From eb6ec97ec404576215c23115b9078615913375d2 Mon Sep 17 00:00:00 2001
From: Hardik Kaurani <hardikkaurani1@gmail.com>
Date: Thu, 22 Oct 2026 00:30:07 +0530
Subject: [PATCH] fix(models): preserve tag collection types and comparers in
 OpenApiDocument copy constructor

---
 .../Models/OpenApiDocument.cs                 | 29 +++++-----
 .../Models/OpenApiDocumentTests.cs            | 56 +++++++++++++++++++
 2 files changed, 71 insertions(+), 14 deletions(-)

diff --git a/src/Microsoft.OpenApi/Models/OpenApiDocument.cs b/src/Microsoft.OpenApi/Models/OpenApiDocument.cs
index 03086600d..de2bb9d16 100644
--- a/src/Microsoft.OpenApi/Models/OpenApiDocument.cs
+++ b/src/Microsoft.OpenApi/Models/OpenApiDocument.cs
@@ -87,21 +87,22 @@ public ISet<OpenApiTag>? Tags
             }
             set
             {
-                if (value is null)
-                {
-                    _tags = null;
-                    return;
-                }
-                _tags = value switch
-                {
-                    HashSet<OpenApiTag> tags when tags.Comparer != EqualityComparer<OpenApiTag>.Default => value,
-                    SortedSet<OpenApiTag> => value,
+                _tags = CopyTags(value);
+            }
+        }
+
+        private static ISet<OpenApiTag>? CopyTags(ISet<OpenApiTag>? tags, bool copy = false)
+        {
+            return tags switch
+            {
+                null => null,
+                HashSet<OpenApiTag> hashSet when !ReferenceEquals(hashSet.Comparer, EqualityComparer<OpenApiTag>.Default) => copy ? new HashSet<OpenApiTag>(hashSet, hashSet.Comparer) : hashSet,
+                SortedSet<OpenApiTag> sorted => copy ? new SortedSet<OpenApiTag>(sorted, sorted.Comparer) : sorted,
 #if NET
-                    ImmutableSortedSet<OpenApiTag> => value,
+                ImmutableSortedSet<OpenApiTag> => tags,
 #endif
-                    _ => new HashSet<OpenApiTag>(value, OpenApiTagComparer.Instance),
-                };
-            }
+                _ => new HashSet<OpenApiTag>(tags, OpenApiTagComparer.Instance),
+            };
         }
 
         /// <summary>
@@ -147,7 +148,7 @@ public OpenApiDocument(OpenApiDocument? document)
             Webhooks = document?.Webhooks != null ? new Dictionary<string, IOpenApiPathItem>(document.Webhooks) : null;
             Components = document?.Components != null ? new(document?.Components) : null;
             Security = document?.Security != null ? [.. document.Security] : null;
-            Tags = document?.Tags != null ? new HashSet<OpenApiTag>(document.Tags, OpenApiTagComparer.Instance) : null;
+            Tags = CopyTags(document?.Tags, copy: true);
             ExternalDocs = document?.ExternalDocs != null ? new(document.ExternalDocs) : null;
             Extensions = document?.Extensions != null ? new Dictionary<string, IOpenApiExtension>(document.Extensions) : null;
             Metadata = document?.Metadata != null ? new Dictionary<string, object>(document.Metadata) : null;
diff --git a/test/Microsoft.OpenApi.Tests/Models/OpenApiDocumentTests.cs b/test/Microsoft.OpenApi.Tests/Models/OpenApiDocumentTests.cs
index 8b4629664..3abfb42fd 100644
--- a/test/Microsoft.OpenApi.Tests/Models/OpenApiDocumentTests.cs
+++ b/test/Microsoft.OpenApi.Tests/Models/OpenApiDocumentTests.cs
@@ -2214,6 +2214,62 @@ public void TagsSupportsImmutableSortedSets()
             Assert.IsType<ImmutableSortedSet<OpenApiTag>>(document.Tags);
         }
 
+        [Fact]
+        public void OpenApiDocumentCopyConstructorPreservesCustomComparer()
+        {
+            var document = new OpenApiDocument
+            {
+                Tags = new HashSet<OpenApiTag>(new CaseInsensitiveOpenApiTagEqualityComparer())
+                {
+                    new() { Name = "Tag1" }
+                }
+            };
+
+            var copy = new OpenApiDocument(document);
+            Assert.NotNull(copy.Tags);
+            Assert.False(copy.Tags.Add(new OpenApiTag { Name = "tag1" }));
+            Assert.Single(copy.Tags);
+        }
+
+        [Fact]
+        public void OpenApiDocumentCopyConstructorPreservesSortedSet()
+        {
+            var document = new OpenApiDocument
+            {
+                Tags = new SortedSet<OpenApiTag>(new DescendingOpenApiTagComparer())
+                {
+                    new() { Name = "tagB" },
+                    new() { Name = "tagA" },
+                    new() { Name = "tagC" },
+                }
+            };
+
+            var copy = new OpenApiDocument(document);
+            Assert.IsType<SortedSet<OpenApiTag>>(copy.Tags);
+            var copyNames = copy.Tags.Select(t => t.Name);
+            Assert.Equal(["tagC", "tagB", "tagA"], copyNames);
+        }
+
+        [Fact]
+        public void OpenApiDocumentCopyConstructorPreservesImmutableSortedSet()
+        {
+            var document = new OpenApiDocument
+            {
+                Tags = ImmutableSortedSet.Create(
+                    new DescendingOpenApiTagComparer(),
+                    [
+                        new OpenApiTag { Name = "tagB" },
+                        new OpenApiTag { Name = "tagA" },
+                        new OpenApiTag { Name = "tagC" },
+                    ]),
+            };
+
+            var copy = new OpenApiDocument(document);
+            Assert.IsType<ImmutableSortedSet<OpenApiTag>>(copy.Tags);
+            var copyNames = copy.Tags.Select(t => t.Name);
+            Assert.Equal(["tagC", "tagB", "tagA"], copyNames);
+        }
+
         public static TheoryData<OpenApiSpecVersion> OpenApiSpecVersions()
         {
             var values = new TheoryData<OpenApiSpecVersion>();