fix: handle relative schema references without fragment and prevent UriFormatException (#3148)

src/Microsoft.OpenApi/Models/BaseOpenApiReference.cs

@@ -254,6 +254,11 @@ public virtual void SerializeAsV2(IOpenApiWriter writer)
                     return Id;
                 }
 
+                if (string.Equals(ExternalResource, Id, StringComparison.Ordinal) && !Id.Contains("#"))
+                {
+                    return ExternalResource;
+                }
+
                 return ExternalResource + "#/components/" + Type.GetDisplayName() + "/"+ Id; 
             }
 
@@ -329,9 +334,8 @@ internal void SetJsonPointerPath(string pointer, string nodeLocation)
                 ReferenceV3 = ResolveRelativePointer(nodeLocation, pointer);
             }
 
-            // Absolute reference or anchor (e.g. "#/components/schemas/..." or full URL)
-            else if ((pointer.Contains('#') || pointer.StartsWith("http", StringComparison.OrdinalIgnoreCase))
-                && !string.Equals(ReferenceV3, pointer, StringComparison.OrdinalIgnoreCase))
+            // Absolute reference, anchor, or relative pointer (e.g. "#/components/schemas/...", full URL, or "schemas/uuid.schema.json")
+            else if (!string.Equals(ReferenceV3, pointer, StringComparison.OrdinalIgnoreCase))
             {
                 ReferenceV3 = pointer;
             }

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
 using System;
@@ -630,14 +630,21 @@ private static string ConvertByteArrayToString(byte[] hash)
 
             string uriLocation;
             var id = reference.Id;
-            if (!string.IsNullOrEmpty(id) && id!.Contains('/')) // this means its a URL reference
+            var referenceV3 = !string.IsNullOrEmpty(reference.ReferenceV3) ? reference.ReferenceV3! : string.Empty;
+            var isStandaloneExternal = reference.IsExternal &&
+                (!string.IsNullOrEmpty(referenceV3) ? !referenceV3.Contains("#") : (id is not null && !id.Contains("#")));
+
+            if (isStandaloneExternal)
+            {
+                uriLocation = id ?? reference.ExternalResource ?? string.Empty;
+            }
+            else if (!string.IsNullOrEmpty(id) && id!.Contains("/"))
             {
                 uriLocation = id!;
             }
             else
             {
                 string relativePath;
-                var referenceV3 = !string.IsNullOrEmpty(reference.ReferenceV3) ? reference.ReferenceV3! : string.Empty;
 
                 if (reference.Type is ReferenceType.Schema && useExternal && TryGetPlainNameFragment(referenceV3, out var plainNameFragment))
                 {
@@ -668,19 +675,28 @@ private static string ConvertByteArrayToString(byte[] hash)
                     : BaseUri + relativePath;
             }
 
-            var absoluteUri =
-#if NETSTANDARD2_1 || NETCOREAPP || NET5_0_OR_GREATER
-            uriLocation.StartsWith('#')
-#else
-            uriLocation.StartsWith("#", StringComparison.OrdinalIgnoreCase)
-#endif
-            switch
+            Uri? resolvedUri = null;
+            if (Uri.TryCreate(uriLocation, UriKind.Absolute, out var absUri))
             {
-                true => new Uri(BaseUri, uriLocation).AbsoluteUri,
-                false => new Uri(uriLocation).AbsoluteUri
-            };
+                resolvedUri = absUri;
+            }
+            else if (useExternal && reference.ExternalResource is not null && Workspace?.GetDocumentId(reference.ExternalResource) is Uri extDocUri)
+            {
+                resolvedUri = extDocUri;
+            }
+            else if (BaseUri is not null && Uri.TryCreate(BaseUri, uriLocation, out var relCombinedUri))
+            {
+                resolvedUri = relCombinedUri;
+            }
+
+            if (resolvedUri is null)
+            {
+                return null;
+            }
+
+            var absoluteUri = resolvedUri.AbsoluteUri;
 
