Add support for JSON schema const keyword in OpenApiSchema

src/Microsoft.OpenApi/Models/OpenApiConstants.cs

@@ -50,6 +50,11 @@ public static class OpenApiConstants
         /// </summary>
         public const string Title = "title";
 
+        /// <summary>
+        /// Field: Const
+        /// </summary>
+        public const string Const = "const";
+
         /// <summary>
         /// Field: Type
         /// </summary>

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -83,6 +83,11 @@
         /// </summary>
         public virtual JsonSchemaType? Type { get; set; }
 
+        /// <summary>
+        /// Follow JSON Schema definition: https://json-schema.org/draft/2020-12/json-schema-validation
+        /// </summary>
+        public virtual string Const { get; set; }
+
         /// <summary>
         /// Follow JSON Schema definition: https://tools.ietf.org/html/draft-fge-json-schema-validation-00
         /// While relying on JSON Schema's defined formats,
@@ -347,6 +352,7 @@
         {
             Title = schema?.Title ?? Title;
             Id = schema?.Id ?? Id;
+            Const = schema?.Const ?? Const;
             Schema = schema?.Schema ?? Schema;
             Comment = schema?.Comment ?? Comment;
             Vocabulary = schema?.Vocabulary != null ? new Dictionary<string, bool>(schema.Vocabulary) : null;
@@ -563,6 +569,7 @@
             writer.WriteProperty(OpenApiConstants.Id, Id);
             writer.WriteProperty(OpenApiConstants.DollarSchema, Schema);
             writer.WriteProperty(OpenApiConstants.Comment, Comment);
+            writer.WriteProperty(OpenApiConstants.Const, Const);
             writer.WriteOptionalMap(OpenApiConstants.Vocabulary, Vocabulary, (w, s) => w.WriteValue(s));
             writer.WriteOptionalMap(OpenApiConstants.Defs, Definitions, (w, s) => s.SerializeAsV31(w));
             writer.WriteProperty(OpenApiConstants.DynamicRef, DynamicRef);

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

@@ -17,6 +17,9 @@ public class OpenApiSchemaReference : OpenApiSchema
         internal OpenApiSchema _target;
         private readonly OpenApiReference _reference;
         private string _description;
+        private JsonNode _default;
+        private JsonNode _example;
+        private IList<JsonNode> _examples;
 
         private OpenApiSchema Target
         {
@@ -90,6 +93,8 @@ internal OpenApiSchemaReference(OpenApiSchema target, string referenceId)
         /// <inheritdoc/>
         public override JsonSchemaType? Type { get => Target.Type; set => Target.Type = value; }
         /// <inheritdoc/>
+        public override string Const { get => Target.Const; set => Target.Const = value; }
+        /// <inheritdoc/>
         public override string Format { get => Target.Format; set => Target.Format = value; }
         /// <inheritdoc/>
         public override string Description
@@ -114,7 +119,11 @@ public override string Description
         /// <inheritdoc/>
         public override decimal? MultipleOf { get => Target.MultipleOf; set => Target.MultipleOf = value; }
         /// <inheritdoc/>
-        public override JsonNode Default { get => Target.Default; set => Target.Default = value; }
+        public override JsonNode Default 
+        { 
+            get => _default ??= Target.Default; 
+            set => _default = value; 
+        }
         /// <inheritdoc/>
         public override bool ReadOnly { get => Target.ReadOnly; set => Target.ReadOnly = value; }
         /// <inheritdoc/>
@@ -152,9 +161,17 @@ public override string Description
         /// <inheritdoc/>
         public override OpenApiDiscriminator Discriminator { get => Target.Discriminator; set => Target.Discriminator = value; }
         /// <inheritdoc/>
-        public override JsonNode Example { get => Target.Example; set => Target.Example = value; }
+        public override JsonNode Example 
+        { 
+            get => _example ??= Target.Example; 
+            set => _example = value; 
+        }
         /// <inheritdoc/>
-        public override IList<JsonNode> Examples { get => Target.Examples; set => Target.Examples = value; }
+        public override IList<JsonNode> Examples 
+        { 
+            get => _examples ??= Target.Examples; 
+            set => Target.Examples = value; 
+        }
         /// <inheritdoc/>
         public override IList<JsonNode> Enum { get => Target.Enum; set => Target.Enum = value; }
         /// <inheritdoc/>

