How to Return a File Using Web API

How to return a file (FileContentResult) in ASP.NET WebAPI

Instead of returning StreamContent as the Content, I can make it work with ByteArrayContent.

[HttpGet]
public HttpResponseMessage Generate()
{
    var stream = new MemoryStream();
    // processing the stream.

    var result = new HttpResponseMessage(HttpStatusCode.OK)
    {
        Content = new ByteArrayContent(stream.ToArray())
    };
    result.Content.Headers.ContentDisposition =
        new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment")
    {
        FileName = "CertificationCard.pdf"
    };
    result.Content.Headers.ContentType =
        new MediaTypeHeaderValue("application/octet-stream");

    return result;
}

How to return a file in ASP.NET WebAPI

You can (should?) use ActionResults to return data from your API.

[HttpGet]
[Route("GetReport")]
public ActionResult GetReport()
{
     return File(System.IO.File.OpenRead(PDF_TEMP_PATH), "application/pdf");
}

You'll just have to set your ContentType accordingly.

Another good practice is to make your method async to a void blocking.

How to return a file using Web API?

Better to return HttpResponseMessage with StreamContent inside of it.

Here is example:

public HttpResponseMessage GetFile(string id)
{
    if (String.IsNullOrEmpty(id))
        return Request.CreateResponse(HttpStatusCode.BadRequest);

    string fileName;
    string localFilePath;
    int fileSize;

    localFilePath = getFileFromID(id, out fileName, out fileSize);
       
    HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
    response.Content = new StreamContent(new FileStream(localFilePath, FileMode.Open, FileAccess.Read));
    response.Content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment");
    response.Content.Headers.ContentDisposition.FileName = fileName;
    response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");

    return response;
}

UPDATE from comment by patridge:
Should anyone else get here looking to send out a response from a byte array instead of an actual file, you're going to want to use new ByteArrayContent(someData) instead of StreamContent (see here).

Return file in ASP.Net Core Web API

If this is ASP.net-Core then you are mixing web API versions. Have the action return a derived IActionResult because in your current code the framework is treating HttpResponseMessage as a model.

[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"); // returns a FileStreamResult
    }    
}

Note:

The framework will dispose of the stream used in this case when the response is completed. If a using statement is used, the stream will be disposed before the response has been sent and result in an exception or corrupt response.

Returning binary file from controller in ASP.NET Web API

Try using a simple HttpResponseMessage with its Content property set to a StreamContent:

// using System.IO;
// using System.Net.Http;
// using System.Net.Http.Headers;

public HttpResponseMessage Post(string version, string environment,
    string filetype)
{
    var path = @"C:\Temp\test.exe";
    HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
    var stream = new FileStream(path, FileMode.Open, FileAccess.Read);
    result.Content = new StreamContent(stream);
    result.Content.Headers.ContentType = 
        new MediaTypeHeaderValue("application/octet-stream");
    return result;
}

A few things to note about the stream used:

You must not call stream.Dispose(), since Web API still needs to be able to access it when it processes the controller method's result to send data back to the client. Therefore, do not use a using (var stream = …) block. Web API will dispose the stream for you.
Make sure that the stream has its current position set to 0 (i.e. the beginning of the stream's data). In the above example, this is a given since you've only just opened the file. However, in other scenarios (such as when you first write some binary data to a MemoryStream), make sure to stream.Seek(0, SeekOrigin.Begin); or set stream.Position = 0;
With file streams, explicitly specifying FileAccess.Read permission can help prevent access rights issues on web servers; IIS application pool accounts are often given only read / list / execute access rights to the wwwroot.

ASP.NET Minimal API How to Return/Download Files from URL

You can use Results.File to return file to download from your Minimal APIs handler:

app.MapGet("/download", () =>
{
    var mimeType = "image/png";
    var path = @"path_to_png.png";
    return Results.File(path, contentType: mimeType);
});