-            if (reference.Type is ReferenceType.Schema && absoluteUri.Contains('#') && parentSchema is not null)
+            if (reference.Type is ReferenceType.Schema && absoluteUri.Contains("#") && parentSchema is not null)
             {
                 return Workspace?.ResolveJsonSchemaReference(absoluteUri, parentSchema);
             }

src/Microsoft.OpenApi/Reader/Services/OpenApiWorkspaceLoader.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -45,18 +45,25 @@ internal async Task<OpenApiDiagnostic> LoadAsync(BaseOpenApiReference reference,
                 // If not already in workspace, load it and process references
                 if (item.ExternalResource is not null && !_workspace.Contains(item.ExternalResource))
                 {
-                    var uri = new Uri(item.ExternalResource, UriKind.RelativeOrAbsolute);
-                    var input = await _loader.LoadAsync(item.HostDocument!.BaseUri, uri, cancellationToken).ConfigureAwait(false);
-                    var result = await OpenApiDocument.LoadAsync(input, format, _readerSettings, cancellationToken).ConfigureAwait(false);
-                    // Merge diagnostics
-                    if (result.Diagnostic != null)
+                    try
                     {
-                        diagnostic.AppendDiagnostic(result.Diagnostic, item.ExternalResource);
+                        var uri = new Uri(item.ExternalResource, UriKind.RelativeOrAbsolute);
+                        var input = await _loader.LoadAsync(item.HostDocument!.BaseUri, uri, cancellationToken).ConfigureAwait(false);
+                        var result = await OpenApiDocument.LoadAsync(input, format, _readerSettings, cancellationToken).ConfigureAwait(false);
+                        // Merge diagnostics
+                        if (result.Diagnostic != null)
+                        {
+                            diagnostic.AppendDiagnostic(result.Diagnostic, item.ExternalResource);
+                        }
+                        if (result.Document != null)
+                        {
+                            var loadDiagnostic = await LoadAsync(item, result.Document, format, diagnostic, cancellationToken).ConfigureAwait(false);
+                            diagnostic = loadDiagnostic;
+                        }
                     }
-                    if (result.Document != null)
+                    catch (Exception ex)
                     {
-                        var loadDiagnostic = await LoadAsync(item, result.Document, format, diagnostic, cancellationToken).ConfigureAwait(false);
-                        diagnostic = loadDiagnostic;
+                        diagnostic.Errors.Add(new OpenApiError(string.Empty, ex.Message));
                     }
                 }
             }

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

@@ -56,8 +56,28 @@ public static OpenApiDiscriminator LoadDiscriminator(JsonNode node, OpenApiDocum
         public static OpenApiSchemaReference LoadMapping(JsonNode node, OpenApiDocument hostDocument, ParsingContext context)
         {
             var pointer = node.GetScalarValue() ?? throw new InvalidOperationException("Could not get a pointer reference");
+            if (IsBareSchemaName(pointer))
+            {
+                var schemaRef = new OpenApiSchemaReference(pointer, hostDocument);
+                schemaRef.Reference.SetJsonPointerPath(pointer, context.GetLocation());
+                return schemaRef;
+            }
+
             var reference = GetReferenceIdAndExternalResource(pointer);
-            return new OpenApiSchemaReference(reference.Item1, hostDocument, reference.Item2);
+            var result = new OpenApiSchemaReference(reference.Item1, hostDocument, reference.Item2);
+            result.Reference.SetJsonPointerPath(pointer, context.GetLocation());
+            return result;
+        }
+
+        private static bool IsBareSchemaName(string pointer)
+        {
+            return !pointer.StartsWith("#", StringComparison.OrdinalIgnoreCase)
+                && !pointer.Contains("/")
+                && !pointer.Contains("#")
+                && !pointer.Contains(":")
+                && !pointer.EndsWith(".json", StringComparison.OrdinalIgnoreCase)
+                && !pointer.EndsWith(".yaml", StringComparison.OrdinalIgnoreCase)
+                && !pointer.EndsWith(".yml", StringComparison.OrdinalIgnoreCase);
         }
     }
 }

src/Microsoft.OpenApi/Reader/V31/OpenApiV31Deserializer.cs

@@ -165,7 +165,7 @@ private static (string, string?) GetReferenceIdAndExternalResource(string pointe
 
