OpenAPI schema generation fails for second IList<T> property in ASP.NET Core 9

[kvanover]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When using ASP.NET Core 9’s built-in OpenAPI support (Microsoft.AspNetCore.OpenApi), the schema generator incorrectly handles multiple properties of the same generic type (IList). The first property is generated correctly with a $ref to the component schema, but the second property produces an empty items object.

It will generate:

"weatherPoints1": {
  "type": "array",
  "items": {
    "$ref": "#/components/schemas/WeatherPoints"
  }
},
"weatherPoints2": {
  "type": "array",
  "items": { }
}

Expected Behavior

"weatherPoints2": {
  "type": "array",
  "items": {
    "$ref": "#/components/schemas/WeatherPoints"
  }
}

Steps To Reproduce

1. Create a minimal API project in .NET 9.
2. Add the following classes:

public class WeatherForecast
{
    public DateOnly Date { get; set; }
    public int TemperatureC { get; set; }
    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);
    public IList<WeatherPoints> WeatherPoints1 { get; set; } = new List<WeatherPoints>();
    public IList<WeatherPoints> WeatherPoints2 { get; set; } = new List<WeatherPoints>();
    public string? Summary { get; set; }
}

public class WeatherPoints
{
    public int Point { get; set; }
}

3. Map an endpoint returning WeatherForecast:

app.MapGet("/weatherforecast", () => new WeatherForecast())
.WithName("GetWeatherForecast")
.WithOpenApi();

4. Run the app and inspect /openapi/v1.json.

Exceptions (if any)

No response

.NET Version

9.0.306

Anything else?

• .NET SDK: 9.0.x
• ASP.NET Core: 9.0
• Package: Microsoft.AspNetCore.OpenApi

[MihuBot]

I'm a bot. Here are possible related and/or duplicate issues (I may be wrong):

• [OpenApi] $ref not generated for identical lists in same class #63096
• OpenApi: Multiple array properties of the same type do not get the correct type reference #63353
• Incorrect OpenAPI generation #63460
• OpenApi document generation does not generate reference to array items #63735
• OpenApi 9.0.9 - Incorrect reference types for classes with more than one collection properties of the same reference type #63790

[kvanover]

my bad, but there is a duplicate issue that I missed -> https://github.com/dotnet/aspnetcore/issues/62079
Closing issue