internal void Save()

in src/Microsoft.VisualStudio.SlnGen/SlnFile.cs [386:538]
120 lines of code
35 McCabe index (conditional complexity)

        internal void Save(string rootPath, TextWriter writer, bool useFolders, bool collapseFolders = false)
        {
            writer.WriteLine(Header, _fileFormatVersion);

            if (VisualStudioVersion != null)
            {
                writer.WriteLine($"# Visual Studio Version {VisualStudioVersion.Major}");
                writer.WriteLine($"VisualStudioVersion = {VisualStudioVersion}");
                writer.WriteLine($"MinimumVisualStudioVersion = {MinumumVisualStudioVersion}");
            }

            if (SolutionItems.Count > 0)
            {
                writer.WriteLine($@"Project(""{SlnFolder.FolderProjectTypeGuidString}"") = ""Solution Items"", ""Solution Items"", ""{{B283EBC2-E01F-412D-9339-FD56EF114549}}"" ");
                writer.WriteLine("	ProjectSection(SolutionItems) = preProject");
                foreach (string solutionItem in SolutionItems.Select(i => i.ToRelativePath(rootPath).ToSolutionPath()))
                {
                    writer.WriteLine($"		{solutionItem} = {solutionItem}");
                }

                writer.WriteLine("	EndProjectSection");
                writer.WriteLine("EndProject");
            }

            List<SlnProject> sortedProjects = _projects.OrderBy(i => i.IsMainProject ? 0 : 1).ThenBy(i => i.FullPath).ToList();

            foreach (SlnProject project in sortedProjects)
            {
                string solutionPath = project.FullPath.ToRelativePath(rootPath).ToSolutionPath();

                if (ExistingProjectGuids != null && ExistingProjectGuids.TryGetValue(solutionPath, out Guid existingProjectGuid))
                {
                    project.ProjectGuid = existingProjectGuid;
                }

                writer.WriteLine($@"Project(""{project.ProjectTypeGuid.ToSolutionString()}"") = ""{project.Name}"", ""{solutionPath}"", ""{project.ProjectGuid.ToSolutionString()}""");
                writer.WriteLine("EndProject");
            }

            SlnHierarchy hierarchy = null;

            if (useFolders && sortedProjects.Any(i => !i.IsMainProject))
            {
                hierarchy = SlnHierarchy.CreateFromProjectDirectories(sortedProjects, collapseFolders);
            }
            else if (sortedProjects.Any(i => !string.IsNullOrWhiteSpace(i.SolutionFolder)))
            {
                hierarchy = SlnHierarchy.CreateFromProjectSolutionFolder(sortedProjects);
            }

            if (hierarchy != null)
            {
                foreach (SlnFolder folder in hierarchy.Folders)
                {
                    string projectSolutionPath = (useFolders ? folder.FullPath.ToRelativePath(rootPath) : folder.Name).ToSolutionPath();

                    // Try to preserve the folder GUID if a matching relative folder path was parsed from an existing solution
                    if (ExistingProjectGuids != null && ExistingProjectGuids.TryGetValue(projectSolutionPath, out Guid projectGuid))
                    {
                        folder.FolderGuid = projectGuid;
                    }

                    writer.WriteLine($@"Project(""{folder.ProjectTypeGuidString}"") = ""{folder.Name}"", ""{projectSolutionPath}"", ""{folder.FolderGuid.ToSolutionString()}""");
                    writer.WriteLine("EndProject");
                }
            }

            writer.WriteLine("Global");

            writer.WriteLine("	GlobalSection(SolutionConfigurationPlatforms) = preSolution");

            HashSet<string> solutionPlatforms = Platforms != null && Platforms.Any()
                ? new HashSet<string>(GetValidSolutionPlatforms(Platforms), StringComparer.OrdinalIgnoreCase)
                : new HashSet<string>(GetValidSolutionPlatforms(sortedProjects.SelectMany(i => i.Platforms)), StringComparer.OrdinalIgnoreCase);

            HashSet<string> solutionConfigurations = Configurations != null && Configurations.Any()
                ? new HashSet<string>(Configurations, StringComparer.OrdinalIgnoreCase)
                : new HashSet<string>(sortedProjects.SelectMany(i => i.Configurations).Where(i => !i.IsNullOrWhiteSpace()), StringComparer.OrdinalIgnoreCase);

            foreach (string configuration in solutionConfigurations)
            {
                foreach (string platform in solutionPlatforms)
                {
                    if (!string.IsNullOrWhiteSpace(configuration) && !string.IsNullOrWhiteSpace(platform))
                    {
                        writer.WriteLine($"		{configuration}|{platform} = {configuration}|{platform}");
                    }
                }
            }

            writer.WriteLine("	EndGlobalSection");

            writer.WriteLine("	GlobalSection(ProjectConfigurationPlatforms) = postSolution");

            foreach (SlnProject project in sortedProjects)
            {
                string projectGuid = project.ProjectGuid.ToSolutionString();

                foreach (string configuration in solutionConfigurations)
                {
                    bool foundConfiguration = TryGetProjectSolutionConfiguration(configuration, project, out string projectSolutionConfiguration);

                    foreach (string platform in solutionPlatforms)
                    {
                        bool foundPlatform = TryGetProjectSolutionPlatform(platform, project, out string projectSolutionPlatform, out string projectBuildPlatform);

                        writer.WriteLine($@"		{projectGuid}.{configuration}|{platform}.ActiveCfg = {projectSolutionConfiguration}|{projectSolutionPlatform}");

                        if (foundPlatform && foundConfiguration)
                        {
                            writer.WriteLine($@"		{projectGuid}.{configuration}|{platform}.Build.0 = {projectSolutionConfiguration}|{projectBuildPlatform}");
                        }

                        if (project.IsDeployable)
                        {
                            writer.WriteLine($@"		{projectGuid}.{configuration}|{platform}.Deploy.0 = {projectSolutionConfiguration}|{projectSolutionPlatform}");
                        }
                    }
                }
            }

            writer.WriteLine("	EndGlobalSection");

            writer.WriteLine("	GlobalSection(SolutionProperties) = preSolution");
            writer.WriteLine("		HideSolutionNode = FALSE");
            writer.WriteLine("	EndGlobalSection");

            if (hierarchy != null)
            {
                writer.WriteLine(@"	GlobalSection(NestedProjects) = preSolution");

                foreach (SlnFolder folder in hierarchy.Folders.Where(i => i.Parent != null))
                {
                    foreach (SlnProject project in folder.Projects)
                    {
                        writer.WriteLine($@"		{project.ProjectGuid.ToSolutionString()} = {folder.FolderGuid.ToSolutionString()}");
                    }

                    if (useFolders)
                    {
                        writer.WriteLine($@"		{folder.FolderGuid.ToSolutionString()} = {folder.Parent.FolderGuid.ToSolutionString()}");
                    }
                }

                writer.WriteLine("	EndGlobalSection");
            }

            writer.WriteLine("	GlobalSection(ExtensibilityGlobals) = postSolution");
            writer.WriteLine($"		SolutionGuid = {SolutionGuid.ToSolutionString()}");
            writer.WriteLine("	EndGlobalSection");

            writer.WriteLine("EndGlobal");
        }