Fix deserializing of const null

Author: Youssef1313

src/Microsoft.OpenApi/Reader/JsonNodeHelper.cs

@@ -157,6 +157,11 @@ public static Dictionary<string, HashSet<T>> CreateArrayMap<T>(this JsonNode? no
 
         public static string? GetScalarValue(this JsonNode? node)
         {
+            if (node.IsJsonNullSentinel())
+            {
+                return null;
+            }
+
             var scalarNode = node is JsonValue value ? value : throw new OpenApiException("Expected scalar value.");
 
             return Convert.ToString(scalarNode.GetValue<object>(), CultureInfo.InvariantCulture);

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

@@ -632,6 +632,55 @@ public async Task ParseSchemaWithConstWorks()
             Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), schemaString.MakeLineBreaksEnvironmentNeutral());
         }
 
+        [Fact]
+        public async Task ParseSchemaWithConstNullWorks()
+        {
+            var expected = @"{
+  ""$schema"": ""https://json-schema.org/draft/2020-12/schema"",
+  ""required"": [
+    ""status""
+  ],
+  ""type"": ""object"",
+  ""properties"": {
+    ""status"": {
+      ""const"": null,
+      ""type"": ""string""
+    },
+    ""user"": {
+      ""required"": [
+        ""role""
+      ],
+      ""type"": ""object"",
+      ""properties"": {
+        ""role"": {
+          ""const"": null,
+          ""type"": ""string""
+        }
+      }
+    }
+  }
+}";
+
+            var path = Path.Combine(SampleFolderPath, "schemaWithConstNull.json");
+
+            // Act
+            var schema = await OpenApiModelFactory.LoadAsync<OpenApiSchema>(path, OpenApiSpecVersion.OpenApi3_1, new(), SettingsFixture.ReaderSettings);
+
+            var statusSchema = Assert.IsType<OpenApiSchema>(schema.Properties["status"]);
+            Assert.Null(statusSchema.Const);
+            Assert.True(statusSchema.WasConstExplicitlySet);
+
+            var userRoleSchema = Assert.IsType<OpenApiSchema>(schema.Properties["user"].Properties["role"]);
+            Assert.Null(userRoleSchema.Const);
+            Assert.True(userRoleSchema.WasConstExplicitlySet);
+
+            // serialization
+            var writer = new StringWriter();
+            schema.SerializeAsV31(new OpenApiJsonWriter(writer));
+            var schemaString = writer.ToString();
+            Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), schemaString.MakeLineBreaksEnvironmentNeutral());
+        }
+
         [Fact]
         public void ParseSchemaWithUnrecognizedKeywordsWorks()
         {

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

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

test/Microsoft.OpenApi.Readers.Tests/V32Tests/OpenApiSchemaTests.cs

@@ -534,6 +534,55 @@ public async Task ParseSchemaWithConstWorks()
             Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), schemaString.MakeLineBreaksEnvironmentNeutral());
         }
 
+        [Fact]
+        public async Task ParseSchemaWithConstNullWorks()
+        {
+            var expected = @"{
+  ""$schema"": ""https://json-schema.org/draft/2020-12/schema"",
+  ""required"": [
+    ""status""
+  ],
+  ""type"": ""object"",
+  ""properties"": {
+    ""status"": {
+      ""const"": null,
+      ""type"": ""string""
+    },
+    ""user"": {
+      ""required"": [
+        ""role""
+      ],
+      ""type"": ""object"",
+      ""properties"": {
+        ""role"": {
+          ""const"": null,
+          ""type"": ""string""
+        }
+      }
+    }
+  }
+}";
+
+            var path = Path.Combine(SampleFolderPath, "schemaWithConstNull.json");
+
+            // Act
+            var schema = await OpenApiModelFactory.LoadAsync<OpenApiSchema>(path, OpenApiSpecVersion.OpenApi3_2, new(), SettingsFixture.ReaderSettings);
+            
+            var statusSchema = Assert.IsType<OpenApiSchema>(schema.Properties["status"]);
+            Assert.Null(statusSchema.Const);
+            Assert.True(statusSchema.WasConstExplicitlySet);
+
+            var userRoleSchema = Assert.IsType<OpenApiSchema>(schema.Properties["user"].Properties["role"]);
+            Assert.Null(userRoleSchema.Const);
+            Assert.True(userRoleSchema.WasConstExplicitlySet);
+
+            // serialization
+            var writer = new StringWriter();
+            schema.SerializeAsV32(new OpenApiJsonWriter(writer));
+            var schemaString = writer.ToString();
+            Assert.Equal(expected.MakeLineBreaksEnvironmentNeutral(), schemaString.MakeLineBreaksEnvironmentNeutral());
+        }
+
         [Fact]
         public void ParseSchemaWithUnrecognizedKeywordsWorks()
         {

test/Microsoft.OpenApi.Readers.Tests/V32Tests/Samples/OpenApiSchema/schemaWithConstNull.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "object",
+  "properties": {
+    "status": {
+      "type": "string",
+      "const": null
+    },
+    "user": {
+      "type": "object",
+      "properties": {
+        "role": {
+          "type": "string",
+          "const": null
+        }
+      },
+      "required": [ "role" ]
+    }
+  },
+  "required": [ "status" ]
+}
+