fix/unique tags

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -76,10 +76,32 @@ public void RegisterComponents()
         public IList<OpenApiSecurityRequirement>? SecurityRequirements { get; set; } =
             new List<OpenApiSecurityRequirement>();
 
+        private HashSet<OpenApiTag>? _tags;
         /// <summary>
         /// A list of tags used by the specification with additional metadata.
         /// </summary>
-        public IList<OpenApiTag>? Tags { get; set; } = new List<OpenApiTag>();
+        public ISet<OpenApiTag>? Tags 
+        { 
+            get
+            {
+                return _tags;
+            }
+            set
+            {
+                if (value is null)
+                {
+                    return;
+                }
+                if (value is HashSet<OpenApiTag> tags && tags.Comparer is OpenApiTagComparer)
+                {
+                    _tags = tags;
+                }
+                else
+                {
+                    _tags = new HashSet<OpenApiTag>(value, OpenApiTagComparer.Instance);
+                }
+            }
+        }
 
         /// <summary>
         /// Additional external documentation.
@@ -123,7 +145,7 @@ public OpenApiDocument(OpenApiDocument? document)
             Webhooks = document?.Webhooks != null ? new Dictionary<string, IOpenApiPathItem>(document.Webhooks) : null;
             Components = document?.Components != null ? new(document?.Components) : null;
             SecurityRequirements = document?.SecurityRequirements != null ? new List<OpenApiSecurityRequirement>(document.SecurityRequirements) : null;
-            Tags = document?.Tags != null ? new List<OpenApiTag>(document.Tags) : null;
+            Tags = document?.Tags != null ? new HashSet<OpenApiTag>(document.Tags, OpenApiTagComparer.Instance) : null;
             ExternalDocs = document?.ExternalDocs != null ? new(document?.ExternalDocs) : null;
             Extensions = document?.Extensions != null ? new Dictionary<string, IOpenApiExtension>(document.Extensions) : null;
             Annotations = document?.Annotations != null ? new Dictionary<string, object>(document.Annotations) : null;

src/Microsoft.OpenApi/Models/References/OpenApiTagReference.cs

@@ -21,7 +21,7 @@ public override OpenApiTag Target
         {
             get
             {
-                return Reference.HostDocument?.Tags.FirstOrDefault(t => StringComparer.Ordinal.Equals(t.Name, Reference.Id));
+                return Reference.HostDocument?.Tags.FirstOrDefault(t => OpenApiTagComparer.StringComparer.Equals(t.Name, Reference.Id));
             }
         }
 

src/Microsoft.OpenApi/OpenApiTagComparer.cs

@@ -0,0 +1,47 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.OpenApi.Models;
+
+namespace Microsoft.OpenApi;
+
+#nullable enable
+/// <summary>
+/// This comparer is used to maintain a globally unique list of tags encountered
+/// in a particular OpenAPI document.
+/// </summary>
+internal sealed class OpenApiTagComparer : IEqualityComparer<OpenApiTag>
+{
+    private static readonly Lazy<OpenApiTagComparer> _lazyInstance = new(() => new OpenApiTagComparer());
+    /// <summary>
+    /// Default instance for the comparer.
+    /// </summary>
+    internal static OpenApiTagComparer Instance { get => _lazyInstance.Value; }
+
+    /// <inheritdoc/>
+    public bool Equals(OpenApiTag? x, OpenApiTag? y)
+    {
+        if (x is null && y is null)
+        {
+            return true;
+        }
+        if (x is null || y is null)
+        {
+            return false;
+        }
+        if (ReferenceEquals(x, y))
+        {
+            return true;
+        }
+        return StringComparer.Equals(x.Name, y.Name);
+    }
+
+    // Tag comparisons are case-sensitive by default. Although the OpenAPI specification
+    // only outlines case sensitivity for property names, we extend this principle to
+    // property values for tag names as well.
+    // See https://spec.openapis.org/oas/v3.1.0#format.
+    internal static readonly StringComparer StringComparer = StringComparer.Ordinal;
+
+    /// <inheritdoc/>
+    public int GetHashCode(OpenApiTag obj) => obj?.Name is null ? 0 : StringComparer.GetHashCode(obj.Name);
+}
+#nullable restore

