Failed to load API definition when a function returns a file

Question

I have my API project with ASP.NET Core (.NET5). Everything is working fine and the Swagger page shows all the APIs. Now, in a controller I want to add a function to export an Excel file. The code in the controller is this:

[HttpGet]
public IActionResult DownloadExcel()
{
    byte[] reportBytes;

    using (var package = _utils.CreateCommentPackage())
    {
        reportBytes = package.GetAsByteArray();
    }

    return File(reportBytes, XlsxContentType, 
        $"Comments-{DateTime.Now.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture)}.xlsx");
}

Pretty simple but the Swagger definition is not created. When I open the Swagger page I see that

If I remove this function, Swagger is working again.

Answer 1

You need add a route, change your code like following；

[HttpGet]
[Route("api/DownloadExcel")]
public IActionResult DownloadExcel()
{
    byte[] reportBytes;

    using (var package = _utils.CreateCommentPackage())
    {
        reportBytes = package.GetAsByteArray();
    }

    return File(reportBytes, XlsxContentType, 
        $"Comments-{DateTime.Now.ToString("yyyy-MM-dd", CultureInfo.InvariantCulture)}.xlsx");
}