoteBuildEngine(connection, solutionFile);
                } catch {
                    if (connection != null)
                    {
                        try {
                            connection.Dispose();
                        } catch {
                        }
                    }
                    throw;
                }

                builders.Add(builderKey, builder);
                builder.ReferenceCount = 0;
                builder.BuildSessionId = buildSessionId;
                builder.Disconnected += async delegate {
                    using (await buildersLock.EnterAsync().ConfigureAwait(false))
                        builders.Remove(builder);
                };
                if (setBusy)
                {
                    builder.SetBusy();
                }
                if (buildSessionId != null)
                {
                    var si = (SessionInfo)buildSessionId;
                    await builder.BeginBuildOperation(si.Monitor, si.Logger, si.Verbosity, si.Configurations);
                }
                return builder;
            }));
        }
 internal static async Task ReleaseProjectBuilder(RemoteBuildEngine engine)
 {
     using (await buildersLock.EnterAsync().ConfigureAwait(false)) {
         await ReleaseProjectBuilderNoLock(engine);
     }
 }