src/Microsoft.OpenApi/Reader/V2/OpenApiDocumentDeserializer.cs

@@ -104,7 +104,7 @@ internal static partial class OpenApiV2Deserializer
                 }
             },
             {"security", (o, n, _) => o.SecurityRequirements = n.CreateList(LoadSecurityRequirement, o)},
-            {"tags", (o, n, _) => o.Tags = n.CreateList(LoadTag, o)},
+            {"tags", (o, n, _) => { if (n.CreateList(LoadTag, o) is {Count:> 0} tags) {o.Tags = new HashSet<OpenApiTag>(tags, OpenApiTagComparer.Instance); } } },
             {"externalDocs", (o, n, _) => o.ExternalDocs = LoadExternalDocs(n, o)}
         };
 

src/Microsoft.OpenApi/Reader/V3/OpenApiDocumentDeserializer.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 
 using System;
+using System.Collections.Generic;
 using Microsoft.OpenApi.Extensions;
 using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.Reader.ParseNodes;
@@ -26,7 +27,7 @@ internal static partial class OpenApiV3Deserializer
             {"servers", (o, n, _) => o.Servers = n.CreateList(LoadServer, o)},
             {"paths", (o, n, _) => o.Paths = LoadPaths(n, o)},
             {"components", (o, n, _) => o.Components = LoadComponents(n, o)},
-            {"tags", (o, n, _) => o.Tags = n.CreateList(LoadTag, o) },
+            {"tags", (o, n, _) => { if (n.CreateList(LoadTag, o) is {Count:> 0} tags) {o.Tags = new HashSet<OpenApiTag>(tags, OpenApiTagComparer.Instance); } } },
             {"externalDocs", (o, n, _) => o.ExternalDocs = LoadExternalDocs(n, o)},
             {"security", (o, n, _) => o.SecurityRequirements = n.CreateList(LoadSecurityRequirement, o)}
         };

src/Microsoft.OpenApi/Reader/V31/OpenApiDocumentDeserializer.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using Microsoft.OpenApi.Extensions;
 using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.Reader.ParseNodes;
@@ -24,7 +25,7 @@ internal static partial class OpenApiV31Deserializer
             {"paths", (o, n, _) => o.Paths = LoadPaths(n, o)},
             {"webhooks", (o, n, _) => o.Webhooks = n.CreateMap(LoadPathItem, o)},
             {"components", (o, n, _) => o.Components = LoadComponents(n, o)},
-            {"tags", (o, n, _) => o.Tags = n.CreateList(LoadTag, o) },
+            {"tags", (o, n, _) => { if (n.CreateList(LoadTag, o) is {Count:> 0} tags) {o.Tags = new HashSet<OpenApiTag>(tags, OpenApiTagComparer.Instance); } } },
             {"externalDocs", (o, n, _) => o.ExternalDocs = LoadExternalDocs(n, o)},
             {"security", (o, n, _) => o.SecurityRequirements = n.CreateList(LoadSecurityRequirement, o)}
         };

src/Microsoft.OpenApi/Services/OpenApiVisitorBase.cs

@@ -309,7 +309,7 @@ public virtual void Visit(IOpenApiExample example)
         /// <summary>
         /// Visits list of <see cref="OpenApiTag"/>
         /// </summary>
-        public virtual void Visit(IList<OpenApiTag> openApiTags)
+        public virtual void Visit(ISet<OpenApiTag> openApiTags)
         {
         }
 

src/Microsoft.OpenApi/Services/OpenApiWalker.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text.Json.Nodes;
 using Microsoft.OpenApi.Any;
 using Microsoft.OpenApi.Extensions;
@@ -60,7 +61,7 @@ public void Walk(OpenApiDocument doc)
         /// <summary>
         /// Visits list of <see cref="OpenApiTag"/> and child objects
         /// </summary>
-        internal void Walk(IList<OpenApiTag> tags)
+        internal void Walk(ISet<OpenApiTag> tags)
         {
             if (tags == null)
             {
@@ -72,9 +73,10 @@ internal void Walk(IList<OpenApiTag> tags)
             // Visit tags
             if (tags != null)
             {
-                for (var i = 0; i < tags.Count; i++)
+                var tagsAsArray = tags.ToArray();
+                for (var i = 0; i < tagsAsArray.Length; i++)
                 {
-                    Walk(i.ToString(), () => Walk(tags[i]));
+                    Walk(i.ToString(), () => Walk(tagsAsArray[i]));
                 }
             }
         }
@@ -1213,7 +1215,7 @@ internal void Walk(IOpenApiElement element)
                 case OpenApiServer e: Walk(e); break;
                 case OpenApiServerVariable e: Walk(e); break;
                 case OpenApiTag e: Walk(e); break;
-                case IList<OpenApiTag> e: Walk(e); break;
+                case ISet<OpenApiTag> e: Walk(e); break;
                 case IOpenApiExtensible e: Walk(e); break;
                 case IOpenApiExtension e: Walk(e); break;
             }

