Fix handling of nested solution folders with same name.

src/Microsoft.VisualStudio.SolutionPersistence/Model/SolutionItemModel.cs

@@ -122,6 +122,13 @@ public void MoveToFolder(SolutionFolderModel? folder)
         try
         {
             this.Parent = folder;
+
+            // Reevaulate the id.
+            if (this.id == this.DefaultId)
+            {
+                this.id = null;
+            }
+
             if (this is SolutionProjectModel thisProject)
             {
                 this.Solution.ValidateProjectName(thisProject);

src/Microsoft.VisualStudio.SolutionPersistence/Model/SolutionModel.cs

@@ -193,7 +193,12 @@ public SolutionFolderModel AddFolder(string path)
         string? parentItemRef = lastSlash > 0 ? folderPath.Slice(0, lastSlash + 1).ToString() : null;
         StringSpan newName = lastSlash > 0 ? folderPath.Slice(lastSlash + 1) : folderPath.Slice(1);
 
-        return this.AddFolder(newName, parentItemRef);
+        SolutionFolderModel folder = this.AddFolder(newName, parentItemRef);
+
+        // Ensure the project type is in the project type table, if it is not already.
+        this.solutionItemsById[folder.Id] = folder;
+
+        return folder;
     }
 
     /// <summary>
@@ -629,8 +634,6 @@ private SolutionFolderModel AddFolder(StringSpan name, string? parentItemRef)
         this.solutionFolders.Add(folder);
         this.solutionItems.Add(folder);
 
-        // Ensure the project type is in the project type table, if it is not already.
-        this.solutionItemsById[folder.Id] = folder;
         return folder;
     }
 

src/Microsoft.VisualStudio.SolutionPersistence/Utilities/ListBuilderStruct`1.cs

@@ -111,7 +111,7 @@ internal readonly T[] ToArray()
             2 => [this.item0, this.item1],
             3 => [this.item0, this.item1, this.item2],
             4 => [this.item0, this.item1, this.item2, this.item3],
-            _ => [this.item0, this.item1, this.item2, this.item3, .. this.items],
+            _ => [this.item0, this.item1, this.item2, this.item3, .. this.items!],
         };
     }
 

test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSln.cs

@@ -39,6 +39,9 @@ public class RoundTripClassicSln
     [Fact]
     public Task MissingConfigurationsAsync() => TestRoundTripSerializerAsync(SlnAssets.ClassicSlnMissingConfigurations);
 
+    [Fact]
+    public Task FolderIdAsync() => TestRoundTripSerializerAsync(SlnAssets.LoadResource("FolderId.sln"));
+
     [Theory]
     [MemberData(nameof(ClassicSlnFiles))]
     public Task AllClassicSolutionAsync(ResourceName sampleFile)

test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/FolderId.sln.txt

@@ -0,0 +1,19 @@
+Microsoft Visual Studio Solution File, Format Version 12.00
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Foo", "Foo", "{A340195B-735B-34A1-F5D5-445C9F08470D}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Foo", "Foo", "{F508AC4D-2574-0F7F-5175-5C248303655F}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Foo", "Foo", "{45DA1D9E-8A3F-8D58-BB91-EFD6A5B29A17}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Foo", "Foo", "{6647119C-3B2A-3090-A1B3-560120CC3977}"
+EndProject
+Global
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(NestedProjects) = preSolution
+		{F508AC4D-2574-0F7F-5175-5C248303655F} = {A340195B-735B-34A1-F5D5-445C9F08470D}
+		{45DA1D9E-8A3F-8D58-BB91-EFD6A5B29A17} = {F508AC4D-2574-0F7F-5175-5C248303655F}
+		{6647119C-3B2A-3090-A1B3-560120CC3977} = {45DA1D9E-8A3F-8D58-BB91-EFD6A5B29A17}
+	EndGlobalSection
+EndGlobal

test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/FolderId.slnx.xml

@@ -0,0 +1,6 @@
+<Solution>
+    <Folder Name="/Foo/" />
+    <Folder Name="/Foo/Foo/" />
+    <Folder Name="/Foo/Foo/Foo/" />
+    <Folder Name="/Foo/Foo/Foo/Foo/" />
+</Solution>