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
             string? externalResource = null;
-            if (isExternalResource && pointer.Contains('#'))
+            if (isExternalResource)
             {
                 externalResource = pointer.Split('#')[0].TrimEnd('#');
             }

src/Microsoft.OpenApi/Reader/V32/OpenApiDiscriminatorDeserializer.cs

@@ -51,8 +51,28 @@ public static OpenApiDiscriminator LoadDiscriminator(JsonNode node, OpenApiDocum
         public static OpenApiSchemaReference LoadMapping(JsonNode node, OpenApiDocument hostDocument, ParsingContext context)
         {
             var pointer = node.GetScalarValue() ?? throw new InvalidOperationException("Could not get a pointer reference");
+            if (IsBareSchemaName(pointer))
+            {
+                var schemaRef = new OpenApiSchemaReference(pointer, hostDocument);
+                schemaRef.Reference.SetJsonPointerPath(pointer, context.GetLocation());
+                return schemaRef;
+            }
+
             var reference = GetReferenceIdAndExternalResource(pointer);
-            return new OpenApiSchemaReference(reference.Item1, hostDocument, reference.Item2);
+            var result = new OpenApiSchemaReference(reference.Item1, hostDocument, reference.Item2);
+            result.Reference.SetJsonPointerPath(pointer, context.GetLocation());
+            return result;
+        }
+
+        private static bool IsBareSchemaName(string pointer)
+        {
+            return !pointer.StartsWith("#", StringComparison.OrdinalIgnoreCase)
+                && !pointer.Contains("/")
+                && !pointer.Contains("#")
+                && !pointer.Contains(":")
+                && !pointer.EndsWith(".json", StringComparison.OrdinalIgnoreCase)
+                && !pointer.EndsWith(".yaml", StringComparison.OrdinalIgnoreCase)
+                && !pointer.EndsWith(".yml", StringComparison.OrdinalIgnoreCase);
         }
     }
 }

src/Microsoft.OpenApi/Reader/V32/OpenApiV32Deserializer.cs

@@ -165,7 +165,7 @@ private static (string, string?) GetReferenceIdAndExternalResource(string pointe
 
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
             string? externalResource = null;
-            if (isExternalResource && pointer.Contains('#'))
+            if (isExternalResource)
             {
                 externalResource = pointer.Split('#')[0].TrimEnd('#');
             }

src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
 using System;
@@ -949,10 +949,16 @@ public bool Contains(string location)
 
         private Uri? ToLocationUrl(string location)
         {
-            if (BaseUrl is not null)
+            if (BaseUrl is not null && Uri.TryCreate(BaseUrl, location, out var relativeUri))
             {
-                return new(BaseUrl, location);
+                return relativeUri;
             }
+
+            if (Uri.TryCreate(location, UriKind.Absolute, out var absoluteUri))
+            {
+                return absoluteUri;
+            }
+
             return null;
         }
     }

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiComponentsTests.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. 
 
 using Microsoft.OpenApi.Reader;
@@ -14,7 +14,7 @@ public class OpenApiComponentsTests
         [InlineData("A:\\Dir\\File.json#/components/schemas/ExternalAbsWindowsPathModel", "ExternalAbsWindowsPathModel", "A:\\Dir\\File.json")]
         [InlineData("/Dir/File.json#/components/schemas/ExternalAbsUnixPathModel", "ExternalAbsUnixPathModel", "/Dir/File.json")]
         [InlineData("https://host.lan:1234/path/to/file/resource.json#/components/schemas/ExternalHttpsModel", "ExternalHttpsModel", "https://host.lan:1234/path/to/file/resource.json")]
-        [InlineData("File.json", "File.json", null)]
+        [InlineData("File.json", "File.json", "File.json")]
         public void ParseExternalSchemaReferenceShouldSucceed(string reference, string referenceId, string externalResource)
         {
             var input = $@"{{

