Emit x-nullable for required nullable properties in Swagger2

src/NJsonSchema.Tests/Generation/AttributeGenerationTests.cs

@@ -378,6 +378,20 @@ public void When_property_has_required_keyword_and_nullable_type_then_it_is_requ
             Assert.True(schema.Properties["Name"].IsNullable(SchemaType.JsonSchema));
             Assert.Null(schema.Properties["Name"].MinLength);
         }
+
+        [Fact]
+        public void When_property_has_required_keyword_and_nullable_type_then_it_is_nullable_in_Swagger2()
+        {
+            // Act
+            var schema = NewtonsoftJsonSchemaGenerator.FromType<ClassWithRequiredNullableKeyword>(
+                new NewtonsoftJsonSchemaGeneratorSettings { SchemaType = SchemaType.Swagger2 });
+
+            // Assert: the required keyword forces the nullable property into Swagger2's required
+            // array, which by itself would signal "non-null". x-nullable must be emitted so the
+            // declared nullability is not silently lost.
+            Assert.Contains("Name", schema.RequiredProperties);
+            Assert.True(schema.Properties["Name"].IsNullable(SchemaType.Swagger2));
+        }
 #endif
 
 #if NET6_0_OR_GREATER

src/NJsonSchema.Tests/Generation/DataContractTests.cs

@@ -41,19 +41,19 @@ public void When_DataContractRequired_is_set_property_is_nullable_in_OpenApi3()
         }
 
         [Fact]
-        public void When_DataContractRequired_is_set_property_is_not_nullable_in_Swagger2()
+        public void When_DataContractRequired_is_set_property_is_nullable_in_Swagger2()
         {
             // Act
             var schema = NewtonsoftJsonSchemaGenerator.FromType<Person>(new NewtonsoftJsonSchemaGeneratorSettings { SchemaType = SchemaType.Swagger2 });
             var json = schema.ToJson();
 
             // Assert
             Assert.True(schema.ActualProperties["middleName"].IsRequired);
-            Assert.False(schema.ActualProperties["middleName"].IsNullable(SchemaType.Swagger2));
+            Assert.True(schema.ActualProperties["middleName"].IsNullable(SchemaType.Swagger2));
 
-            // not nullable, because Swagger2 does not know null but it has to set "required" 
-            // because not setting the property would result in a serialization error, 
-            // see When_DataContractRequired_is_set_then_undefined_is_not_allowed
+            // The property must be "required" because not setting it would result in a serialization
+            // error, but its value may still be null. Swagger2 has no native null, so x-nullable 
+            // is emitted to express that the required value can be null.
         }
     }
 }

src/NJsonSchema.Tests/Generation/SystemTextJson/SystemTextJsonTests.cs

@@ -176,6 +176,20 @@ public void When_property_has_required_keyword_and_nullable_type_then_it_is_requ
             Assert.Null(schema.Properties["Name"].MinLength);
         }
 
+        [Fact]
+        public void When_property_has_required_keyword_and_nullable_type_then_it_is_nullable_in_Swagger2()
+        {
+            // Act
+            var schema = JsonSchema.FromType<ClassWithRequiredNullableKeyword>(
+                new SystemTextJsonSchemaGeneratorSettings { SchemaType = SchemaType.Swagger2 });
+
+            // Assert: the required keyword forces the nullable property into Swagger2's required
+            // array, which by itself would signal "non-null". x-nullable must be emitted so the
+            // declared nullability is not silently lost.
+            Assert.Contains("Name", schema.RequiredProperties);
+            Assert.True(schema.Properties["Name"].IsNullable(SchemaType.Swagger2));
+        }
+
         public class ClassWithJsonRequired
         {
             [System.Text.Json.Serialization.JsonRequired]

src/NJsonSchema/Generation/JsonSchemaGenerator.cs

@@ -1222,9 +1222,17 @@ public void AddProperty(
                     propertySchema.MinLength = 1;
                 }
 
-                if (!isNullable && Settings.SchemaType == SchemaType.Swagger2)
+                if (Settings.SchemaType == SchemaType.Swagger2)
                 {
-                    if (!parentSchema.RequiredProperties.Contains(propertyName))
+                    if (parentSchema.RequiredProperties.Contains(propertyName))
+                    {
+                        if (isNullable)
+                        {
+                            // Swagger2 has no native nullable
+                            propertySchema.IsNullableRaw = true;
+                        }
+                    }
+                    else if (!isNullable)
                     {
                         parentSchema.RequiredProperties.Add(propertyName);
                     }