Other 5xx codes
All HTTP status codes in ASP.NET Core
5xx Server error
501 Not Implemented
The server does not support the functionality needed, such as an unknown method.
In ASP.NET Core: Retrying never helps, yet HttpClient's standard resilience handler retries it.
501 in .NET
The StatusCodes constant, HttpStatusCode enum name, reason phrase, class, and how EnsureSuccessStatusCode() and the standard resilience handler treat 501.
| Constant | StatusCodes.Status501NotImplemented |
|---|---|
| HttpStatusCode | HttpStatusCode.NotImplemented |
| Reason phrase | Kestrel sends Not Implemented |
| Class | 5xx, server error |
| IsSuccessStatusCode | false |
| EnsureSuccessStatusCode() | throws HttpRequestException: Response status code does not indicate success: 501 (Not Implemented). |
| Standard resilience handler | retries it (treated as transient); retrying will not help here |
Returning 501
Minimal APIs
Results.StatusCode(StatusCodes.Status501NotImplemented)
Results.Problem(statusCode: StatusCodes.Status501NotImplemented, detail: "...")Controllers
StatusCode(StatusCodes.Status501NotImplemented)
Problem(statusCode: StatusCodes.Status501NotImplemented, detail: "...")The ProblemDetails body
What Results.Problem(statusCode: 501) sends (with AddProblemDetails()), as application/problem+json:
{
"type": "https://tools.ietf.org/html/rfc9110#section-15.6.2",
"title": "Not Implemented",
"status": 501,
"traceId": "0HNA1B2C3D4E5:00000001"
}A controller's Problem(statusCode: 501) sends the same body.
A bare StatusCode(501) in an [ApiController] also gets a ProblemDetails body automatically; Results.StatusCode(501) in a minimal API sends an empty body unless you add UseStatusCodePages().
Every response on this page was recorded from ASP.NET Core 10.0.12 on Kestrel in the Production environment; the HttpClient rows come from .NET 10.0.12 and Microsoft.Extensions.Http.Resilience 10.10.0.