How to Return a FileContentResult (PDF) from an ASP.NET Web API?

Returning a PDF File from an ASP.NET Web API

In standard MVC controllers, returning a PDF using FileContentResult is simple. However, ApiController requires a different approach.

The Problem

Using the typical IHttpActionResult return type in ApiController presents challenges when returning a PDF. Simply returning a StreamContent, as some examples suggest, often results in the browser displaying headers instead of the PDF file itself.

The Correct Method

The key is to use ByteArrayContent instead of StreamContent. This converts the stream data into a byte array for proper transmission. Here's an improved example:

<code class="language-csharp">[HttpGet]
public HttpResponseMessage Generate()
{
    MemoryStream stream = new MemoryStream();
    // ... PDF generation code to populate the stream ...

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

    return result;
}</code>

This code uses ByteArrayContent to send the PDF as a byte array. Crucially, it sets the ContentDisposition header to attachment (telling the browser to download the file) and the ContentType to application/octet-stream (indicating a binary file). This ensures the PDF is correctly downloaded instead of being rendered in the browser.

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