ConflictObjectResult.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;

public class ConflictObjectResult : ObjectResult
{
    public ConflictObjectResult(string detail)
        : base(detail)
        => StatusCode = StatusCodes.Status409Conflict;
}

public static class ControllerExtensions
{
    public static IActionResult Conflict(this Controller controller, string detail)
        => new ConflictObjectResult(detail);
}