Preserve folder GUIDs in existing solutions

src/Microsoft.VisualStudio.SlnGen.UnitTests/SlnFileTests.cs

@@ -410,6 +410,12 @@ public void TryParseExistingSolution()
                 [@"test\Microsoft.VisualStudio.SlnGen.UnitTests\Microsoft.VisualStudio.SlnGen.UnitTests.csproj"] = new Guid("B55ACBF0-DC34-44BA-8535-8F81325B6D70"),
             };
 
+            Dictionary<string, Guid> folders = new Dictionary<string, Guid>
+            {
+                [@"FolderA"] = new Guid("9C915FE4-72A5-4368-8979-32B3983E6041"),
+                [@"FolderB"] = new Guid("D3A9F802-38CC-4F8D-8DE9-8DF9C8B7EADC"),
+            };
+
             Dictionary<FileInfo, Guid> projectFiles = projects.ToDictionary(i => new FileInfo(Path.Combine(solutionFilePath.DirectoryName!, i.Key)), i => i.Value);
 
             foreach (KeyValuePair<FileInfo, Guid> item in projectFiles)
@@ -465,7 +471,7 @@ public void TryParseExistingSolution()
 
             solutionGuid.ShouldBe(Guid.Parse("CFFC4187-96EE-4465-B5B3-0BAFD3C14BB6"));
 
-            projectGuidsByPath.ShouldBe(projectFiles.Select(i => new KeyValuePair<string, Guid>(i.Key.FullName, i.Value)));
+            projectGuidsByPath.ShouldBe(projectFiles.Select(i => new KeyValuePair<string, Guid>(i.Key.FullName, i.Value)).Concat(folders));
         }
 
         [Fact]

src/Microsoft.VisualStudio.SlnGen/SlnFile.cs

@@ -8,6 +8,7 @@
 using System.IO;
 using System.Linq;
 using System.Text;
+using System.Text.RegularExpressions;
 
 namespace Microsoft.VisualStudio.SlnGen
 {
@@ -51,6 +52,11 @@ public sealed class SlnFile
         /// </summary>
         private static readonly string[] ProjectSectionSeparator = { "\", \"" };
 
+        /// <summary>
+        /// A regular expression used to parse the project section.
+        /// </summary>
+        private static readonly Regex GuidRegex = new (@"(?<Guid>\{[0-9a-fA-F\-]+\})");
+
         /// <summary>
         /// The file format version.
         /// </summary>
@@ -227,11 +233,41 @@ public static bool TryParseExistingSolution(string path, out Guid solutionGuid,
 
                         if (projectDetails.Length == 3)
                         {
-                            FileInfo projectFullPath = new FileInfo(Path.Combine(solutionFileDirectory, projectDetails[1].Trim().Trim('\"')));
+                            Match projectGuidMatch = GuidRegex.Match(projectDetails[2]);
+
+                            if (!projectGuidMatch.Groups["Guid"].Success)
+                            {
+                                continue;
+                            }
+
+                            string projectGuidString = projectGuidMatch.Groups["Guid"].Value;
+
+                            Match projectTypeGuidMatch = GuidRegex.Match(projectDetails[0]);
+
+                            if (!projectTypeGuidMatch.Groups["Guid"].Success)
+                            {
+                                continue;
+                            }
 
-                            if (projectFullPath.Exists && Guid.TryParse(projectDetails[2].Trim('\"'), out Guid projectGuid))
+                            if (!Guid.TryParse(projectGuidString, out Guid projectGuid) || !Guid.TryParse(projectTypeGuidMatch.Groups["Guid"].Value, out Guid projectTypeGuid))
                             {
-                                projectGuids[projectFullPath.FullName] = projectGuid;
+                                continue;
+                            }
+
+                            string projectPath = projectDetails[1].Trim().Trim('\"');
+
+                            if (projectTypeGuid.Equals(SlnFolder.FolderProjectTypeGuid))
+                            {
+                                projectGuids[projectPath] = projectGuid;
+                            }
+                            else
+                            {
+                                FileInfo projectFileInfo = new FileInfo(Path.Combine(solutionFileDirectory, projectPath));
+
+                                if (projectFileInfo.Exists)
+                                {
+                                    projectGuids[projectFileInfo.FullName] = projectGuid;
+                                }
                             }
                         }
 