test/Microsoft.OpenApi.Hidi.Tests/UtilityFiles/OpenApiDocumentMock.cs

@@ -629,7 +629,7 @@ public static OpenApiDocument CreateOpenApiDocument()
                         }
                     }
                 },
-                Tags = new List<OpenApiTag>
+                Tags = new HashSet<OpenApiTag>
                 {
                     new()
                     {

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiDocumentTests.cs

@@ -1000,7 +1000,7 @@ public async Task ParseModifiedPetStoreDocumentWithTagAndSecurityShouldSucceed()
                     }
                 },
                 Components = components,
-                Tags = new List<OpenApiTag>
+                Tags = new HashSet<OpenApiTag>
                     {
                         new OpenApiTag
                         {

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiOperationTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
@@ -35,7 +36,7 @@ public async Task ParseOperationWithParameterWithNoLocationShouldSucceed()
         {
             var openApiDocument = new OpenApiDocument
             {
-                Tags = { new OpenApiTag() { Name = "user" } }
+                Tags = new HashSet<OpenApiTag> { new() { Name = "user" } }
             };
             // Act
             var operation = await OpenApiModelFactory.LoadAsync<OpenApiOperation>(Path.Combine(SampleFolderPath, "operationWithParameterWithNoLocation.json"), OpenApiSpecVersion.OpenApi3_0, openApiDocument);

test/Microsoft.OpenApi.Tests/Models/OpenApiDocumentTests.cs

@@ -2078,7 +2078,7 @@ public async Task SerializeDocumentTagsWithMultipleExtensionsWorks()
                     Version = "1.0.0"
                 },
                 Paths = new OpenApiPaths(),
-                Tags = new List<OpenApiTag>
+                Tags = new HashSet<OpenApiTag>
                 {
                     new OpenApiTag
                     {
@@ -2102,5 +2102,42 @@ public async Task SerializeDocumentTagsWithMultipleExtensionsWorks()
             var actual = await doc.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
             Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), actual.MakeLineBreaksEnvironmentNeutral());
         }
+        [Fact]
+        public void DeduplicatesTags()
+        {
+            var document = new OpenApiDocument
+            {
+                Tags = new HashSet<OpenApiTag>
+                {
+                    new OpenApiTag
+                    {
+                        Name = "tag1",
+                        Extensions = new Dictionary<string, IOpenApiExtension>
+                        {
+                            ["x-tag1"] = new OpenApiAny("tag1")
+                        }
+                    },
+                    new OpenApiTag
+                    {
+                        Name = "tag2",
+                        Extensions = new Dictionary<string, IOpenApiExtension>
+                        {
+                            ["x-tag2"] = new OpenApiAny("tag2")
+                        }
+                    },
+                    new OpenApiTag
+                    {
+                        Name = "tag1",
+                        Extensions = new Dictionary<string, IOpenApiExtension>
+                        {
+                            ["x-tag1"] = new OpenApiAny("tag1")
+                        }
+                    }
+                }
+            };
+            Assert.Equal(2, document.Tags.Count);
+            Assert.Contains(document.Tags, t => t.Name == "tag1");
+            Assert.Contains(document.Tags, t => t.Name == "tag2");
+        }
     }
 }

test/Microsoft.OpenApi.Tests/Models/OpenApiOperationTests.cs

@@ -91,7 +91,7 @@ public class OpenApiOperationTests
         {
             Tags = new List<OpenApiTagReference>
             {
-                new OpenApiTagReference("tagId1", new OpenApiDocument{ Tags = new List<OpenApiTag>() { new OpenApiTag{Name = "tagId1"}} })
+                new OpenApiTagReference("tagId1", new OpenApiDocument{ Tags = new HashSet<OpenApiTag>() { new OpenApiTag{Name = "tagId1"}} })
             },
             Summary = "summary1",
             Description = "operationDescription",

