[c#] How to return a file using Web API?

I am using ASP.NET Web API.
I want to download a PDF with C# from the API (that the API generates).

Can I just have the API return a byte[]? and for the C# application can I just do:

byte[] pdf = client.DownloadData("urlToAPI");? 

and

File.WriteAllBytes()?

Another way to download file is to write the stream content to the response's body directly:

[HttpGet("pdfstream/{id}")]
public async Task  GetFile(long id)
{        
    var stream = GetStream(id);
    Response.StatusCode = (int)HttpStatusCode.OK;
    Response.Headers.Add( HeaderNames.ContentDisposition, $"attachment; filename=\"{Guid.NewGuid()}.pdf\"" );
    Response.Headers.Add( HeaderNames.ContentType, "application/pdf"  );            
    await stream.CopyToAsync(Response.Body);
    await Response.Body.FlushAsync();           
}

Just a note for .Net Core: We can use the FileContentResult and set the contentType to application/octet-stream if we want to send the raw bytes. Example:

[HttpGet("{id}")]
public IActionResult GetDocumentBytes(int id)
{
    byte[] byteArray = GetDocumentByteArray(id);
    return new FileContentResult(byteArray, "application/octet-stream");
}

Example with IHttpActionResult in ApiController.

[HttpGet]
[Route("file/{id}/")]
public IHttpActionResult GetFileForCustomer(int id)
{
    if (id == 0)
      return BadRequest();

    var file = GetFile(id);

    IHttpActionResult response;
    HttpResponseMessage responseMsg = new HttpResponseMessage(HttpStatusCode.OK);
    responseMsg.Content = new ByteArrayContent(file.SomeData);
    responseMsg.Content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment");
    responseMsg.Content.Headers.ContentDisposition.FileName = file.FileName;
    responseMsg.Content.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");
    response = ResponseMessage(responseMsg);
    return response;
}

If you don't want to download the PDF and use a browsers built in PDF viewer instead remove the following two lines:

responseMsg.Content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment");
responseMsg.Content.Headers.ContentDisposition.FileName = file.FileName;

I've been wondering if there was a simple way to download a file in a more ... "generic" way. I came up with this.

It's a simple ActionResult that will allow you to download a file from a controller call that returns an IHttpActionResult. The file is stored in the byte[] Content. You can turn it into a stream if needs be.

I used this to return files stored in a database's varbinary column.

    public class FileHttpActionResult : IHttpActionResult
    {
        public HttpRequestMessage Request { get; set; }

        public string FileName { get; set; }
        public string MediaType { get; set; }
        public HttpStatusCode StatusCode { get; set; }

        public byte[] Content { get; set; }

        public Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
        {
            HttpResponseMessage response = new HttpResponseMessage(StatusCode);

            response.StatusCode = StatusCode;
            response.Content = new StreamContent(new MemoryStream(Content));
            response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");
            response.Content.Headers.ContentDisposition.FileName = FileName;
            response.Content.Headers.ContentType = new MediaTypeHeaderValue(MediaType);

            return Task.FromResult(response);
        }
    }

I made the follow action:

[HttpGet]
[Route("api/DownloadPdfFile/{id}")]
public HttpResponseMessage DownloadPdfFile(long id)
{
    HttpResponseMessage result = null;
    try
    {
        SQL.File file = db.Files.Where(b => b.ID == id).SingleOrDefault();

        if (file == null)
        {
            result = Request.CreateResponse(HttpStatusCode.Gone);
        }
        else
        {
            // sendo file to client
            byte[] bytes = Convert.FromBase64String(file.pdfBase64);


            result = Request.CreateResponse(HttpStatusCode.OK);
            result.Content = new ByteArrayContent(bytes);
            result.Content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment");
            result.Content.Headers.ContentDisposition.FileName = file.name + ".pdf";
        }

        return result;
    }
    catch (Exception ex)
    {
        return Request.CreateResponse(HttpStatusCode.Gone);
    }
}