diff --git a/src/Microsoft.OpenApi.Hidi/Formatters/PowerShellFormatter.cs b/src/Microsoft.OpenApi.Hidi/Formatters/PowerShellFormatter.cs
index fbfb1b716..8749e4537 100644
--- a/src/Microsoft.OpenApi.Hidi/Formatters/PowerShellFormatter.cs
+++ b/src/Microsoft.OpenApi.Hidi/Formatters/PowerShellFormatter.cs
@@ -52,7 +52,8 @@ public override void Visit(OpenApiSchema schema)
 
         public override void Visit(OpenApiPathItem pathItem)
         {
-            if (pathItem.Operations.TryGetValue(OperationType.Put, out var value))
+            if (pathItem.Operations.TryGetValue(OperationType.Put, out var value) &&
+                value.OperationId != null)
             {
                 var operationId = value.OperationId;
                 pathItem.Operations[OperationType.Put].OperationId = ResolvePutOperationId(operationId);
@@ -67,14 +68,14 @@ public override void Visit(OpenApiOperation operation)
                 throw new ArgumentException($"OperationId is required {PathString}", nameof(operation));
 
             var operationId = operation.OperationId;
-            var operationTypeExtension = operation.Extensions.GetExtension("x-ms-docs-operation-type");
+            var operationTypeExtension = operation.Extensions?.GetExtension("x-ms-docs-operation-type");
             if (operationTypeExtension.IsEquals("function"))
-                operation.Parameters = ResolveFunctionParameters(operation.Parameters);
+                operation.Parameters = ResolveFunctionParameters(operation.Parameters ?? new List<OpenApiParameter>());
 
             // Order matters. Resolve operationId.
             operationId = RemoveHashSuffix(operationId);
             if (operationTypeExtension.IsEquals("action") || operationTypeExtension.IsEquals("function"))
-                operationId = RemoveKeyTypeSegment(operationId, operation.Parameters);
+                operationId = RemoveKeyTypeSegment(operationId, operation.Parameters ?? new List<OpenApiParameter>());
             operationId = SingularizeAndDeduplicateOperationId(operationId.SplitByChar('.'));
             operationId = ResolveODataCastOperationId(operationId);
             operationId = ResolveByRefOperationId(operationId);
diff --git a/src/Microsoft.OpenApi.Hidi/OpenApiService.cs b/src/Microsoft.OpenApi.Hidi/OpenApiService.cs
index fd53086d2..df3bf0e67 100644
--- a/src/Microsoft.OpenApi.Hidi/OpenApiService.cs
+++ b/src/Microsoft.OpenApi.Hidi/OpenApiService.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;
diff --git a/src/Microsoft.OpenApi/Models/OpenApiComponents.cs b/src/Microsoft.OpenApi/Models/OpenApiComponents.cs
index 5079e9915..f672b7dd1 100644
--- a/src/Microsoft.OpenApi/Models/OpenApiComponents.cs
+++ b/src/Microsoft.OpenApi/Models/OpenApiComponents.cs
@@ -8,6 +8,7 @@
 using Microsoft.OpenApi.Models.References;
 using Microsoft.OpenApi.Writers;
 
+#nullable enable
 
 namespace Microsoft.OpenApi.Models
 {
@@ -19,60 +20,60 @@ public class OpenApiComponents : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiSchema"/> Objects.
         /// </summary>
-        public IDictionary<string, OpenApiSchema> Schemas { get; set; } = new Dictionary<string, OpenApiSchema>();
+        public IDictionary<string, OpenApiSchema>? Schemas { get; set; } = new Dictionary<string, OpenApiSchema>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiResponse"/> Objects.
         /// </summary>
-        public virtual IDictionary<string, OpenApiResponse> Responses { get; set; } = new Dictionary<string, OpenApiResponse>();
+        public virtual IDictionary<string, OpenApiResponse>? Responses { get; set; } = new Dictionary<string, OpenApiResponse>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiParameter"/> Objects.
         /// </summary>
-        public virtual IDictionary<string, OpenApiParameter> Parameters { get; set; } =
+        public virtual IDictionary<string, OpenApiParameter>? Parameters { get; set; } =
             new Dictionary<string, OpenApiParameter>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiExample"/> Objects.
         /// </summary>
-        public virtual IDictionary<string, OpenApiExample> Examples { get; set; } = new Dictionary<string, OpenApiExample>();
+        public virtual IDictionary<string, OpenApiExample>? Examples { get; set; } = new Dictionary<string, OpenApiExample>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiRequestBody"/> Objects.
         /// </summary>
-        public virtual IDictionary<string, OpenApiRequestBody> RequestBodies { get; set; } =
+        public virtual IDictionary<string, OpenApiRequestBody>? RequestBodies { get; set; } =
             new Dictionary<string, OpenApiRequestBody>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiHeader"/> Objects.
         /// </summary>
-        public virtual IDictionary<string, OpenApiHeader> Headers { get; set; } = new Dictionary<string, OpenApiHeader>();
+        public virtual IDictionary<string, OpenApiHeader>? Headers { get; set; } = new Dictionary<string, OpenApiHeader>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiSecurityScheme"/> Objects.
         /// </summary>
-        public virtual IDictionary<string, OpenApiSecurityScheme> SecuritySchemes { get; set; } =
+        public virtual IDictionary<string, OpenApiSecurityScheme>? SecuritySchemes { get; set; } =
             new Dictionary<string, OpenApiSecurityScheme>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiLink"/> Objects.
         /// </summary>
-        public virtual IDictionary<string, OpenApiLink> Links { get; set; } = new Dictionary<string, OpenApiLink>();
+        public virtual IDictionary<string, OpenApiLink>? Links { get; set; } = new Dictionary<string, OpenApiLink>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiCallback"/> Objects.
         /// </summary>
-        public virtual IDictionary<string, OpenApiCallback> Callbacks { get; set; } = new Dictionary<string, OpenApiCallback>();
+        public virtual IDictionary<string, OpenApiCallback>? Callbacks { get; set; } = new Dictionary<string, OpenApiCallback>();
 