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

@@ -131,6 +131,10 @@ internal static partial class OpenApiV31Deserializer
                     }
                 }
             },
+            {
+                "const",
+                (o, n, _) => o.Const = n.GetScalarValue()
+            },
             {
                 "allOf",
                 (o, n, t) => o.AllOf = n.CreateList(LoadSchema, t)

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

@@ -452,5 +452,48 @@ public void SerializeSchemaWithJsonSchemaKeywordsWorks()
             schema.Vocabulary.Keys.Count.Should().Be(5);
             schemaString.MakeLineBreaksEnvironmentNeutral().Should().Be(expected.MakeLineBreaksEnvironmentNeutral());
         }
+
+        [Fact]
+        public void ParseSchemaWithConstWorks()
+        {
+            var expected = @"{
+  ""$schema"": ""https://json-schema.org/draft/2020-12/schema"",
+  ""required"": [
+    ""status""
+  ],
+  ""type"": ""object"",
+  ""properties"": {
+    ""status"": {
+      ""const"": ""active"",
+      ""type"": ""string""
+    },
+    ""user"": {
+      ""required"": [
+        ""role""
+      ],
+      ""type"": ""object"",
+      ""properties"": {
+        ""role"": {
+          ""const"": ""admin"",
+          ""type"": ""string""
+        }
+      }
+    }
+  }
+}";
+
+            var path = Path.Combine(SampleFolderPath, "schemaWithConst.json");
+
+            // Act
+            var schema = OpenApiModelFactory.Load<OpenApiSchema>(path, OpenApiSpecVersion.OpenApi3_1, out _);
+            schema.Properties["status"].Const.Should().Be("active");
+            schema.Properties["user"].Properties["role"].Const.Should().Be("admin");
+
+            // serialization
+            var writer = new StringWriter();
+            schema.SerializeAsV31(new OpenApiJsonWriter(writer));
+            var schemaString = writer.ToString();
+            schemaString.MakeLineBreaksEnvironmentNeutral().Should().Be(expected.MakeLineBreaksEnvironmentNeutral());
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V31Tests/Samples/OpenApiSchema/schemaWithConst.json

@@ -0,0 +1,21 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "object",
+  "properties": {
+    "status": {
+      "type": "string",
+      "const": "active"
+    },
+    "user": {
+      "type": "object",
+      "properties": {
+        "role": {
+          "type": "string",
+          "const": "admin"
+        }
+      },
+      "required": [ "role" ]
+    }
+  },
+  "required": [ "status" ]
+}

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

@@ -398,6 +398,7 @@ namespace Microsoft.OpenApi.Models
         public const string Comment = "$comment";
         public const string Components = "components";
         public const string ComponentsSegment = "/components/";
+        public const string Const = "const";
         public const string Consumes = "consumes";
         public const string Contact = "contact";
         public const string Content = "content";
@@ -882,6 +883,7 @@ namespace Microsoft.OpenApi.Models
         public virtual System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiSchema> AllOf { get; set; }
         public virtual System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiSchema> AnyOf { get; set; }
         public virtual string Comment { get; set; }
+        public virtual string Const { get; set; }
         public virtual System.Text.Json.Nodes.JsonNode Default { get; set; }
         public virtual System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiSchema> Definitions { get; set; }
         public virtual bool Deprecated { get; set; }
@@ -1222,6 +1224,7 @@ namespace Microsoft.OpenApi.Models.References
         public override System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiSchema> AllOf { get; set; }
         public override System.Collections.Generic.IList<Microsoft.OpenApi.Models.OpenApiSchema> AnyOf { get; set; }
         public override string Comment { get; set; }
+        public override string Const { get; set; }
         public override System.Text.Json.Nodes.JsonNode Default { get; set; }
         public override System.Collections.Generic.IDictionary<string, Microsoft.OpenApi.Models.OpenApiSchema> Definitions { get; set; }
         public override bool Deprecated { get; set; }