Return PDF to the Browser using ASP.NET Core

I couldn't comment the answer by CodeCaster since my reputation isn't high enough. When trying

public IActionResult Get(int id)
{
    using (var stream = new FileStream(@"path\to\file", FileMode.Open))
    {
        return File(stream, "application/pdf", "FileDownloadName.ext");
    }       
} 

we got a

ObjectDisposedException: Cannot access a disposed object. Object name: 'Cannot access a closed file.'. System.IO.FileStream.BeginRead(byte[] array, int offset, int numBytes, AsyncCallback callback, object state)

We removed the using

   [HttpGet]
   [Route("getImageFile")]
   public IActionResult GetWorkbook()
   {
        var stream = new FileStream(@"pathToFile", FileMode.Open);
        return File(stream, "image/png", "image.png");
   }

And that worked. This is ASP.NET Core 2.1 running in IIS Express.

As explained in ASP.NET Core HTTPRequestMessage returns strange JSON message, ASP.NET Core does not support returning an HttpResponseMessage (what package did you install to get access to that type?).

Because of this, the serializer is simply writing all public properties of the HttpResponseMessage to the output, as it would with any other unsupported response type.

To support custom responses, you must return an IActionResult-implementing type. There's plenty of those. In your case, I'd look into the FileStreamResult:

public IActionResult Get(int id)
{
    var stream = new FileStream(@"path\to\file", FileMode.Open);
    return new FileStreamResult(stream, "application/pdf");     
}

Or simply use a PhysicalFileResult, where the stream is handled for you:

public IActionResult Get(int id)
{
    return new PhysicalFileResult(@"path\to\file", "application/pdf");
}

Of course all of this can be simplified using helper methods, such as Controller.File():

public IActionResult Get(int id)
{
    var stream = new FileStream(@"path\to\file", FileMode.Open);
    return File(stream, "application/pdf", "FileDownloadName.ext");
}

This simply abstracts the creation of a FileContentResult or FileStreamResult (for this overload, the latter).

Or if you're converting an older MVC or Web API application and don't want to convert all your code at once, add a reference to WebApiCompatShim (NuGet) and wrap your current code in a ResponseMessageResult:

public IActionResult Get(int id)
{
    var response = new HttpResponseMessage(HttpStatusCode.OK);           
    var stream = ...
    response.Content...

    return new ResponseMessageResult(response);
}

If you don't want to use return File(fileName, contentType, fileDownloadName), then the FileStreamResult doesn't support setting the content-disposition header from the constructor or through properties.

In that case you'll have to add that response header to the response yourself before returning the file result:

var contentDisposition = new ContentDispositionHeaderValue("attachment");
contentDisposition.SetHttpFileName("foo.txt");
Response.Headers[HeaderNames.ContentDisposition] = contentDisposition.ToString();

I don't have enough reputation to post this as a comment, so posting as an answer. The first 3 solutions from @CodeCaster and the solution from @BernhardMaertl are correct.

However, for someone who may not work with files often (like me), please note that if the process running this code (e.g. the API) only has read permissions to the file, you will need to specify that as the third parameter when creating your FileStream, otherwise the default behavior is to open the file for read/write and you will get an exception since you do not have write permissions.

The 3rd solution from @CodeCaster would then look like this:

public IActionResult Get(int id)
{
    var stream = new FileStream(@"path\to\file", FileMode.Open, FileAccess.Read);
    return File(stream, "application/pdf", "FileDownloadName.ext");
}