         /// <summary>
         /// An object to hold reusable <see cref="OpenApiPathItem"/> Object.
         /// </summary>
-        public virtual IDictionary<string, OpenApiPathItem> PathItems { get; set; } = new Dictionary<string, OpenApiPathItem>();
+        public virtual IDictionary<string, OpenApiPathItem>? PathItems { get; set; } = new Dictionary<string, OpenApiPathItem>();
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public virtual IDictionary<string, IOpenApiExtension> Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public virtual IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
 
         /// <summary>
         /// Parameter-less constructor
@@ -82,7 +83,7 @@ public OpenApiComponents() { }
         /// <summary>
         /// Initializes a copy of an <see cref="OpenApiComponents"/> object
         /// </summary>
-        public OpenApiComponents(OpenApiComponents components)
+        public OpenApiComponents(OpenApiComponents? components)
         {
             Schemas = components?.Schemas != null ? new Dictionary<string, OpenApiSchema>(components.Schemas) : null;
             Responses = components?.Responses != null ? new Dictionary<string, OpenApiResponse>(components.Responses) : null;
diff --git a/src/Microsoft.OpenApi/Models/OpenApiDocument.cs b/src/Microsoft.OpenApi/Models/OpenApiDocument.cs
index 5fee30ac2..8b80fe958 100644
--- a/src/Microsoft.OpenApi/Models/OpenApiDocument.cs
+++ b/src/Microsoft.OpenApi/Models/OpenApiDocument.cs
@@ -16,6 +16,8 @@
 using Microsoft.OpenApi.Services;
 using Microsoft.OpenApi.Writers;
 
+#nullable enable
+
 namespace Microsoft.OpenApi.Models
 {
     /// <summary>
@@ -26,7 +28,7 @@ public class OpenApiDocument : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// Related workspace containing OpenApiDocuments that are referenced in this document
         /// </summary>
-        public OpenApiWorkspace Workspace { get; set; }
+        public OpenApiWorkspace? Workspace { get; set; }
 
         /// <summary>
         /// REQUIRED. Provides metadata about the API. The metadata MAY be used by tooling as required.
@@ -36,12 +38,12 @@ public class OpenApiDocument : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// The default value for the $schema keyword within Schema Objects contained within this OAS document. This MUST be in the form of a URI.
         /// </summary>
-        public string JsonSchemaDialect { get; set; }
+        public string? JsonSchemaDialect { get; set; }
 
         /// <summary>
         /// An array of Server Objects, which provide connectivity information to a target server.
         /// </summary>
-        public IList<OpenApiServer> Servers { get; set; } = new List<OpenApiServer>();
+        public IList<OpenApiServer>? Servers { get; set; } = new List<OpenApiServer>();
 
         /// <summary>
         /// REQUIRED. The available paths and operations for the API.
@@ -53,33 +55,33 @@ public class OpenApiDocument : IOpenApiSerializable, IOpenApiExtensible
         /// A map of requests initiated other than by an API call, for example by an out of band registration. 
         /// The key name is a unique string to refer to each webhook, while the (optionally referenced) Path Item Object describes a request that may be initiated by the API provider and the expected responses
         /// </summary>
-        public IDictionary<string, OpenApiPathItem> Webhooks { get; set; } = new Dictionary<string, OpenApiPathItem>();
+        public IDictionary<string, OpenApiPathItem>? Webhooks { get; set; } = new Dictionary<string, OpenApiPathItem>();
 
         /// <summary>
         /// An element to hold various schemas for the specification.
         /// </summary>
-        public OpenApiComponents Components { get; set; }
+        public OpenApiComponents? Components { get; set; }
 
         /// <summary>
         /// A declaration of which security mechanisms can be used across the API.
         /// </summary>
-        public IList<OpenApiSecurityRequirement> SecurityRequirements { get; set; } =
+        public IList<OpenApiSecurityRequirement>? SecurityRequirements { get; set; } =
             new List<OpenApiSecurityRequirement>();
 
         /// <summary>
         /// A list of tags used by the specification with additional metadata.
         /// </summary>
-        public IList<OpenApiTag> Tags { get; set; } = new List<OpenApiTag>();
+        public IList<OpenApiTag>? Tags { get; set; } = new List<OpenApiTag>();
 
         /// <summary>
         /// Additional external documentation.
         /// </summary>
-        public OpenApiExternalDocs ExternalDocs { get; set; }
+        public OpenApiExternalDocs? ExternalDocs { get; set; }
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension> Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
 
         /// <summary>
         /// The unique hash code of the generated OpenAPI document
@@ -97,25 +99,28 @@ public class OpenApiDocument : IOpenApiSerializable, IOpenApiExtensible
         public OpenApiDocument() 
         {
             Workspace = new OpenApiWorkspace();
-            BaseUri = new(OpenApiConstants.BaseRegistryUri + Guid.NewGuid().ToString());            
+            BaseUri = new(OpenApiConstants.BaseRegistryUri + Guid.NewGuid());
+            Info = new OpenApiInfo();
+            Paths = new OpenApiPaths();
         }
                 
         /// <summary>
         /// Initializes a copy of an an <see cref="OpenApiDocument"/> object
         /// </summary>
-        public OpenApiDocument(OpenApiDocument document)
+        public OpenApiDocument(OpenApiDocument? document)
         {
             Workspace = document?.Workspace != null ? new(document?.Workspace) : null;
-            Info = document?.Info != null ? new(document?.Info) : null;
+            Info = document?.Info != null ? new(document?.Info) : new OpenApiInfo();
             JsonSchemaDialect = document?.JsonSchemaDialect ?? JsonSchemaDialect;
             Servers = document?.Servers != null ? new List<OpenApiServer>(document.Servers) : null;
-            Paths = document?.Paths != null ? new(document?.Paths) : null;
+            Paths = document?.Paths != null ? new(document?.Paths) : new OpenApiPaths();
             Webhooks = document?.Webhooks != null ? new Dictionary<string, OpenApiPathItem>(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;
             ExternalDocs = document?.ExternalDocs != null ? new(document?.ExternalDocs) : null;
             Extensions = document?.Extensions != null ? new Dictionary<string, IOpenApiExtension>(document.Extensions) : null;
+            BaseUri = document?.BaseUri != null ? document.BaseUri : new(OpenApiConstants.BaseRegistryUri + Guid.NewGuid());
         }
 