test/Microsoft.OpenApi.Tests/OpenApiTagComparerTests.cs

@@ -0,0 +1,40 @@
+using Microsoft.OpenApi.Models;
+using Xunit;
+
+namespace Microsoft.OpenApi.Tests;
+
+public class OpenApiTagComparerTests
+{
+    private readonly OpenApiTagComparer _comparer = OpenApiTagComparer.Instance;
+    [Fact]
+    public void Defensive()
+    {
+        Assert.NotNull(_comparer);
+
+        Assert.True(_comparer.Equals(null, null));
+        Assert.False(_comparer.Equals(null, new OpenApiTag()));
+        Assert.Equal(0, _comparer.GetHashCode(null));
+        Assert.Equal(0, _comparer.GetHashCode(new OpenApiTag()));
+    }
+    [Fact]
+    public void SameNamesAreEqual()
+    {
+        var openApiTag1 = new OpenApiTag { Name = "tag" };
+        var openApiTag2 = new OpenApiTag { Name = "tag" };
+        Assert.True(_comparer.Equals(openApiTag1, openApiTag2));
+    }
+    [Fact]
+    public void SameInstanceAreEqual()
+    {
+        var openApiTag = new OpenApiTag { Name = "tag" };
+        Assert.True(_comparer.Equals(openApiTag, openApiTag));
+    }
+
+    [Fact]
+    public void DifferentCasingAreNotEquals()
+    {
+        var openApiTag1 = new OpenApiTag { Name = "tag" };
+        var openApiTag2 = new OpenApiTag { Name = "TAG" };
+        Assert.False(_comparer.Equals(openApiTag1, openApiTag2));
+    }
+}

test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt

@@ -722,7 +722,7 @@ namespace Microsoft.OpenApi.Models
         public Microsoft.OpenApi.Models.OpenApiPaths Paths { get; set; }
         public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiSecurityRequirement>? SecurityRequirements { get; set; }
         public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiServer>? Servers { get; set; }
-        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiTag>? Tags { get; set; }
+        public System.Collections.Generic.ISet<Microsoft.OpenApi.Models.OpenApiTag>? Tags { get; set; }
         public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.Interfaces.IOpenApiPathItem>? Webhooks { get; set; }
         public Microsoft.OpenApi.Services.OpenApiWorkspace? Workspace { get; set; }
         public bool AddComponent<T>(string id, T componentToRegister) { }
@@ -1664,8 +1664,8 @@ namespace Microsoft.OpenApi.Services
         public virtual void Visit(System.Collections.Generic.IList<Microsoft.OpenApi.Models.Interfaces.IOpenApiParameter> parameters) { }
         public virtual void Visit(System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiSecurityRequirement> openApiSecurityRequirements) { }
         public virtual void Visit(System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiServer> servers) { }
-        public virtual void Visit(System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiTag> openApiTags) { }
         public virtual void Visit(System.Collections.Generic.IList<Microsoft.OpenApi.Models.References.OpenApiTagReference> openApiTags) { }
+        public virtual void Visit(System.Collections.Generic.ISet<Microsoft.OpenApi.Models.OpenApiTag> openApiTags) { }
         public virtual void Visit(System.Text.Json.Nodes.JsonNode node) { }
     }
     public class OpenApiWalker

test/Microsoft.OpenApi.Tests/Visitors/InheritanceTests.cs

@@ -53,7 +53,7 @@ public void ExpectedVirtualsInvolved()
             visitor.Visit(default(OpenApiSecurityRequirement));
             visitor.Visit(default(IOpenApiSecurityScheme));
             visitor.Visit(default(IOpenApiExample));
-            visitor.Visit(default(IList<OpenApiTag>));
+            visitor.Visit(default(ISet<OpenApiTag>));
             visitor.Visit(default(IList<OpenApiSecurityRequirement>));
             visitor.Visit(default(IOpenApiExtensible));
             visitor.Visit(default(IOpenApiExtension));
@@ -292,7 +292,7 @@ public override void Visit(IOpenApiExample example)
                 base.Visit(example);
             }
 
-            public override void Visit(IList<OpenApiTag> openApiTags)
+            public override void Visit(ISet<OpenApiTag> openApiTags)
             {
                 EncodeCall();
                 base.Visit(openApiTags);