1 am using c# NSwag.AspNetCore. And I have a polymorphic model. The definition is:

components:
  schemas:
    TestModel:
      type: object
      discriminator:
        propertyName: $type
        mapping:
          subModel1: '#/components/schemas/SubModel1'
          subModel2: '#/components/schemas/SubModel2'
      properties:
        id:
          type: string
          readOnly: true
    SubModel1:
      type: object
      properties:
        substrateBuildVersion:
          type: string
      allOf:
        - $ref: '#/components/schemas/TestModel'
    SubModel2:
      type: object
      properties:
        isLevOrchestrated:
          type: boolean
      allOf:
        - $ref: '#/components/schemas/TestModel'

the api definition is:

        /// <inheritdoc />
        [ProducesResponseType(StatusCodes.Status200OK)]
        [ProducesResponseType(StatusCodes.Status401Unauthorized)]
        [ProducesResponseType(StatusCodes.Status403Forbidden)]
        [ProducesResponseType(StatusCodes.Status404NotFound)]
#pragma warning disable CS8625 // Cannot convert null literal to non-nullable reference type.
        public override async Task<ActionResult<TestModel>> TestAPI2([FromBody] TestModel body = null, CancellationToken cancellationToken = default(System.Threading.CancellationToken))
#pragma warning restore CS8625 // Cannot convert null literal to non-nullable reference type.
        {
            await Task.Delay(1).ConfigureAwait(false);
            TestModel model = (SubModel1)body!;
            return Ok(model);
        }

        /// <inheritdoc />
        [ProducesResponseType(StatusCodes.Status200OK)]
        [ProducesResponseType(StatusCodes.Status401Unauthorized)]
        [ProducesResponseType(StatusCodes.Status403Forbidden)]
        [ProducesResponseType(StatusCodes.Status404NotFound)]
        public override async Task<ActionResult<ICollection<TestModel>>> TestAPI3(CancellationToken cancellationToken = default)
        {
            await Task.Delay(1).ConfigureAwait(false);
            TestModel model = new SubModel1
            {
                Id = "test",
                SubstrateBuildVersion = "test",
            };
            return Ok(new List<TestModel> { model });

And I found that if the response is a collection (TestAPI3), the response will contain $type, but if not, the response will not contain $type.

{
  "substrateBuildVersion": "test",
  "id": "test"
}

{
  "substrateBuildVersion": "test",
  "id": "test",
  "$type": "subModel1"
}

Why have this result? what if I want the single object has the $type too?

what if I want the single object has the $type too?