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiDiscriminatorTests.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
 using System.Collections.Generic;
@@ -42,5 +42,59 @@ public async Task ParseBasicDiscriminatorShouldSucceed()
                    DefaultMapping = new OpenApiSchemaReference("Animal", openApiDocument)  
                }, discriminator);
         }
+
+        [Fact]
+        public void ParseDiscriminatorWithBareSchemaNameMappingResolvesLocalTarget()
+        {
+            var openApi = """
+            {
+              "openapi": "3.1.1",
+              "info": { "title": "Repro", "version": "1" },
+              "paths": {},
+              "components": {
+                "schemas": {
+                  "Dog": {
+                    "type": "object",
+                    "properties": { "bark": { "type": "boolean" } }
+                  },
+                  "Pet": {
+                    "type": "object",
+                    "required": [ "petType" ],
+                    "properties": {
+                      "petType": { "type": "string" }
+                    },
+                    "discriminator": {
+                      "propertyName": "petType",
+                      "mapping": {
+                        "dog": "Dog"
+                      },
+                      "x-oas-default-mapping": "Dog"
+                    }
+                  }
+                }
+              }
+            }
+            """;
+
+            var readResult = OpenApiDocument.Parse(openApi, "json");
+            var doc = readResult.Document;
+            Assert.NotNull(doc);
+            Assert.Empty(readResult.Diagnostic.Errors);
+
+            var petSchema = Assert.IsType<OpenApiSchema>(doc.Components.Schemas["Pet"]);
+            Assert.NotNull(petSchema.Discriminator);
+
+            var dogMapping = petSchema.Discriminator.Mapping["dog"];
+            Assert.False(dogMapping.Reference.IsExternal);
+            Assert.Null(dogMapping.Reference.ExternalResource);
+            Assert.Equal("Dog", dogMapping.Reference.Id);
+
+            var dogSchema = Assert.IsType<OpenApiSchema>(doc.Components.Schemas["Dog"]);
+            Assert.Same(dogSchema, dogMapping.Target);
+
+            Assert.NotNull(petSchema.Discriminator.DefaultMapping);
+            Assert.False(petSchema.Discriminator.DefaultMapping.Reference.IsExternal);
+            Assert.Same(dogSchema, petSchema.Discriminator.DefaultMapping.Target);
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiSchemaReferenceDeserializerTests.cs

@@ -57,4 +57,31 @@ public void ShouldDeserializeSchemaReferenceAnnotations()
         Assert.Equal("example reference value", resultReference.Examples[0]?.ToString());
         Assert.NotNull(resultReference.Target);
     }
+
+    [Theory]
+    [InlineData("schemas/uuid.schema.json", "schemas/uuid.schema.json", "schemas/uuid.schema.json")]
+    [InlineData("./schemas/uuid.schema.json", "./schemas/uuid.schema.json", "./schemas/uuid.schema.json")]
+    [InlineData("uuid.schema.json", "uuid.schema.json", "uuid.schema.json")]
+    [InlineData("https://example.com/schemas/resource.json", "https://example.com/schemas/resource.json", "https://example.com/schemas/resource.json")]
+    public void ShouldDeserializeExternalSchemaReferenceWithoutFragment(string refPointer, string expectedId, string expectedExternalResource)
+    {
+        var json = $$"""
+        {
+            "$ref": "{{refPointer}}"
+        }
+        """;
+
+        var hostDocument = new OpenApiDocument();
+        var jsonNode = JsonNode.Parse(json);
+
+        var result = OpenApiV31Deserializer.LoadSchema(jsonNode, hostDocument, new ParsingContext(new()));
+
+        Assert.NotNull(result);
+        var resultReference = Assert.IsType<OpenApiSchemaReference>(result);
+
+        Assert.True(resultReference.Reference.IsExternal);
+        Assert.Equal(expectedId, resultReference.Reference.Id);
+        Assert.Equal(expectedExternalResource, resultReference.Reference.ExternalResource);
+        Assert.Equal(refPointer, resultReference.Reference.ReferenceV3);
+    }
 }