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.

For Web API 2, you can implement IHttpActionResult. Here's mine:

using System;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading;
using System.Threading.Tasks;
using System.Web;
using System.Web.Http;

class FileResult : IHttpActionResult
{
    private readonly string _filePath;
    private readonly string _contentType;

    public FileResult(string filePath, string contentType = null)
    {
        if (filePath == null) throw new ArgumentNullException("filePath");

        _filePath = filePath;
        _contentType = contentType;
    }

    public Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
    {
        var response = new HttpResponseMessage(HttpStatusCode.OK)
        {
            Content = new StreamContent(File.OpenRead(_filePath))
        };

        var contentType = _contentType ?? MimeMapping.GetMimeMapping(Path.GetExtension(_filePath));
        response.Content.Headers.ContentType = new MediaTypeHeaderValue(contentType);

        return Task.FromResult(response);
    }
}

Then something like this in your controller:

[Route("Images/{*imagePath}")]
public IHttpActionResult GetImage(string imagePath)
{
    var serverPath = Path.Combine(_rootPath, imagePath);
    var fileInfo = new FileInfo(serverPath);

    return !fileInfo.Exists
        ? (IHttpActionResult) NotFound()
        : new FileResult(fileInfo.FullName);
}

And here's one way you can tell IIS to ignore requests with an extension so that the request will make it to the controller:

<!-- web.config -->
<system.webServer>
  <modules runAllManagedModulesForAllRequests="true"/>

For those using .NET Core:

You can make use of the IActionResult interface in an API controller method, like so.

[HttpGet("GetReportData/{year}")]
public async Task<IActionResult> GetReportData(int year)
{
    // Render Excel document in memory and return as Byte[]
    Byte[] file = await this._reportDao.RenderReportAsExcel(year);

    return File(file, "application/vnd.openxmlformats", "fileName.xlsx");
}

This example is simplified, but should get the point across. In .NET Core this process is so much simpler than in previous versions of .NET - i.e. no setting response type, content, headers, etc.

Also, of course the MIME type for the file and the extension will depend on individual needs.

Reference: SO Post Answer by @NKosi

While the suggested solution works fine, there is another way to return a byte array from the controller, with response stream properly formatted :

In the request, set header "Accept: application/octet-stream".

Server-side, add a media type formatter to support this mime type.

Unfortunately, WebApi does not include any formatter for "application/octet-stream". There is an implementation here on GitHub: BinaryMediaTypeFormatter (there are minor adaptations to make it work for webapi 2, method signatures changed).

You can add this formatter into your global config :

HttpConfiguration config;
// ...
config.Formatters.Add(new BinaryMediaTypeFormatter(false));

WebApi should now use BinaryMediaTypeFormatter if the request specifies the correct Accept header.

I prefer this solution because an action controller returning byte[] is more comfortable to test. Though, the other solution allows you more control if you want to return another content-type than "application/octet-stream" (for example "image/gif").

For anyone having the problem of the API being called more than once while downloading a fairly large file using the method in the accepted answer, please set response buffering to true System.Web.HttpContext.Current.Response.Buffer = true;

This makes sure that the entire binary content is buffered on the server side before it is sent to the client. Otherwise you will see multiple request being sent to the controller and if you do not handle it properly, the file will become corrupt.

The overload that you're using sets the enumeration of serialization formatters. You need to specify the content type explicitly like:

httpResponseMessage.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");

You could try

httpResponseMessage.Content.Headers.Add("Content-Type", "application/octet-stream");

You can try the following code snippet

httpResponseMessage.Content.Headers.Add("Content-Type", "application/octet-stream");

Hope it will work for you.