How to Return a File Instead of JSON in ASP.Net Core Web API?

Returning a File in ASP.Net Core Web API

Problem:

When attempting to return a file in an ASP.Net Core Web API controller, the HttpResponseMessage is being returned as JSON with an application/json content header, rather than as a file.

Code Attempt:

public async Task<HttpResponseMessage> DownloadAsync(string id)
{
    var response = new HttpResponseMessage(HttpStatusCode.OK);
    response.Content = new StreamContent({{__insert_stream_here__}});
    response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
    return response;
}

Cause:

The HttpResponseMessage is being treated as a model by the web API framework because it is being returned from an action that is decorated with the [HttpGet] attribute.

Solution:

To correctly return a file, modify the controller action to return an IActionResult:

[Route("api/[controller]")]
public class DownloadController : Controller
{
    //GET api/download/12345abc
    [HttpGet("{id}")]
    public async Task<IActionResult> Download(string id)
    {
        Stream stream = await {{__get_stream_based_on_id_here__}};

        if(stream == null)
            return NotFound(); // returns a NotFoundResult with Status404NotFound response.

        return File(stream, "application/octet-stream", "{{filename.ext}}"); // returns a FileStreamResult
    }    
}

Note:

The framework will dispose of the used stream after the response is completed. Using a using statement to dispose of the stream before the response is sent will result in an exception or corrupt response.

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