Send and Receive Media Messages with WhatsApp in C#/ASP.NET

In this tutorial, we'll set up a C#/ASP.NET application that uses the WhatsApp Channel to:

• Send media messages
• Reply to incoming messages with media

The code samples in this tutorial use Twilio’s C# helper library and ASP.NET MVC 5.

Send outbound media messages through WhatsApp

Just like when sending an MMS, sending an outbound WhatsApp message uses Twilio's Message resource. This section walks you through the setting up and sending media in a WhatsApp message. Media can consist of images, audio files, and PDFs.

Sign up for (or log in to) your Twilio Account and activate the Sandbox

Gather your Twilio account information

Before you can send any messages, you'll need to gather your Twilio account credentials. You can find these in the Twilio Console.

• Account SID - Used to identify yourself in API requests
• Auth Token - Used to authenticate REST API requests

For all of our code snippets and Python examples, you need to authenticate with the Account SID and Auth Token.

Send a media WhatsApp message via the REST API

To send an outgoing media message via WhatsApp from your Twilio account you’ll need to make an HTTP POST to Twilio's Message resource.

Sending a media message through WhatsApp is similar to sending a text-based message over WhatsApp with one important addition: the media_url parameter. The media_url parameter in this code tells Twilio where to retrieve the media that we want to include in the WhatsApp message. (You can prevent unauthorized access to your media by turning on HTTP Basic Auth for media URLs in the Twilio Console.)

Twilio's C# helper library helps you to create a new instance of the Message resource. When you do this, you'll specify the to, from, and mediaUrl parameters of your message.

If you don’t already have the C# helper library installed you can install it using nuget:

Install-Package Twilio -ProjectName MyProject

Now, create a file named program.cs and include the following code:

Replace the placeholder values for accountSid and authToken with your unique values. You can find these in your Twilio console.

The to number should be the phone number for the destination WhatsApp account in the E.164 format. If you are using the WhatsApp Sandbox, you can only send messages to numbers that have joined the Sandbox.

You’ll tell Twilio which phone number to use to send this message by replacing the from number with the whatsapp: channel identifier followed by the Sandbox number in E.164 format.

Save the file, compile it and then run the executable:

csc program.cs
program

In a few moments, you should receive a WhatsApp message with an image!

Please note that WhatsApp does not support including a text body in the same message as a video, audio file, document, contact (vCard), or location. If you pass the Body parameter on a message with one of these media types, the body will be ignored and not delivered to the device.

Understanding Twilio’s Response

When you send an outbound WhatsApp media message, Twilio sends data about the message in its response to your request. The JSON response contains the unique SID and URI for your media resource:

"subresource_uris": {"media": "/2010-04 01/Accounts/ACxxxxxxxx/Messages/SMxxxxxxxxxxxxx/Media.json"}

When the Twilio REST API creates your new Message resource, it saves the image found at the specified mediaUrl as a Media resource. Once created, you can access this resource at any time via the API.

You can print this value from your C# code to see where the image is stored. The following line to the end of your program.cs file prints out your newly provisioned Media URI:

Debug.WriteLine(message.SubresourceUris["media"]);

Respond with media in WhatsApp

To reply using media to incoming WhatsApp messages, you’ll need to provide Twilio with a webhook URL that points to a server that runs code to inspect and save the media files.

What are webhooks?

Webhooks are user-defined HTTP callbacks. They are usually triggered by some event, such as receiving an SMS message or an incoming phone call. When that event occurs, Twilio makes an HTTP request (usually a POST or a GET) to the URL configured for the webhook.

To handle a webhook, you only need to build a small web application that can accept the HTTP requests. Almost all server-side programming languages offer some framework for you to do this. In this tutorial, we'll be setting up our web application with ASP.NET.

Webhook functionality is the same for every Twilio application. First, a webhook makes an HTTP request to a URI that you provide to Twilio. When it receives this request, your application performs pre-defined logic. This could be something like database read/writes or calling another API. Finally, your application sends a TwiML response to Twilio in which it specifies the instructions for Twilio to follow.

What is TwiML?

TwiML is the Twilio Markup Language, which is just to say that it's an XML document with special tags defined by Twilio to help you build your messaging and voice applications.

TwiML is easier shown than explained:

<?xml version="1.0" encoding="UTF-8"?>
<Response>
   <Message>Thanks for the message!</Message>
</Response>

Every TwiML document has the <Response> element, which can contain one or more verbs. Verbs are actions you'd like Twilio to take, such as <Say> a greeting to a caller, or send an SMS <Message> in reply to an incoming message. For a full reference on everything you can do with TwiML, refer to our TwiML API Reference.

To send back a media in your WhatsApp reply, you need to include the media TwiML element with the URL to the media file. One media attachment is supported per message, with a size limit of 5MB.

Generate TwiML in your application

To reply to an incoming WhatsApp message, you can either write raw TwiML or use a helper library. When you use the helper library, you don't have to worry about generating the raw XML yourself.

You have the code – now you need a URL you can give to Twilio.

Twilio can only access public servers on the Internet. That means you need to take your web application and publish it to a web or cloud hosting provider (of which there are many), you can host it on your own server, or you can use a service such as ngrok to expose your local development machine to the internet. (We only recommend the latter for development and testing purposes and not for production deployments.)

To send media in response to an incoming WhatsApp message, simply add an image URL. If necessary, restart your server, then send a message to your WhatsApp number again. You should receive a WhatsApp message that includes an image. Check out the API Reference for more details.

Configure your webhook URL

Now that you have a URL for your web application's TwiML reply generating routine, you can configure your Twilio phone number to call your webhook URL whenever a new WhatsApp message comes in for you.

You can set the webhook URL for incoming messages to your server in the Sandbox.

Make sure you choose HTTP POST or HTTP GET to correspond to what your web application is expecting. Usually, the default of POST is fine.

And that's all there is to it; receiving and responding is exactly the same as you would do in any SMS app with our Messaging API. Cool, right?

Examine media on incoming WhatsApp messages

Viewing, saving, or manipulating the media on incoming WhatsApp messages also involves configuring a Webhook URL. The URL points to a server generating TwiML instructions including the media you want to send.

Get incoming message details

When Twilio calls your webhook, it sends a number of parameters about the message you just received. Most of these, such as the To phone number, the From phone number, and the Body of the message are available as properties of the request parameter to our action method.

Get URLs to the media

Twilio sends form variables named MediaUrlX, where X is a zero-based index. WhatsApp messages will only contain one media file per incoming message, so you can access the file at MediaUrl0 on the incoming request from Twilio to your webhook URL.

Determine Content-Type of media

Attachments to WhatsApp messages can be of many different file types, including JPG, MP3, and PDF. (See more about the supported file types in the FAQs.) Twilio handles the determination of the file type for you and you can get the standard mime type from the MediaContentTypeX parameter. If you are expecting photos and images, then you will likely see a lot of attachments with the mime type image/jpeg.

What’s next?

All the code, in a complete working project, is available on GitHub. To dig deeper, head over to the Messaging API Reference and learn more about the Twilio webhook request and the REST API Media resource. Also, you should be aware of the pricing for storage of all the media files that you keep on Twilio’s servers.

We’d love to hear what you build with this!