Return stream of downloaded file without allocating it to memory (if possible)

I have Web API endpoint that serves a file to me by first downloading it, and then returning the byte array.

This code works, but loads the entire file (no matter how big it is) into memory first.

Is there a way to simply return the same stream that I have used to download the file with the httpClient?

Simplified example of working code:

[HttpGet("file")]
public async Task<IActionResult> DownloadFile(string url)
{
    using var httpClient = new HttpClient();
    using var response = await httpClient.GetAsync(url);
    if (response.IsSuccessStatusCode)
    {
        var content = await response.Content.ReadAsByteArrayAsync();

        var cd = new System.Net.Mime.ContentDisposition
        {
             FileName = "myFile.pdf",

             // always prompt the user for downloading, set to true if you want 
             // the browser to try to show the file inline
             Inline = false, 
        };
                
        Response.Headers.Add("Content-Disposition", cd.ToString());
                
        var mediaType = response.Content.Headers.ContentType.MediaType;

        return File(content, mediaType);
    }

    return NotFound();
}

What I have tried (and kind of expected to work):

using var httpClient = new HttpClient();
using var response = await httpClient.GetAsync(url);
if (response.IsSuccessStatusCode)
{
   using (var content = await response.Content.ReadAsStreamAsync())
   {
      content.Position = 0;

      var cd = new System.Net.Mime.ContentDisposition
      {
         FileName = "myFile.pdf",

         // always prompt the user for downloading, set to true if you want 
         // the browser to try to show the file inline
         Inline = false,
      };

      Response.Headers.Add("Content-Disposition", cd.ToString());

      var mediaType = response.Content.Headers.ContentType.MediaType;

      return File(content, mediaType);
}

but this gives me the ObjectDisposedException: cannot access a closed stream.

Is there any way of accomplishing what I'm intending to do please?

To handle multiple requests at the same time, I added the [ApiController] attribute on the controller class and the [Produces("application/pdf")] attribute to the DownloadFile action method. This allows multiple requests to be handled simultaneously and efficiently.

To prevent the file content from being overwritten when multiple requests are made, I generated a unique file name for each request using a GUID and appended it to the file name. This ensures that each request gets its own unique file name and the file content won't be overwritten.

The unique file name that is generated is used only to set the file name in the Content-Disposition header, which tells the user's browser how to handle the downloaded file.

So, the file is not actually written to the disk, but it is streamed directly to the response stream and returned to the user as a downloadable attachment with a unique file name.

The EmptyResult class is a built-in class in ASP.NET Core that represents an empty response with a 200 status code. It is used in cases where the action does not need to return any data, but still needs to indicate that the action has completed successfully.

[ApiController]
public class FileController : ControllerBase
{
    [HttpGet("file")]
    [Produces("application/pdf")]
    public async Task<IActionResult> DownloadFile(string url)
    {
        using var httpClient = new HttpClient();
        var response = await httpClient.GetAsync(url);

        if (response.IsSuccessStatusCode)
        {
            var fileName = $"myFile_{Guid.NewGuid()}.pdf";
            var cd = new System.Net.Mime.ContentDisposition
            {
                FileName = fileName,
                Inline = false,
            };

            Response.Headers.Add("Content-Disposition", cd.ToString());
            var mediaType = response.Content.Headers.ContentType.MediaType;

            await response.Content.CopyToAsync(Response.Body);

            return new EmptyResult();
        }

        return NotFound();
    }
}