chore: code refactor and cleanup

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/Interfaces/IOpenApiSchema.cs

@@ -303,5 +303,5 @@ public interface IOpenApiSchema : IOpenApiDescribedElement, IOpenApiSerializable
     /// <summary>
     /// Follow JSON Schema definition:https://json-schema.org/draft/2020-12/json-schema-validation#section-6.5.4
     /// </summary>
-    public IDictionary<string, ISet<string>> DependentRequired { get; }
+    public IDictionary<string, ISet<string>>? DependentRequired { get; }
 }

src/Microsoft.OpenApi/Models/OpenApiExtensibleDictionary.cs

@@ -20,14 +20,14 @@ public abstract class OpenApiExtensibleDictionary<T> : Dictionary<string, T>,
         /// <summary>
         /// Parameterless constructor
         /// </summary>
-        protected OpenApiExtensibleDictionary():this(null) { }
+        protected OpenApiExtensibleDictionary():this([]) { }
         /// <summary>
         /// Initializes a copy of <see cref="OpenApiExtensibleDictionary{T}"/> class.
         /// </summary>
         /// <param name="dictionary">The generic dictionary.</param>
         /// <param name="extensions">The dictionary of <see cref="IOpenApiExtension"/>.</param>
         protected OpenApiExtensibleDictionary(
-            Dictionary<string, T>? dictionary,
+            Dictionary<string, T> dictionary,
             IDictionary<string, IOpenApiExtension>? extensions = null) : base(dictionary is null ? [] : dictionary)
         {
             Extensions = extensions != null ? new Dictionary<string, IOpenApiExtension>(extensions) : [];

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -177,7 +177,7 @@ public class OpenApiSchema : IOpenApiReferenceable, IOpenApiExtensible, IOpenApi
         public IDictionary<string, object>? Annotations { get; set; }
 
         /// <inheritdoc />
-        public IDictionary<string, ISet<string>> DependentRequired { get; set; } = new Dictionary<string, ISet<string>>();
+        public IDictionary<string, ISet<string>>? DependentRequired { get; set; } = new Dictionary<string, ISet<string>>();
 
         /// <summary>
         /// Parameterless constructor

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

@@ -155,7 +155,7 @@ public string? Description
         public IDictionary<string, object>? Annotations { get => Target?.Annotations; }
 
         /// <inheritdoc/>
-        public IDictionary<string, ISet<string>> DependentRequired { get => Target?.DependentRequired; }
+        public IDictionary<string, ISet<string>>? DependentRequired { get => Target?.DependentRequired; }
 
         /// <inheritdoc/>
         public override void SerializeAsV31(IOpenApiWriter writer)

src/Microsoft.OpenApi/Reader/ParseNodes/MapNode.cs

@@ -103,7 +103,7 @@ public override Dictionary<string, T> CreateSimpleMap<T>(Func<ValueNode, T> map)
             return nodes.ToDictionary(k => k.key, v => v.value);
         }
 
-        public override Dictionary<string, ISet<T>> CreateArrayMap<T>(Func<ValueNode, OpenApiDocument, T> map, OpenApiDocument openApiDocument)
+        public override Dictionary<string, ISet<T>> CreateArrayMap<T>(Func<ValueNode, OpenApiDocument?, T> map, OpenApiDocument? openApiDocument)
         {
             var jsonMap = _node ?? throw new OpenApiReaderException($"Expected map while parsing {typeof(T).Name}", Context);
 

src/Microsoft.OpenApi/Reader/ParseNodes/ParseNode.cs

@@ -86,7 +86,7 @@ public virtual List<JsonNode> CreateListOfAny()
             throw new OpenApiReaderException("Cannot create a list from this type of node.", Context);
         }
 
-        public virtual Dictionary<string, ISet<T>> CreateArrayMap<T>(Func<ValueNode, OpenApiDocument, T> map, OpenApiDocument openApiDocument)
+        public virtual Dictionary<string, ISet<T>> CreateArrayMap<T>(Func<ValueNode, OpenApiDocument?, T> map, OpenApiDocument? openApiDocument)
         {
             throw new OpenApiReaderException("Cannot create array map from this type of node.", Context);
         }

src/Microsoft.OpenApi/Reader/ParseNodes/ValueNode.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Globalization;
 using System.Text.Json.Nodes;
+using System.Xml.Linq;
 using Microsoft.OpenApi.Exceptions;
 
 namespace Microsoft.OpenApi.Reader.ParseNodes
@@ -22,10 +23,11 @@ public ValueNode(ParsingContext context, JsonNode? node) : base(
             _node = scalarNode;
         }
 
-        public override string? GetScalarValue()
+        public override string GetScalarValue()
         {
             var scalarValue = _node.GetValue<object>();
-            return Convert.ToString(scalarValue, CultureInfo.InvariantCulture);
+            return Convert.ToString(scalarValue, CultureInfo.InvariantCulture) 
+                ?? throw new OpenApiReaderException("Expected a value.", _node);
         }
 
         /// <summary>

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

@@ -265,7 +265,7 @@ internal static partial class OpenApiV31Deserializer
                 {
                     if (n is ValueNode)
                     {
-                         var value = n.GetScalarValue();
+                        var value = n.GetScalarValue();
                         if (value is not null)
                         {
                             o.AdditionalPropertiesAllowed = bool.Parse(value);

src/Microsoft.OpenApi/Writers/OpenApiWriterBase.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Globalization;
 using System.IO;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.OpenApi.Exceptions;
@@ -229,7 +230,7 @@ public virtual void WriteEnumerable<T>(IEnumerable<T> collection)
         /// Write object value.
         /// </summary>
         /// <param name="value">The object value.</param>
-        public virtual void WriteValue(object value)
+        public virtual void WriteValue(object? value)
         {
             if (value == null)
             {

src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs

@@ -321,7 +321,7 @@ public static void WriteOptionalMap(
         public static void WriteOptionalMap(
             this IOpenApiWriter writer,
             string name,
-            IDictionary<string, ISet<string>> elements,
+            IDictionary<string, ISet<string>>? elements,
             Action<IOpenApiWriter, ISet<string>> action)
         {
             if (elements != null && elements.Any())