         /// <summary>
@@ -360,7 +365,7 @@ private static string ParseServerUrl(OpenApiServer server)
             return parsedUrl;
         }
 
-        private static void WriteHostInfoV2(IOpenApiWriter writer, IList<OpenApiServer> servers)
+        private static void WriteHostInfoV2(IOpenApiWriter writer, IList<OpenApiServer>? servers)
         {
             if (servers == null || !servers.Any())
             {
@@ -440,7 +445,7 @@ public void SetReferenceHostDocument()
         /// <summary>
         /// Load the referenced <see cref="IOpenApiReferenceable"/> object from a <see cref="OpenApiReference"/> object
         /// </summary>
-        internal T ResolveReferenceTo<T>(OpenApiReference reference) where T : class, IOpenApiReferenceable
+        internal T? ResolveReferenceTo<T>(OpenApiReference reference) where T : class, IOpenApiReferenceable
         {
             if (reference.IsExternal)
             {
@@ -489,7 +494,7 @@ private static string ConvertByteArrayToString(byte[] hash)
         /// <summary>
         /// Load the referenced <see cref="IOpenApiReferenceable"/> object from a <see cref="OpenApiReference"/> object
         /// </summary>
-        internal IOpenApiReferenceable ResolveReference(OpenApiReference reference, bool useExternal)
+        internal IOpenApiReferenceable? ResolveReference(OpenApiReference? reference, bool useExternal)
         {
             if (reference == null)
             {
@@ -504,7 +509,7 @@ internal IOpenApiReferenceable ResolveReference(OpenApiReference reference, bool
             // Special case for Tag
             if (reference.Type == ReferenceType.Tag)
             {
-                foreach (var tag in this.Tags)
+                foreach (var tag in this.Tags ?? Enumerable.Empty<OpenApiTag>())
                 {
                     if (tag.Name == reference.Id)
                     {
@@ -526,11 +531,11 @@ internal IOpenApiReferenceable ResolveReference(OpenApiReference reference, bool
                 string relativePath = OpenApiConstants.ComponentsSegment + reference.Type.GetDisplayName() + "/" + reference.Id;
 
                 uriLocation = useExternal
-                    ? Workspace.GetDocumentId(reference.ExternalResource)?.OriginalString + relativePath
+                    ? Workspace?.GetDocumentId(reference.ExternalResource)?.OriginalString + relativePath
                     : BaseUri + relativePath;
             }
 
-            return Workspace.ResolveReference<IOpenApiReferenceable>(uriLocation);
+            return Workspace?.ResolveReference<IOpenApiReferenceable>(uriLocation);
         }
 
         /// <summary>
@@ -539,7 +544,7 @@ internal IOpenApiReferenceable ResolveReference(OpenApiReference reference, bool
         /// <param name="url"> The path to the OpenAPI file.</param>
         /// <param name="settings"></param>
         /// <returns></returns>
-        public static ReadResult Load(string url, OpenApiReaderSettings settings = null)
+        public static ReadResult Load(string url, OpenApiReaderSettings? settings = null)
         {
             return OpenApiModelFactory.Load(url, settings);
         }
@@ -553,7 +558,7 @@ public static ReadResult Load(string url, OpenApiReaderSettings settings = null)
         /// <returns></returns>
         public static ReadResult Load(Stream stream,
                                       string format,
-                                      OpenApiReaderSettings settings = null)
+                                      OpenApiReaderSettings? settings = null)
         {
             return OpenApiModelFactory.Load(stream, format, settings);
         }
@@ -567,7 +572,7 @@ public static ReadResult Load(Stream stream,
         /// <returns></returns>
         public static ReadResult Load(TextReader input,
                                            string format,
-                                           OpenApiReaderSettings settings = null)
+                                           OpenApiReaderSettings? settings = null)
         {
             return OpenApiModelFactory.Load(input, format, settings);
         }
@@ -578,7 +583,7 @@ public static ReadResult Load(TextReader input,
         /// <param name="url"> The path to the OpenAPI file.</param>
         /// <param name="settings">The OpenApi reader settings.</param>
         /// <returns></returns>
-        public static async Task<ReadResult> LoadAsync(string url, OpenApiReaderSettings settings = null)
+        public static async Task<ReadResult> LoadAsync(string url, OpenApiReaderSettings? settings = null)
         {
             return await OpenApiModelFactory.LoadAsync(url, settings);
         }
@@ -591,7 +596,7 @@ public static async Task<ReadResult> LoadAsync(string url, OpenApiReaderSettings
         /// <param name="settings">The OpenApi reader settings.</param>
         /// <param name="cancellationToken">Propagates information about operation cancelling.</param>
         /// <returns></returns>
-        public static async Task<ReadResult> LoadAsync(Stream stream, string format, OpenApiReaderSettings settings = null, CancellationToken cancellationToken = default)
+        public static async Task<ReadResult> LoadAsync(Stream stream, string format, OpenApiReaderSettings? settings = null, CancellationToken cancellationToken = default)
         {
             return await OpenApiModelFactory.LoadAsync(stream, format, settings, cancellationToken);
         }
@@ -603,7 +608,7 @@ public static async Task<ReadResult> LoadAsync(Stream stream, string format, Ope
         /// <param name="format"> The OpenAPI format to use during parsing.</param>
         /// <param name="settings">The OpenApi reader settings.</param>
         /// <returns></returns>
-        public static async Task<ReadResult> LoadAsync(TextReader input, string format, OpenApiReaderSettings settings = null)
+        public static async Task<ReadResult> LoadAsync(TextReader input, string format, OpenApiReaderSettings? settings = null)
         {
             return await OpenApiModelFactory.LoadAsync(input, format, settings);
         }
@@ -616,8 +621,8 @@ public static async Task<ReadResult> LoadAsync(TextReader input, string format,
         /// <param name="settings"></param>
         /// <returns></returns>
         public static ReadResult Parse(string input,
-                                       string format = null,
-                                       OpenApiReaderSettings settings = null)
+                                       string? format = null,
+                                       OpenApiReaderSettings? settings = null)
         {
             return OpenApiModelFactory.Parse(input, format, settings);
         }
@@ -625,9 +630,9 @@ public static ReadResult Parse(string input,
 
     internal class FindSchemaReferences : OpenApiVisitorBase
     {
-        private Dictionary<string, OpenApiSchema> Schemas;
+        private Dictionary<string, OpenApiSchema> Schemas = new();
 
