Different approach

Author: Youssef1313

src/Microsoft.OpenApi/Models/JsonSchemaReference.cs

@@ -114,10 +114,22 @@ public class JsonSchemaReference : OpenApiReferenceWithDescription
     /// </summary>
     public JsonSchemaType? SchemaType { get; set; }
 
+    internal bool WasConstExplicitlySet { get; private set; }
+
     /// <summary>
     /// Follow <see href="https://json-schema.org/draft/2020-12/json-schema-validation">JSON Schema definition</see>.
     /// </summary>
-    public string? Const { get; set; } = OpenApiUnsetValues.UnsetString;
+    public string? Const
+    {
+        get => field;
+        set
+        {
+            // TODO: In the next major release, Const should be made a JsonNode.
+            // See https://github.com/microsoft/OpenAPI.NET/issues/2935 for more information.
+            WasConstExplicitlySet = true;
+            field = value;
+        }
+    }
 
     /// <summary>
     /// Follow <see href="https://json-schema.org/draft/2020-12/json-schema-validation">JSON Schema definition</see>.
@@ -365,6 +377,7 @@ public JsonSchemaReference(JsonSchemaReference reference) : base(reference)
         ExclusiveMinimum = reference.ExclusiveMinimum;
         SchemaType = reference.SchemaType;
         Const = reference.Const;
+        WasConstExplicitlySet = reference.WasConstExplicitlySet;
         Format = reference.Format;
         Maximum = reference.Maximum;
         Minimum = reference.Minimum;
@@ -436,7 +449,11 @@ private void SerializeAdditionalV3XProperties(IOpenApiWriter writer, Action<IOpe
         writer.WriteProperty(OpenApiConstants.DynamicRef, DynamicRef);
         writer.WriteProperty(OpenApiConstants.DynamicAnchor, DynamicAnchor);
 
-        writer.WriteRequiredProperty(OpenApiConstants.Const, Const);
+        if (WasConstExplicitlySet)
+        {
+            writer.WriteRequiredProperty(OpenApiConstants.Const, Const);
+        }
+
         WriteSchemaType(writer, OpenApiConstants.Type, SchemaType, allowMultipleTypes: true);
         writer.WriteProperty(OpenApiConstants.Format, Format);
         writer.WriteProperty(OpenApiConstants.MultipleOf, MultipleOf);
@@ -607,6 +624,7 @@ internal void ApplySchemaMetadata(OpenApiSchema schema, JsonObject jsonObject)
         ExclusiveMinimum = schema.ExclusiveMinimum;
         SchemaType = schema.Type;
         Const = schema.Const;
+        WasConstExplicitlySet = schema.WasConstExplicitlySet;
         Format = schema.Format;
         Maximum = schema.Maximum;
         Minimum = schema.Minimum;

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -117,8 +117,20 @@ Extensions is not null &&
             nullExtRawValue is JsonNodeExtension { Node: JsonNode jsonNode } &&
             jsonNode.GetValueKind() is JsonValueKind.True;
 
+        internal bool WasConstExplicitlySet { get; private set; }
+
         /// <inheritdoc />
-        public string? Const { get; set; } = OpenApiUnsetValues.UnsetString;
+        public string? Const
+        {
+            get => field;
+            set
+            {
+                // TODO: In the next major release, Const should be made a JsonNode.
+                // See https://github.com/microsoft/OpenAPI.NET/issues/2935 for more information.
+                WasConstExplicitlySet = true;
+                field = value;
+            }
+        }
 
         /// <inheritdoc />
         public string? Format { get; set; }
@@ -316,6 +328,15 @@ internal OpenApiSchema(IOpenApiSchema schema)
             Title = schema.Title ?? Title;
             Id = schema.Id ?? Id;
             Const = schema.Const ?? Const;
+            if (schema is OpenApiSchema concreteSchema)
+            {
+                WasConstExplicitlySet = concreteSchema.WasConstExplicitlySet;
+            }
+            else if (Const is null)
+            {
+                WasConstExplicitlySet = false;
+            }
+
             Schema = schema.Schema ?? Schema;
             Comment = schema.Comment ?? Comment;
             Vocabulary = schema.Vocabulary != null ? new Dictionary<string, bool>(schema.Vocabulary) : null;
@@ -511,7 +532,7 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
 
             // enum
             var enumValue = Enum is not { Count: > 0 }
-                && !ReferenceEquals(Const, OpenApiUnsetValues.UnsetString)
+                && WasConstExplicitlySet
                 && version < OpenApiSpecVersion.OpenApi3_1
                 ? new List<JsonNode> { JsonValue.Create(Const)! }
                 : Enum;
@@ -685,7 +706,12 @@ internal void WriteJsonSchemaKeywords(IOpenApiWriter writer, Action<IOpenApiWrit
             writer.WriteProperty(OpenApiConstants.Id, Id);
             writer.WriteProperty(OpenApiConstants.DollarSchema, Schema?.ToString());
             writer.WriteProperty(OpenApiConstants.Comment, Comment);
-            writer.WriteRequiredProperty(OpenApiConstants.Const, Const);
+            
+            if (WasConstExplicitlySet)
+            {
+                writer.WriteRequiredProperty(OpenApiConstants.Const, Const);
+            }
+
             writer.WriteOptionalMap(OpenApiConstants.Vocabulary, Vocabulary, (w, s) => w.WriteValue(s));
             writer.WriteOptionalMap(OpenApiConstants.Defs, Definitions, callback);
             writer.WriteProperty(OpenApiConstants.Anchor, Anchor);
@@ -891,7 +917,7 @@ private void SerializeAsV2(
             });
 
             // enum
-            var enumValue = Enum is not { Count: > 0 } && !ReferenceEquals(Const, OpenApiUnsetValues.UnsetString)
+            var enumValue = Enum is not { Count: > 0 } && WasConstExplicitlySet
                 ? new List<JsonNode> { JsonValue.Create(Const)! }
                 : Enum;
             writer.WriteOptionalCollection(OpenApiConstants.Enum, enumValue, (nodeWriter, s) => nodeWriter.WriteAny(s));

src/Microsoft.OpenApi/Models/OpenApiUnsetValues.cs

@@ -71,7 +71,7 @@ public string? Title
         /// <inheritdoc/>
         public JsonSchemaType? Type { get => Reference.SchemaType ?? Target?.Type; set => Reference.SchemaType = value; }
         /// <inheritdoc/>
-        public string? Const { get => string.IsNullOrEmpty(Reference.Const) ? Target?.Const : Reference.Const; set => Reference.Const = value; }
+        public string? Const { get => Reference.WasConstExplicitlySet ? Reference.Const : Target?.Const; set => Reference.Const = value; }
         /// <inheritdoc/>
         public string? Format { get => string.IsNullOrEmpty(Reference.Format) ? Target?.Format : Reference.Format; set => Reference.Format = value; }
         /// <inheritdoc/>

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

@@ -1,3 +1 @@
 #nullable enable
-Microsoft.OpenApi.OpenApiUnsetValues
-static readonly Microsoft.OpenApi.OpenApiUnsetValues.UnsetString -> string!

src/Microsoft.OpenApi/PublicAPI.Unshipped.txt

@@ -42,11 +42,6 @@ public static void WriteRequiredProperty(this IOpenApiWriter writer, string name
         {
             Utils.CheckArgumentNullOrEmpty(name);
 
-            if (ReferenceEquals(value, OpenApiUnsetValues.UnsetString))
-            {
-                return;
-            }
-
             writer.WritePropertyName(name);
             if (value == null)
             {