MapOpenApi writes invalid json if example contains OpenApiDouble and CurrentCulture formats doubles with comma

[park-jasper]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Examples that are returned as part of .MapOpenApi() are formatted using CultureInfo.CurrentCulture in such a way that on a system with e.g. a german locale floating point numbers are output with a comma as a decimal symbol, which results in an invalid JSON response.

Expected Behavior

OpenApi examples should be written using a fixed IFormatProvider which results in valid JSON.

Steps To Reproduce

park-jasper/aspnetcore-openapi-format-issue

run the single project in this solution and query /openapi/v1.json

This returns the following document:

{
  "openapi": "3.0.1",
  "info": {
    "title": "OpenApiFormatIssue | v1",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "https://localhost:7126"
    },
    {
      "url": "http://localhost:5058"
    }
  ],
  "paths": {
    "/": {
      "get": {
        "tags": [
          "OpenApiFormatIssue"
        ],
        "operationId": "ExampleEndpoint",
        "responses": {
          "200": {
            "description": "OK",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/ApiResponse"
                },
                "examples": {
                  "FormatIssue": {
                    "value": 1,234
                  }
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "ApiResponse": {
        "required": [
          "value"
        ],
        "type": "object",
        "properties": {
          "value": {
            "type": "number",
            "format": "double"
          }
        }
      }
    }
  },
  "tags": [
    {
      "name": "OpenApiFormatIssue"
    }
  ]
}

Exceptions (if any)

No response

.NET Version

9.0.103

Anything else?

ASP.NET Core version 9.0.2

-- Workaround --
I can set CultureInfo.DefaultThreadCultureInfo to e.g. CultureInfo.InvariantCulture to circumvent this, because the TextWriter takes that as a fallback, but having to globally set this is not my ideal solution

-- Suggestion --
The code of interest is this one: https://github.com/dotnet/aspnetcore/blob/main/src/OpenApi/src/Extensions/OpenApiEndpointRouteBuilderExtensions.cs#L57
Here an Utf8BufferTextWriter is retrieved, and it in turn inherits from TextWriter.
TextWriter offers a constructor that takes an IFormatProvider, but Utf8BufferTextWriter implicitly calls the default constructor instead which does not set an IFormatProvider, and the default of CultureInfo.CurrentCulture is used

[martincostello]

Where does OpenApiExampleOperationTransformer come from in your repro? I don't seem to be able to find it anywhere.

Sorry, GitHub hadn't indexed the code yet.

[martincostello]

The code of interest is this one: https://github.com/dotnet/aspnetcore/blob/main/src/OpenApi/src/Extensions/OpenApiEndpointRouteBuilderExtensions.cs#L57
Here an Utf8BufferTextWriter is retrieved, and it in turn inherits from TextWriter.
TextWriter offers a constructor that takes an IFormatProvider, but Utf8BufferTextWriter implicitly calls the default constructor instead which does not set an IFormatProvider, and the default of CultureInfo.CurrentCulture is used

That looks correct. The code path eventually gets to here, which calls TextWriter.Write(), so if that's backed with the current culture it'll format it that way.