-        public static void ResolveSchemas(OpenApiComponents components, Dictionary<string, OpenApiSchema> schemas)
+        public static void ResolveSchemas(OpenApiComponents? components, Dictionary<string, OpenApiSchema> schemas)
         {
             var visitor = new FindSchemaReferences();
             visitor.Schemas = schemas;
diff --git a/src/Microsoft.OpenApi/Models/OpenApiMediaType.cs b/src/Microsoft.OpenApi/Models/OpenApiMediaType.cs
index 7183d5808..17af31e64 100644
--- a/src/Microsoft.OpenApi/Models/OpenApiMediaType.cs
+++ b/src/Microsoft.OpenApi/Models/OpenApiMediaType.cs
@@ -8,6 +8,8 @@
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Writers;
 
+#nullable enable
+
 namespace Microsoft.OpenApi.Models
 {
     /// <summary>
@@ -15,12 +17,12 @@ namespace Microsoft.OpenApi.Models
     /// </summary>
     public class OpenApiMediaType : IOpenApiSerializable, IOpenApiExtensible
     {
-        private OpenApiSchema _schema;
+        private OpenApiSchema? _schema;
 
         /// <summary>
         /// The schema defining the type used for the request body.
         /// </summary>
-        public virtual OpenApiSchema Schema
+        public virtual OpenApiSchema? Schema
         {
             get => _schema;
             set => _schema = value;
@@ -30,13 +32,13 @@ public virtual OpenApiSchema Schema
         /// Example of the media type.
         /// The example object SHOULD be in the correct format as specified by the media type.
         /// </summary>
-        public JsonNode Example { get; set; }
+        public JsonNode? Example { get; set; }
 
         /// <summary>
         /// Examples of the media type.
         /// Each example object SHOULD match the media type and specified schema if present.
         /// </summary>
-        public IDictionary<string, OpenApiExample> Examples { get; set; } = new Dictionary<string, OpenApiExample>();
+        public IDictionary<string, OpenApiExample>? Examples { get; set; } = new Dictionary<string, OpenApiExample>();
 
         /// <summary>
         /// A map between a property name and its encoding information.
@@ -44,12 +46,12 @@ public virtual OpenApiSchema Schema
         /// The encoding object SHALL only apply to requestBody objects
         /// when the media type is multipart or application/x-www-form-urlencoded.
         /// </summary>
-        public IDictionary<string, OpenApiEncoding> Encoding { get; set; } = new Dictionary<string, OpenApiEncoding>();
+        public IDictionary<string, OpenApiEncoding>? Encoding { get; set; } = new Dictionary<string, OpenApiEncoding>();
 
         /// <summary>
         /// Serialize <see cref="OpenApiExternalDocs"/> to Open Api v3.0.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension> Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
 
         /// <summary>
         /// Parameterless constructor
@@ -59,7 +61,7 @@ public OpenApiMediaType() { }
         /// <summary>
         /// Initializes a copy of an <see cref="OpenApiMediaType"/> object
         /// </summary>
-        public OpenApiMediaType(OpenApiMediaType mediaType)
+        public OpenApiMediaType(OpenApiMediaType? mediaType)
         {
             _schema = mediaType?.Schema != null ? new(mediaType.Schema) : null;
             Example = mediaType?.Example != null ? JsonNodeCloneHelper.Clone(mediaType.Example) : null;
diff --git a/src/Microsoft.OpenApi/Models/OpenApiOperation.cs b/src/Microsoft.OpenApi/Models/OpenApiOperation.cs
index 498e93306..4da68d082 100644
--- a/src/Microsoft.OpenApi/Models/OpenApiOperation.cs
+++ b/src/Microsoft.OpenApi/Models/OpenApiOperation.cs
@@ -8,6 +8,8 @@
 using Microsoft.OpenApi.Models.References;
 using Microsoft.OpenApi.Writers;
 
+#nullable enable
+
 namespace Microsoft.OpenApi.Models
 {
     /// <summary>
@@ -24,30 +26,30 @@ public class OpenApiOperation : IOpenApiSerializable, IOpenApiExtensible
         /// A list of tags for API documentation control.
         /// Tags can be used for logical grouping of operations by resources or any other qualifier.
         /// </summary>
-        public IList<OpenApiTag> Tags { get; set; } = new List<OpenApiTag>();
+        public IList<OpenApiTag>? Tags { get; set; } = new List<OpenApiTag>();
 
         /// <summary>
         /// A short summary of what the operation does.
         /// </summary>
-        public string Summary { get; set; }
+        public string? Summary { get; set; }
 
         /// <summary>
         /// A verbose explanation of the operation behavior.
         /// CommonMark syntax MAY be used for rich text representation.
         /// </summary>
-        public string Description { get; set; }
+        public string? Description { get; set; }
 
         /// <summary>
         /// Additional external documentation for this operation.
         /// </summary>
-        public OpenApiExternalDocs ExternalDocs { get; set; }
+        public OpenApiExternalDocs? ExternalDocs { get; set; }
 
         /// <summary>
         /// Unique string used to identify the operation. The id MUST be unique among all operations described in the API.
         /// Tools and libraries MAY use the operationId to uniquely identify an operation, therefore,
         /// it is RECOMMENDED to follow common programming naming conventions.
         /// </summary>
-        public string OperationId { get; set; }
+        public string? OperationId { get; set; }
 
         /// <summary>
         /// A list of parameters that are applicable for this operation.
@@ -55,7 +57,7 @@ public class OpenApiOperation : IOpenApiSerializable, IOpenApiExtensible
         /// The list MUST NOT include duplicated parameters. A unique parameter is defined by a combination of a name and location.
         /// The list can use the Reference Object to link to parameters that are defined at the OpenAPI Object's components/parameters.
         /// </summary>
-        public IList<OpenApiParameter> Parameters { get; set; } = new List<OpenApiParameter>();
+        public IList<OpenApiParameter>? Parameters { get; set; } = new List<OpenApiParameter>();
 