@@ -355,7 +391,7 @@ internal void Save(string rootPath, TextWriter writer, bool useFolders, bool col
 
             if (SolutionItems.Count > 0)
             {
-                writer.WriteLine($@"Project(""{SlnFolder.FolderProjectTypeGuid}"") = ""Solution Items"", ""Solution Items"", ""{Guid.NewGuid().ToSolutionString()}"" ");
+                writer.WriteLine($@"Project(""{SlnFolder.FolderProjectTypeGuidString}"") = ""Solution Items"", ""Solution Items"", ""{Guid.NewGuid().ToSolutionString()}"" ");
                 writer.WriteLine("	ProjectSection(SolutionItems) = preProject");
                 foreach (string solutionItem in SolutionItems.Select(i => i.ToRelativePath(rootPath)))
                 {
@@ -387,7 +423,15 @@ internal void Save(string rootPath, TextWriter writer, bool useFolders, bool col
             {
                 foreach (SlnFolder folder in hierarchy.Folders)
                 {
-                    writer.WriteLine($@"Project(""{folder.ProjectTypeGuid}"") = ""{folder.Name}"", ""{(useFolders ? folder.FullPath.ToRelativePath(rootPath) : folder.Name)}"", ""{folder.FolderGuid.ToSolutionString()}""");
+                    string projectPath = useFolders ? folder.FullPath.ToRelativePath(rootPath) : folder.Name;
+
+                    // Try to preserve the folder GUID if a matching relative folder path was parsed from an existing solution
+                    if (ExistingProjectGuids != null && ExistingProjectGuids.TryGetValue(projectPath, out Guid projectGuid))
+                    {
+                        folder.FolderGuid = projectGuid;
+                    }
+
+                    writer.WriteLine($@"Project(""{folder.ProjectTypeGuidString}"") = ""{folder.Name}"", ""{projectPath}"", ""{folder.FolderGuid.ToSolutionString()}""");
                     writer.WriteLine("EndProject");
                 }
             }

src/Microsoft.VisualStudio.SlnGen/SlnFolder.cs

@@ -16,7 +16,12 @@ public sealed class SlnFolder
         /// <summary>
         /// The project type GUID for a folder.
         /// </summary>
-        public static readonly string FolderProjectTypeGuid = new Guid(VisualStudioProjectTypeGuids.SolutionFolder).ToSolutionString();
+        public static readonly Guid FolderProjectTypeGuid = new (VisualStudioProjectTypeGuids.SolutionFolder);
+
+        /// <summary>
+        /// The project type GUID for a folder as a solution string.
+        /// </summary>
+        public static readonly string FolderProjectTypeGuidString = FolderProjectTypeGuid.ToSolutionString();
 
         /// <summary>
         /// Initializes a new instance of the <see cref="SlnFolder"/> class.
@@ -32,7 +37,7 @@ public SlnFolder(string path)
         /// <summary>
         /// Gets the <see cref="Guid" /> of the folder.
         /// </summary>
-        public Guid FolderGuid { get; }
+        public Guid FolderGuid { get; set; }
 
         /// <summary>
         /// Gets a <see cref="List{SlnFolder}" /> of child folders.
@@ -62,6 +67,6 @@ public SlnFolder(string path)
         /// <summary>
         /// Gets the project type GUID of the folder.
         /// </summary>
-        public string ProjectTypeGuid => FolderProjectTypeGuid;
+        public string ProjectTypeGuidString => FolderProjectTypeGuidString;
     }
 }