async Task <BuildResult> OnBuild(ProgressMonitor monitor, ConfigurationSelector configuration)
        {
            if (!project.OnGetNeedsBuilding(configuration))
            {
                monitor.Log.WriteLine(GettextCatalog.GetString("Skipping project since output files are up to date"));
                return(new BuildResult());
            }

            if (!project.TargetRuntime.IsInstalled(project.TargetFramework))
            {
                BuildResult res = new BuildResult();
                res.AddError(GettextCatalog.GetString("Framework '{0}' not installed.", project.TargetFramework.Name));
                return(res);
            }

            bool hasBuildableFiles = false;

            foreach (ProjectFile pf in project.Files)
            {
                if (pf.BuildAction == BuildAction.Compile || pf.BuildAction == BuildAction.EmbeddedResource)
                {
                    hasBuildableFiles = true;
                    break;
                }
            }
            if (!hasBuildableFiles)
            {
                return(new BuildResult());
            }

            if (project.LanguageBinding == null)
            {
                BuildResult langres = new BuildResult();
                string      msg     = GettextCatalog.GetString("Unknown language '{0}'. You may need to install an additional extension to support this language.", project.LanguageName);
                langres.AddError(msg);
                monitor.ReportError(msg, null);
                return(langres);
            }

            BuildResult           refres         = null;
            HashSet <ProjectItem> itemsToExclude = new HashSet <ProjectItem> ();

            foreach (ProjectReference pr in project.References)
            {
                if (pr.ReferenceType == ReferenceType.Project)
                {
                    // Ignore non-dotnet projects
                    Project p = project.ParentSolution != null?pr.ResolveProject(project.ParentSolution) : null;

                    if (p != null && !(p is DotNetProject))
                    {
                        continue;
                    }

                    if (p == null || pr.GetReferencedFileNames(configuration).Length == 0)
                    {
                        if (refres == null)
                        {
                            refres = new BuildResult();
                        }
                        string msg = GettextCatalog.GetString("Referenced project '{0}' not found in the solution.", pr.Reference);
                        monitor.ReportWarning(msg);
                        refres.AddWarning(msg);
                    }
                }

                if (!pr.IsValid)
                {
                    if (refres == null)
                    {
                        refres = new BuildResult();
                    }
                    string msg;
                    if (!pr.IsExactVersion && pr.SpecificVersion)
                    {
                        msg = GettextCatalog.GetString("Reference '{0}' not found on system. Using '{1}' instead.", pr.StoredReference, pr.Reference);
                        monitor.ReportWarning(msg);
                        refres.AddWarning(msg);
                    }
                    else
                    {
                        bool errorsFound = false;
                        foreach (string asm in pr.GetReferencedFileNames(configuration))
                        {
                            if (!File.Exists(asm))
                            {
                                msg = GettextCatalog.GetString("Assembly '{0}' not found. Make sure that the assembly exists in disk. If the reference is required to build the project you may get compilation errors.", Path.GetFileName(asm));
                                refres.AddWarning(msg);
                                monitor.ReportWarning(msg);
                                errorsFound = true;
                                itemsToExclude.Add(pr);
                            }
                        }
                        msg = null;
                        if (!errorsFound)
                        {
                            msg = GettextCatalog.GetString("The reference '{0}' is not valid for the target framework of the project.", pr.StoredReference, pr.Reference);
                            monitor.ReportWarning(msg);
                            refres.AddWarning(msg);
                            itemsToExclude.Add(pr);
                        }
                    }
                }
            }

            DotNetProjectConfiguration conf = (DotNetProjectConfiguration)project.GetConfiguration(configuration);

            // Create a copy of the data needed to compile the project.
            // This data can be modified by extensions.
            // Also filter out items whose condition evaluates to false

            BuildData            buildData = new BuildData();
            ProjectParserContext ctx       = new ProjectParserContext(project, conf);

            buildData.Items = new ProjectItemCollection();
            foreach (ProjectItem item in project.Items)
            {
                if (!itemsToExclude.Contains(item) && (string.IsNullOrEmpty(item.Condition) || ConditionParser.ParseAndEvaluate(item.Condition, ctx)))
                {
                    buildData.Items.Add(item);
                }
            }
            buildData.Configuration = (DotNetProjectConfiguration)project.CloneConfiguration(conf, conf.Id);
            buildData.Configuration.SetParentItem(project);
            buildData.ConfigurationSelector = configuration;

            var br = await project.Compile(monitor, buildData);

            if (refres != null)
            {
                refres.Append(br);
                return(refres);
            }
            else
            {
                return(br);
            }
        }