         /// <summary>
         /// The request body applicable for this operation.
@@ -63,12 +65,12 @@ public class OpenApiOperation : IOpenApiSerializable, IOpenApiExtensible
         /// has explicitly defined semantics for request bodies.
         /// In other cases where the HTTP spec is vague, requestBody SHALL be ignored by consumers.
         /// </summary>
-        public OpenApiRequestBody RequestBody { get; set; }
+        public OpenApiRequestBody? RequestBody { get; set; }
 
         /// <summary>
         /// REQUIRED. The list of possible responses as they are returned from executing this operation.
         /// </summary>
-        public OpenApiResponses Responses { get; set; } = new();
+        public OpenApiResponses? Responses { get; set; } = new();
 
         /// <summary>
         /// A map of possible out-of band callbacks related to the parent operation.
@@ -78,7 +80,7 @@ public class OpenApiOperation : IOpenApiSerializable, IOpenApiExtensible
         /// The key value used to identify the callback object is an expression, evaluated at runtime,
         /// that identifies a URL to use for the callback operation.
         /// </summary>
-        public IDictionary<string, OpenApiCallback> Callbacks { get; set; } = new Dictionary<string, OpenApiCallback>();
+        public IDictionary<string, OpenApiCallback>? Callbacks { get; set; } = new Dictionary<string, OpenApiCallback>();
 
         /// <summary>
         /// Declares this operation to be deprecated. Consumers SHOULD refrain from usage of the declared operation.
@@ -92,19 +94,19 @@ public class OpenApiOperation : IOpenApiSerializable, IOpenApiExtensible
         /// This definition overrides any declared top-level security.
         /// To remove a top-level security declaration, an empty array can be used.
         /// </summary>
-        public IList<OpenApiSecurityRequirement> Security { get; set; } = new List<OpenApiSecurityRequirement>();
+        public IList<OpenApiSecurityRequirement>? Security { get; set; } = new List<OpenApiSecurityRequirement>();
 
         /// <summary>
         /// An alternative server array to service this operation.
         /// If an alternative server object is specified at the Path Item Object or Root level,
         /// it will be overridden by this value.
         /// </summary>
-        public IList<OpenApiServer> Servers { get; set; } = new List<OpenApiServer>();
+        public IList<OpenApiServer>? Servers { get; set; } = new List<OpenApiServer>();
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.
         /// </summary>
-        public IDictionary<string, IOpenApiExtension> Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
+        public IDictionary<string, IOpenApiExtension>? Extensions { get; set; } = new Dictionary<string, IOpenApiExtension>();
 
         /// <summary>
         /// Parameterless constructor
@@ -114,9 +116,9 @@ public OpenApiOperation() { }
         /// <summary>
         /// Initializes a copy of an <see cref="OpenApiOperation"/> object
         /// </summary>
-        public OpenApiOperation(OpenApiOperation operation)
+        public OpenApiOperation(OpenApiOperation? operation)
         {
-            Tags = operation?.Tags != null ? new List<OpenApiTag>(operation?.Tags) : null;
+            Tags = operation?.Tags != null ? new List<OpenApiTag>(operation.Tags) : null;
             Summary = operation?.Summary ?? Summary;
             Description = operation?.Description ?? Description;
             ExternalDocs = operation?.ExternalDocs != null ? new(operation?.ExternalDocs) : null;
diff --git a/src/Microsoft.OpenApi/Services/OpenApiWalker.cs b/src/Microsoft.OpenApi/Services/OpenApiWalker.cs
index 9cf54f65a..2dd882ce7 100644
--- a/src/Microsoft.OpenApi/Services/OpenApiWalker.cs
+++ b/src/Microsoft.OpenApi/Services/OpenApiWalker.cs
@@ -105,11 +105,11 @@ internal void Walk(OpenApiExternalDocs externalDocs)
 
             _visitor.Visit(externalDocs);
         }
-
+#nullable enable
         /// <summary>
         /// Visits <see cref="OpenApiComponents"/> and child objects
         /// </summary>
-        internal void Walk(OpenApiComponents components)
+        internal void Walk(OpenApiComponents? components)
         {
             if (components == null)
             {
@@ -118,11 +118,6 @@ internal void Walk(OpenApiComponents components)
 
             _visitor.Visit(components);
 
-            if (components == null)
-            {
-                return;
-            }
-
             Walk(OpenApiConstants.Schemas, () =>
             {
                 if (components.Schemas != null)
@@ -236,6 +231,7 @@ internal void Walk(OpenApiComponents components)
             Walk(components as IOpenApiExtensible);
         }
 
+#nullable restore
         /// <summary>
         /// Visits <see cref="OpenApiPaths"/> and child objects
         /// </summary>
diff --git a/src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs b/src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs
index 319a5d63f..a51ccd9c3 100644
--- a/src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs
+++ b/src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs
@@ -121,13 +121,14 @@ public bool Contains(string location)
             return _IOpenApiReferenceableRegistry.ContainsKey(key) || _artifactsRegistry.ContainsKey(key);
         }
 
+#nullable enable
         /// <summary>
         /// Resolves a reference given a key.
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="location"></param>
         /// <returns>The resolved reference.</returns>
-        public T ResolveReference<T>(string location)
+        public T? ResolveReference<T>(string location)
         {
             if (string.IsNullOrEmpty(location)) return default;
 
@@ -143,6 +144,7 @@ public T ResolveReference<T>(string location)
 
             return default;
         }
+#nullable restore
 
         private Uri ToLocationUrl(string location)
         {
diff --git a/src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs b/src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs
index bbf00fef0..f6102f316 100644
--- a/src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs
+++ b/src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs
@@ -125,6 +125,7 @@ public static void WriteProperty<T>(this IOpenApiWriter writer, string name, T v
             writer.WriteValue(value);
         }
 
+#nullable enable
         /// <summary>
         /// Write the optional Open API object/element.
         /// </summary>
@@ -136,7 +137,7 @@ public static void WriteProperty<T>(this IOpenApiWriter writer, string name, T v
         public static void WriteOptionalObject<T>(
             this IOpenApiWriter writer,
             string name,
-            T value,
+            T? value,
             Action<IOpenApiWriter, T> action)
         {
             if (value != null)
@@ -161,7 +162,7 @@ public static void WriteOptionalObject<T>(
         public static void WriteRequiredObject<T>(
             this IOpenApiWriter writer,
             string name,
-            T value,
+            T? value,
             Action<IOpenApiWriter, T> action)
         {
             Utils.CheckArgumentNull(action);
@@ -177,6 +178,7 @@ public static void WriteRequiredObject<T>(
                 writer.WriteEndObject();
             }
         }
+#nullable restore
 
