Solution folders with duplicate ids should not error when reading sln…

src/Microsoft.VisualStudio.SolutionPersistence/Serializer/SlnV12/SlnFileV12Serializer.Reader.cs

@@ -532,6 +532,14 @@ private SolutionItemModel ReadProjectInfo(SolutionModel solution, ref StringToke
 #pragma warning disable CS0618 // Type or member is obsolete (Temporaily create a potentially invalid solution folder until nested projects is interpreted.)
                 SolutionFolderModel folder = solution.CreateSlnFolder(name: displayName.ToString());
 #pragma warning restore CS0618 // Type or member is obsolete
+
+                // Solution folders with duplicate ids should not error when reading sln files to preserve legacy behavior.
+                if (solution.FindItemById(projectId) is not null)
+                {
+                    projectId = Guid.NewGuid();
+                    this.tarnished = true;
+                }
+
                 folder.Id = projectId;
                 return folder;
             }

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

@@ -208,4 +208,35 @@ public async Task InvalidSlnxVersionAsync()
         Assert.Equal(1, ex.Line);
         Assert.Equal(2, ex.Column);
     }
+
+    /// <summary>
+    /// The legacy sln solution parser would ignore duplicate folder guids.
+    /// Ensure this behavior is maintained.
+    /// </summary>
+    [Fact]
+    public async Task DuplicateFolderIdSlnAsync()
+    {
+        ResourceStream duplicateId = SlnAssets.LoadResource("Invalid/DuplicateFolderId.sln");
+        SolutionModel solution = await SolutionSerializers.SlnFileV12.OpenAsync(duplicateId.Stream, CancellationToken.None);
+
+        Assert.Equal(4, solution.SolutionFolders.Count);
+
+        Assert.NotNull(solution.SerializerExtension);
+        Assert.True(solution.SerializerExtension.Tarnished);
+    }
+
+    /// <summary>
+    /// Ensure the slnx parser does fail on duplicate folder guids.
+    /// </summary>
+    [Fact]
+    public async Task DuplicateFolderIdSlnxAsync()
+    {
+        ResourceStream duplicateId = SlnAssets.LoadResource("Invalid/DuplicateFolderId.slnx");
+        SolutionException ex = await Assert.ThrowsAsync<SolutionException>(
+            async () => _ = await SolutionSerializers.SlnXml.OpenAsync(duplicateId.Stream, CancellationToken.None));
+
+        Assert.StartsWith(string.Format(Errors.DuplicateItemRef_Args2, "11111111-1111-1111-1111-111111111111", nameof(SolutionFolderModel)), ex.Message);
+        Assert.Equal(3, ex.Line);
+        Assert.Equal(4, ex.Column);
+    }
 }

test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateFolderId.sln.txt

@@ -0,0 +1,10 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Apple", "Apple", "{11111111-1111-1111-1111-111111111111}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Mango", "Mango", "{11111111-1111-1111-1111-111111111111}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Pear", "Pear", "{11111111-1111-1111-1111-111111111111}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Orange", "Orange", "{11111111-1111-1111-1111-111111111111}"
+EndProject

test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateFolderId.slnx.xml

@@ -0,0 +1,6 @@
+<Solution>
+  <Folder Name="/Apple/" Id="11111111-1111-1111-1111-111111111111" />
+  <Folder Name="/Mango/" Id="11111111-1111-1111-1111-111111111111" />
+  <Folder Name="/Pear/" Id="11111111-1111-1111-1111-111111111111" />
+  <Folder Name="/Orange/" Id="11111111-1111-1111-1111-111111111111" />
+</Solution>