How does an ASP.NET Web API controller return binary data?

Assume you are writing an ASP.NET Web API controller with the aim of returning binary data with .NET 4.x, such as a PDF document or an PNG image. For brevity's sake, let's assume you have a controller action like this:

public class MyApiController : ApiController
{
    // GET api/
    public byte[] Get()
    {
        return new byte[] { 65, 66, 67 };
    }
}

Above, the byte array would present some real binary content, here just three bytes as an example. When the above controller is accessed through HTTP, the response looks like this:

Pragma: no-cache
Cache-Control: no-cache
Date: Wed, 15 Feb 2017 10:40:12 GMT
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Content-Length: 6
Content-Type: application/json; charset=utf-8
Expires: -1

"QUJD"

Notice how the binary data (byte array) is automatically serialized to Base64 format: "QUJD".

However, if you add a content type value manually to the controller and construct a HttpResponseMessage manually, you can more precisely control the output:

public HttpResponseMessage Get()
{
    HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK)
    {
        Content = new ByteArrayContent(new byte[] { 65, 66, 67 })
    };
    result.Content.Headers.ContentType = new MediaTypeHeaderValue(
         "application/octet-stream");
    return result;
}

With this code, the HTTP response looks like this:

Pragma: no-cache
Cache-Control: no-cache
Date: Wed, 15 Feb 2017 10:43:36 GMT
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Content-Length: 3
Content-Type: application/octet-stream
Expires: -1

ABC

Notice how the Content-Type header has now changed, and there is no automatic Base64 encoding anymore.

Hope this helps!