         /// <summary>
         /// Write the optional of collection string.
diff --git a/test/Microsoft.OpenApi.Hidi.Tests/Formatters/PowerShellFormatterTests.cs b/test/Microsoft.OpenApi.Hidi.Tests/Formatters/PowerShellFormatterTests.cs
index 94f99a1d2..214bd47ff 100644
--- a/test/Microsoft.OpenApi.Hidi.Tests/Formatters/PowerShellFormatterTests.cs
+++ b/test/Microsoft.OpenApi.Hidi.Tests/Formatters/PowerShellFormatterTests.cs
@@ -57,18 +57,21 @@ public void RemoveAnyOfAndOneOfFromSchema()
             var walker = new OpenApiWalker(powerShellFormatter);
             walker.Walk(openApiDocument);
 
-            var testSchema = openApiDocument.Components.Schemas["TestSchema"];
-            var averageAudioDegradationProperty = testSchema.Properties["averageAudioDegradation"];
-            var defaultPriceProperty = testSchema.Properties["defaultPrice"];
+            var testSchema = openApiDocument.Components?.Schemas?["TestSchema"];
+            var averageAudioDegradationProperty = testSchema?.Properties["averageAudioDegradation"];
+            var defaultPriceProperty = testSchema?.Properties["defaultPrice"];
 
             // Assert
-            Assert.Null(averageAudioDegradationProperty.AnyOf);
-            Assert.Equal("number", averageAudioDegradationProperty.Type);
-            Assert.Equal("float", averageAudioDegradationProperty.Format);
-            Assert.True(averageAudioDegradationProperty.Nullable);
-            Assert.Null(defaultPriceProperty.OneOf);
-            Assert.Equal("number", defaultPriceProperty.Type);
-            Assert.Equal("double", defaultPriceProperty.Format);
+            Assert.NotNull(openApiDocument.Components);
+            Assert.NotNull(openApiDocument.Components.Schemas);
+            Assert.NotNull(testSchema);
+            Assert.Null(averageAudioDegradationProperty?.AnyOf);
+            Assert.Equal("number", averageAudioDegradationProperty?.Type);
+            Assert.Equal("float", averageAudioDegradationProperty?.Format);
+            Assert.True(averageAudioDegradationProperty?.Nullable);
+            Assert.Null(defaultPriceProperty?.OneOf);
+            Assert.Equal("number", defaultPriceProperty?.Type);
+            Assert.Equal("double", defaultPriceProperty?.Format);
             Assert.NotNull(testSchema.AdditionalProperties);
         }
 
@@ -83,7 +86,7 @@ public void ResolveFunctionParameters()
             var walker = new OpenApiWalker(powerShellFormatter);
             walker.Walk(openApiDocument);
 
-            var idsParameter = openApiDocument.Paths["/foo"].Operations[OperationType.Get].Parameters.Where(static p => p.Name == "ids").FirstOrDefault();
+            var idsParameter = openApiDocument.Paths["/foo"].Operations[OperationType.Get].Parameters?.Where(static p => p.Name == "ids").FirstOrDefault();
 
