Fix AllOf behaviour when nullable

When a property is using `AnyOf` and is nullable, convert it to use `OneOf` with null instead.

Resolves #3649.

Author: martincostello

src/Swashbuckle.AspNetCore.SwaggerGen/SchemaGenerator/SchemaGenerator.cs

@@ -49,12 +49,12 @@ private IOpenApiSchema GenerateSchemaForMember(
             ? GeneratePolymorphicSchema(schemaRepository, knownTypesDataContracts)
             : GenerateConcreteSchema(dataContract, schemaRepository);
 
+        bool usingAllOf = false;
+
         if (_generatorOptions.UseAllOfToExtendReferenceSchemas && schema is OpenApiSchemaReference reference)
         {
-            schema = new OpenApiSchema()
-            {
-                AllOf = [reference],
-            };
+            schema = new OpenApiSchema() { AllOf = [reference] };
+            usingAllOf = true;
         }
 
         if (schema is OpenApiSchema concrete)
@@ -68,7 +68,22 @@ private IOpenApiSchema GenerateSchemaForMember(
 
                 var nullable = IsNullable(requiredAttribute, dataProperty, memberInfo);
 
-                SetNullable(concrete, nullable);
+                if (usingAllOf)
+                {
+                    // When using AllOf to extend reference schemas, we need to adjust the schema to represent
+                    // nullability correctly as a property can't be null AND a specific type at the same time.
+                    // See https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/3649.
+                    if (nullable)
+                    {
+                        concrete.OneOf = schema.AllOf;
+                        concrete.OneOf.Add(new OpenApiSchema { Type = JsonSchemaType.Null });
+                        concrete.AllOf = null;
+                    }
+                }
+                else
+                {
+                    SetNullable(concrete, nullable);
+                }
 
                 concrete.ReadOnly = dataProperty.IsReadOnly;
                 concrete.WriteOnly = dataProperty.IsWriteOnly;

test/Swashbuckle.AspNetCore.IntegrationTests/snapshots/VerifyTests.Swagger_IsValidJson_No_Startup_entryPointType=MinimalAppWithNullableEnums.Program_swaggerRequestUri=v1.DotNet10_0.verified.txt

@@ -48,10 +48,12 @@
         "type": "object",
         "properties": {
           "child": {
-            "type": "null",
-            "allOf": [
+            "oneOf": [
               {
                 "$ref": "#/components/schemas/Child"
+              },
+              {
+                "type": "null"
               }
             ]
           },

test/Swashbuckle.AspNetCore.IntegrationTests/snapshots/VerifyTests.Swagger_IsValidJson_No_Startup_entryPointType=MinimalAppWithNullableEnums.Program_swaggerRequestUri=v1.DotNet8_0.verified.txt

@@ -48,10 +48,12 @@
         "type": "object",
         "properties": {
           "child": {
-            "type": "null",
-            "allOf": [
+            "oneOf": [
               {
                 "$ref": "#/components/schemas/Child"
+              },
+              {
+                "type": "null"
               }
             ]
           },

test/Swashbuckle.AspNetCore.IntegrationTests/snapshots/VerifyTests.Swagger_IsValidJson_No_Startup_entryPointType=MinimalAppWithNullableEnums.Program_swaggerRequestUri=v1.DotNet9_0.verified.txt

@@ -48,10 +48,12 @@
         "type": "object",
         "properties": {
           "child": {
-            "type": "null",
-            "allOf": [
+            "oneOf": [
               {
                 "$ref": "#/components/schemas/Child"
+              },
+              {
+                "type": "null"
               }
             ]
           },