Extend support for nullable reference types in responses

[yohny]

Currently when you have a method returning simple DTO like this:

[HttpGet()]
public SettingsDto Settings()
{
    // implementation details
}

it correctly produces OpenAPI specs like this:

 "/api/Settings": {
      "get": {
        "tags": [
          "Settings"
        ],
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "text/plain": {
                "schema": {
                  "$ref": "#/components/schemas/Viacar.iDispo.Controllers.Shared.SettingsController+SettingsDto"
                }
              },
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/Viacar.iDispo.Controllers.Shared.SettingsController+SettingsDto"
                }
              },
              "text/json": {
                "schema": {
                  "$ref": "#/components/schemas/Viacar.iDispo.Controllers.Shared.SettingsController+SettingsDto"
                }
              }
            }
          }
        }
      }
    },

however if you change the signature to return nullable DTO:

[HttpGet()]
public SettingsDto? Settings() // added ?
{
    // implementation details
}

one would expect that this is reflected in OpenAPI specs (given that SupportNonNullableReferenceTypes() was called when setting up swagger generation), resulting in nullable being added to the specs like so:

 "/api/Settings": {
      "get": {
        "tags": [
          "Settings"
        ],
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "text/plain": {
                "schema": {
                  "$ref": "#/components/schemas/Viacar.iDispo.Controllers.Shared.SettingsController+SettingsDto",
                  "nullable": true // should be added
                }
              },
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/Viacar.iDispo.Controllers.Shared.SettingsController+SettingsDto",
                  "nullable": true // should be added
                }
              },
              "text/json": {
                "schema": {
                  "$ref": "#/components/schemas/Viacar.iDispo.Controllers.Shared.SettingsController+SettingsDto"
                  "nullable": true // should be added
                }
              }
            }
          }
        }
      }
    },

but thats not the case now.

[kpko]

Hi, facing the same issue. Is there a hint what could cause this issue?

[martincostello]

To make issue tracking a bit less overwhelming for the new maintainers (see #2778), I've created a new tracking issue to roll-up various nullability issues here: #2793.

We'll refer back to this issue from there and include it as part of resolving that issue, but I'm going to close this one to help prune the backlog.