OpenApi - Polymorphism information missing when the base class is empty

[DvdKhl]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

If the base class is empty no polymorphism information is included in the document.

Expected Behavior

Include polymorphism information even if the base class is empty.

Steps To Reproduce

Minimal Repo:

using System.Text.Json.Serialization;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddOpenApi();

var app = builder.Build();
app.MapOpenApi();

app.MapGet("/Pet", () => Enumerable.Empty<Pet>).WithName("Pet").Produces<Pet[]>();
app.MapGet("/Shape", () => Enumerable.Empty<Shape>).WithName("Shape").Produces<Shape[]>();

app.Run();

[JsonPolymorphic]
[JsonDerivedType(typeof(Cat), "cat")]
[JsonDerivedType(typeof(Dog), "dog")]
public record Pet(string Name);
public record Dog(string Name, string? Breed) : Pet(Name);
public record Cat(string Name, int? Lives) : Pet(Name);

[JsonPolymorphic]
[JsonDerivedType(typeof(Rectangle), "Rectangle")]
[JsonDerivedType(typeof(Circle), "Circle")]
public record Shape();
public record Rectangle(int Width, int Height) : Shape();
public record Circle(int Radius) : Shape();

Generated document:

{
  "openapi": "3.0.1",
  "info": {
    "title": "OpenApiPolyEmptyBase | v1",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "http://localhost:5129/"
    }
  ],
  "paths": {
    "/Pet": {
      "get": {
        "tags": [
          "OpenApiPolyEmptyBase"
        ],
        "operationId": "Pet",
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "application/json": {
                "schema": {
                  "type": "array",
                  "items": {
                    "$ref": "#/components/schemas/Pet"
                  }
                }
              }
            }
          }
        }
      }
    },
    "/Shape": {
      "get": {
        "tags": [
          "OpenApiPolyEmptyBase"
        ],
        "operationId": "Shape",
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "application/json": {
                "schema": {
                  "type": "array",
                  "items": {
                    "$ref": "#/components/schemas/Shape"
                  }
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Pet": {
        "type": "object",
        "anyOf": [
          {
            "$ref": "#/components/schemas/PetCat"
          },
          {
            "$ref": "#/components/schemas/PetDog"
          },
          {
            "$ref": "#/components/schemas/PetBase"
          }
        ]
      },
      "PetBase": {
        "required": [
          "name"
        ],
        "properties": {
          "name": {
            "type": "string"
          }
        }
      },
      "PetCat": {
        "required": [
          "$type",
          "lives",
          "name"
        ],
        "properties": {
          "$type": {
            "enum": [
              "cat"
            ],
            "type": "string"
          },
          "lives": {
            "type": "integer",
            "format": "int32",
            "nullable": true
          },
          "name": {
            "type": "string"
          }
        }
      },
      "PetDog": {
        "required": [
          "$type",
          "breed",
          "name"
        ],
        "properties": {
          "$type": {
            "enum": [
              "dog"
            ],
            "type": "string"
          },
          "breed": {
            "type": "string",
            "nullable": true
          },
          "name": {
            "type": "string"
          }
        }
      },
      "Shape": {
        "type": "object"
      }
    }
  },
  "tags": [
    {
      "name": "OpenApiPolyEmptyBase"
    }
  ]
}

Exceptions (if any)

No response

.NET Version

9.0.300

Anything else?

Microsoft.AspNetCore.OpenApi: 9.0.5

[mikekistler]

This looks like an issue in the STJ schema generation logic.

var schema = options.GetJsonSchemaAsNode(typeof(Shape));

Console.WriteLine(schema);

[JsonPolymorphic]
[JsonDerivedType(typeof(Rectangle), "Rectangle")]
[JsonDerivedType(typeof(Circle), "Circle")]
public record Shape();
public record Rectangle(int Width, int Height) : Shape();
public record Circle(int Radius) : Shape();

produces

{
  "type": [
    "object",
    "null"
  ]
}

Transferring to runtime to investigate.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

[eiriktsarpalis]

This appears to be caused by a schema simplification transformation that is applied overly aggressively in this case. Because the base type is trivial, it admits any JSON object so the generated schema correspondingly reflects that. I think we could apply tweaks to the transform so that at the very least type discriminator metadata isn't being lost.

A simple workaround is to mark the base class as abstract:

[JsonPolymorphic]
[JsonDerivedType(typeof(Rectangle), "Rectangle")]
[JsonDerivedType(typeof(Circle), "Circle")]
public abstract record Shape();
public record Rectangle(int Width, int Height) : Shape();
public record Circle(int Radius) : Shape();

Yielding

{
  "type": [
    "object",
    "null"
  ],
  "required": [
    "$type"
  ],
  "anyOf": [
    {
      "properties": {
        "$type": {
          "const": "Rectangle"
        },
        "Width": {
          "type": "integer"
        },
        "Height": {
          "type": "integer"
        }
      },
      "required": [
        "Width",
        "Height"
      ]
    },
    {
      "properties": {
        "$type": {
          "const": "Circle"
        },
        "Radius": {
          "type": "integer"
        }
      },
      "required": [
        "Radius"
      ]
    }
  ]
}