             // Assert
             Assert.Null(idsParameter?.Content);
diff --git a/test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt b/test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt
index 7eb01a70c..33c61f484 100755
--- a/test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt
+++ b/test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt
@@ -336,18 +336,18 @@ namespace Microsoft.OpenApi.Models
     public class OpenApiComponents : Microsoft.OpenApi.Interfaces.IOpenApiElement, Microsoft.OpenApi.Interfaces.IOpenApiExtensible, Microsoft.OpenApi.Interfaces.IOpenApiSerializable
     {
         public OpenApiComponents() { }
-        public OpenApiComponents(Microsoft.OpenApi.Models.OpenApiComponents components) { }
-        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiSchema> Schemas { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiCallback> Callbacks { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiExample> Examples { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Interfaces.IOpenApiExtension> Extensions { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiHeader> Headers { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiLink> Links { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiParameter> Parameters { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiPathItem> PathItems { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiRequestBody> RequestBodies { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiResponse> Responses { get; set; }
-        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiSecurityScheme> SecuritySchemes { get; set; }
+        public OpenApiComponents(Microsoft.OpenApi.Models.OpenApiComponents? components) { }
+        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiSchema>? Schemas { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiCallback>? Callbacks { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiExample>? Examples { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Interfaces.IOpenApiExtension>? Extensions { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiHeader>? Headers { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiLink>? Links { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiParameter>? Parameters { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiPathItem>? PathItems { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiRequestBody>? RequestBodies { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiResponse>? Responses { get; set; }
+        public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiSecurityScheme>? SecuritySchemes { get; set; }
         public void SerializeAsV2(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SerializeAsV3(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SerializeAsV31(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
@@ -531,32 +531,32 @@ namespace Microsoft.OpenApi.Models
     public class OpenApiDocument : Microsoft.OpenApi.Interfaces.IOpenApiElement, Microsoft.OpenApi.Interfaces.IOpenApiExtensible, Microsoft.OpenApi.Interfaces.IOpenApiSerializable
     {
         public OpenApiDocument() { }
-        public OpenApiDocument(Microsoft.OpenApi.Models.OpenApiDocument document) { }
+        public OpenApiDocument(Microsoft.OpenApi.Models.OpenApiDocument? document) { }
         public System.Uri BaseUri { get; }
-        public Microsoft.OpenApi.Models.OpenApiComponents Components { get; set; }
-        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Interfaces.IOpenApiExtension> Extensions { get; set; }
-        public Microsoft.OpenApi.Models.OpenApiExternalDocs ExternalDocs { get; set; }
+        public Microsoft.OpenApi.Models.OpenApiComponents? Components { get; set; }
+        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Interfaces.IOpenApiExtension>? Extensions { get; set; }
+        public Microsoft.OpenApi.Models.OpenApiExternalDocs? ExternalDocs { get; set; }
         public string HashCode { get; }
         public Microsoft.OpenApi.Models.OpenApiInfo Info { get; set; }
-        public string JsonSchemaDialect { get; set; }
+        public string? JsonSchemaDialect { get; set; }
         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.IDictionary<string, Microsoft.OpenApi.Models.OpenApiPathItem> Webhooks { get; set; }
-        public Microsoft.OpenApi.Services.OpenApiWorkspace Workspace { 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.IDictionary<string, Microsoft.OpenApi.Models.OpenApiPathItem>? Webhooks { get; set; }
+        public Microsoft.OpenApi.Services.OpenApiWorkspace? Workspace { get; set; }
         public void SerializeAsV2(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SerializeAsV3(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SerializeAsV31(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SetReferenceHostDocument() { }
         public static string GenerateHashValue(Microsoft.OpenApi.Models.OpenApiDocument doc) { }
-        public static Microsoft.OpenApi.Reader.ReadResult Load(string url, Microsoft.OpenApi.Reader.OpenApiReaderSettings settings = null) { }
-        public static Microsoft.OpenApi.Reader.ReadResult Load(System.IO.Stream stream, string format, Microsoft.OpenApi.Reader.OpenApiReaderSettings settings = null) { }
-        public static Microsoft.OpenApi.Reader.ReadResult Load(System.IO.TextReader input, string format, Microsoft.OpenApi.Reader.OpenApiReaderSettings settings = null) { }
-        public static System.Threading.Tasks.Task<Microsoft.OpenApi.Reader.ReadResult> LoadAsync(string url, Microsoft.OpenApi.Reader.OpenApiReaderSettings settings = null) { }
-        public static System.Threading.Tasks.Task<Microsoft.OpenApi.Reader.ReadResult> LoadAsync(System.IO.TextReader input, string format, Microsoft.OpenApi.Reader.OpenApiReaderSettings settings = null) { }
-        public static System.Threading.Tasks.Task<Microsoft.OpenApi.Reader.ReadResult> LoadAsync(System.IO.Stream stream, string format, Microsoft.OpenApi.Reader.OpenApiReaderSettings settings = null, System.Threading.CancellationToken cancellationToken = default) { }
-        public static Microsoft.OpenApi.Reader.ReadResult Parse(string input, string format = null, Microsoft.OpenApi.Reader.OpenApiReaderSettings settings = null) { }
+        public static Microsoft.OpenApi.Reader.ReadResult Load(string url, Microsoft.OpenApi.Reader.OpenApiReaderSettings? settings = null) { }
+        public static Microsoft.OpenApi.Reader.ReadResult Load(System.IO.Stream stream, string format, Microsoft.OpenApi.Reader.OpenApiReaderSettings? settings = null) { }
+        public static Microsoft.OpenApi.Reader.ReadResult Load(System.IO.TextReader input, string format, Microsoft.OpenApi.Reader.OpenApiReaderSettings? settings = null) { }
+        public static System.Threading.Tasks.Task<Microsoft.OpenApi.Reader.ReadResult> LoadAsync(string url, Microsoft.OpenApi.Reader.OpenApiReaderSettings? settings = null) { }
+        public static System.Threading.Tasks.Task<Microsoft.OpenApi.Reader.ReadResult> LoadAsync(System.IO.TextReader input, string format, Microsoft.OpenApi.Reader.OpenApiReaderSettings? settings = null) { }
+        public static System.Threading.Tasks.Task<Microsoft.OpenApi.Reader.ReadResult> LoadAsync(System.IO.Stream stream, string format, Microsoft.OpenApi.Reader.OpenApiReaderSettings? settings = null, System.Threading.CancellationToken cancellationToken = default) { }
+        public static Microsoft.OpenApi.Reader.ReadResult Parse(string input, string? format = null, Microsoft.OpenApi.Reader.OpenApiReaderSettings? settings = null) { }
     }
     public class OpenApiEncoding : Microsoft.OpenApi.Interfaces.IOpenApiElement, Microsoft.OpenApi.Interfaces.IOpenApiExtensible, Microsoft.OpenApi.Interfaces.IOpenApiSerializable
     {
@@ -688,12 +688,12 @@ namespace Microsoft.OpenApi.Models
     public class OpenApiMediaType : Microsoft.OpenApi.Interfaces.IOpenApiElement, Microsoft.OpenApi.Interfaces.IOpenApiExtensible, Microsoft.OpenApi.Interfaces.IOpenApiSerializable
     {
         public OpenApiMediaType() { }
-        public OpenApiMediaType(Microsoft.OpenApi.Models.OpenApiMediaType mediaType) { }
-        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiEncoding> Encoding { get; set; }
-        public System.Text.Json.Nodes.JsonNode Example { get; set; }
-        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiExample> Examples { get; set; }
-        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Interfaces.IOpenApiExtension> Extensions { get; set; }
-        public virtual Microsoft.OpenApi.Models.OpenApiSchema Schema { get; set; }
+        public OpenApiMediaType(Microsoft.OpenApi.Models.OpenApiMediaType? mediaType) { }
+        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiEncoding>? Encoding { get; set; }
+        public System.Text.Json.Nodes.JsonNode? Example { get; set; }
+        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiExample>? Examples { get; set; }
+        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Interfaces.IOpenApiExtension>? Extensions { get; set; }
+        public virtual Microsoft.OpenApi.Models.OpenApiSchema? Schema { get; set; }
         public void SerializeAsV2(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SerializeAsV3(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SerializeAsV31(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
@@ -728,20 +728,20 @@ namespace Microsoft.OpenApi.Models
     {
         public const bool DeprecatedDefault = false;
         public OpenApiOperation() { }
-        public OpenApiOperation(Microsoft.OpenApi.Models.OpenApiOperation operation) { }
-        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiCallback> Callbacks { get; set; }
+        public OpenApiOperation(Microsoft.OpenApi.Models.OpenApiOperation? operation) { }
+        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiCallback>? Callbacks { get; set; }
         public bool Deprecated { get; set; }
-        public string Description { get; set; }
-        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Interfaces.IOpenApiExtension> Extensions { get; set; }
-        public Microsoft.OpenApi.Models.OpenApiExternalDocs ExternalDocs { get; set; }
-        public string OperationId { get; set; }
-        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiParameter> Parameters { get; set; }
-        public Microsoft.OpenApi.Models.OpenApiRequestBody RequestBody { get; set; }
-        public Microsoft.OpenApi.Models.OpenApiResponses Responses { get; set; }
-        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiSecurityRequirement> Security { get; set; }
-        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiServer> Servers { get; set; }
-        public string Summary { get; set; }
-        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiTag> Tags { get; set; }
+        public string? Description { get; set; }
+        public System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Interfaces.IOpenApiExtension>? Extensions { get; set; }
+        public Microsoft.OpenApi.Models.OpenApiExternalDocs? ExternalDocs { get; set; }
+        public string? OperationId { get; set; }
+        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiParameter>? Parameters { get; set; }
+        public Microsoft.OpenApi.Models.OpenApiRequestBody? RequestBody { get; set; }
+        public Microsoft.OpenApi.Models.OpenApiResponses? Responses { get; set; }
+        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiSecurityRequirement>? Security { get; set; }
+        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiServer>? Servers { get; set; }
+        public string? Summary { get; set; }
+        public System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiTag>? Tags { get; set; }
         public void SerializeAsV2(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SerializeAsV3(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
         public void SerializeAsV31(Microsoft.OpenApi.Writers.IOpenApiWriter writer) { }
@@ -1512,7 +1512,7 @@ namespace Microsoft.OpenApi.Services
         public bool Contains(string location) { }
         public System.Uri GetDocumentId(string key) { }
         public bool RegisterComponent<T>(string location, T component) { }
-        public T ResolveReference<T>(string location) { }
+        public T? ResolveReference<T>(string location) { }
     }
     public class OperationSearch : Microsoft.OpenApi.Services.OpenApiVisitorBase
     {
@@ -1837,7 +1837,7 @@ namespace Microsoft.OpenApi.Writers
             where T : Microsoft.OpenApi.Interfaces.IOpenApiElement { }
         public static void WriteOptionalMap<T>(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, System.Collections.Generic.IDictionary<string, T> elements, System.Action<Microsoft.OpenApi.Writers.IOpenApiWriter, string, T> action)
             where T : Microsoft.OpenApi.Interfaces.IOpenApiElement { }
-        public static void WriteOptionalObject<T>(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, T value, System.Action<Microsoft.OpenApi.Writers.IOpenApiWriter, T> action) { }
+        public static void WriteOptionalObject<T>(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, T? value, System.Action<Microsoft.OpenApi.Writers.IOpenApiWriter, T> action) { }
         public static void WriteProperty(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, string value) { }
         public static void WriteProperty(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, bool value, bool defaultValue = false) { }
         public static void WriteProperty(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, bool? value, bool defaultValue = false) { }
@@ -1850,7 +1850,7 @@ namespace Microsoft.OpenApi.Writers
         public static void WriteRequiredMap(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, System.Collections.Generic.IDictionary<string, string> elements, System.Action<Microsoft.OpenApi.Writers.IOpenApiWriter, string> action) { }
         public static void WriteRequiredMap<T>(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, System.Collections.Generic.IDictionary<string, T> elements, System.Action<Microsoft.OpenApi.Writers.IOpenApiWriter, T> action)
             where T : Microsoft.OpenApi.Interfaces.IOpenApiElement { }
-        public static void WriteRequiredObject<T>(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, T value, System.Action<Microsoft.OpenApi.Writers.IOpenApiWriter, T> action) { }
+        public static void WriteRequiredObject<T>(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, T? value, System.Action<Microsoft.OpenApi.Writers.IOpenApiWriter, T> action) { }
         public static void WriteRequiredProperty(this Microsoft.OpenApi.Writers.IOpenApiWriter writer, string name, string value) { }
     }
     public class OpenApiWriterSettings
diff --git a/test/Microsoft.OpenApi.Tests/Walkers/WalkerLocationTests.cs b/test/Microsoft.OpenApi.Tests/Walkers/WalkerLocationTests.cs
index 924364ccd..698d3fc5c 100644
--- a/test/Microsoft.OpenApi.Tests/Walkers/WalkerLocationTests.cs
+++ b/test/Microsoft.OpenApi.Tests/Walkers/WalkerLocationTests.cs
@@ -24,7 +24,9 @@ public void LocateTopLevelObjects()
             walker.Walk(doc);
 
             locator.Locations.Should().BeEquivalentTo(new List<string> {
+                "#/info",
                 "#/servers",
+                "#/paths",
                 "#/tags"
             });
         }
@@ -39,7 +41,6 @@ public void LocateTopLevelArrayItems()
                     new(),
                     new()
                 },
-                Paths = new(),
                 Tags = new List<OpenApiTag>
                 {
                     new()
@@ -51,6 +52,7 @@ public void LocateTopLevelArrayItems()
             walker.Walk(doc);
 
             locator.Locations.Should().BeEquivalentTo(new List<string> {
+                "#/info",
                 "#/servers",
                 "#/servers/0",
                 "#/servers/1",
@@ -63,10 +65,7 @@ public void LocateTopLevelArrayItems()
         [Fact]
         public void LocatePathOperationContentSchema()
         {
-            var doc = new OpenApiDocument
-            {
-                Paths = new()
-            };
+            var doc = new OpenApiDocument();
             doc.Paths.Add("/test", new()
             {
                 Operations = new Dictionary<OperationType, OpenApiOperation>
@@ -98,6 +97,7 @@ public void LocatePathOperationContentSchema()
             walker.Walk(doc);
 
             locator.Locations.Should().BeEquivalentTo(new List<string> {
+                "#/info",
                 "#/servers",
                 "#/paths",
                 "#/paths/~1test",
@@ -131,7 +131,6 @@ public void WalkDOMWithCycles()
 
             var doc = new OpenApiDocument
             {
-                Paths = new(),
                 Components = new()
                 {
                     Schemas = new Dictionary<string, OpenApiSchema>
@@ -146,6 +145,7 @@ public void WalkDOMWithCycles()
             walker.Walk(doc);
 
             locator.Locations.Should().BeEquivalentTo(new List<string> {
+                "#/info",
                 "#/servers",
                 "#/paths",
                 "#/components",