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Custom HTTP Clients for the Twilio C# Helper Library with .NET Core

This article is for applications that utilize .NET Core 2.0 or higher. If you are developing with .NET Framework, please see this article.

If you are working with the Twilio C# / .NET Helper Library, and you need to be able to modify the HTTP requests that the library makes to the Twilio servers, you’re in the right place. The most common need to alter the HTTP request is to connect and authenticate with an enterprise’s proxy server. We’ll provide sample code that you can drop right into your app to handle this use case.

Connect and authenticate with a proxy server

To connect and provide credentials to a proxy server that may be between your app and Twilio, you need a way to modify the HTTP requests that the Twilio helper library makes on your behalf to invoke the Twilio REST API.

On .NET Core, the Twilio helper library uses the HttpClient class (in the System.Net.Http namespace) under the hood to make the HTTP requests. Knowing this, the following two facts should help us arrive at the solution:

  1. Connecting to a proxy server with HttpClient is a solved problem.
  2. The Twilio Helper Library allows you to provide your own HttpClient for making API requests.
    TwilioRestClient Constructor (C#)

So the question becomes how do we apply this to a typical Twilio REST API example?

TwilioClient.Init(accountSid, authToken);

var message = MessageResource.Create(
    to: new PhoneNumber("+15558675309"),
    from: new PhoneNumber("+15017250604"),
    body: "Hello from C#");

Where does a TwilioRestClient get created and used? Out of the box, the helper library is creating a default TwilioRestClient for you, using the Twilio credentials you pass to the Init method. However, there’s nothing stopping you from creating your own and using that.

Once you have your own TwilioRestClient, you can pass it to any Twilio REST API resource action you want. Here’s an example of sending an SMS message with a custom client:


        How to pass a TwilioRestClient

        Create your custom TwilioRestClient

        When you take a closer look at the constructor for TwilioRestClient, you see that the httpClient parameter is actually of type Twilio.Http.HttpClient and not the System.Net.HttpClient we were expecting. Twilio.Http.HttpClient is actually an abstraction that allows plugging in any implementation of an HTTP client you want (or even creating a mocking layer for unit testing).


        However, within the helper library, there is an implementation of Twilio.Http.HttpClient called SystemNetHttpClient. This class wraps the System.Net.HttpClient and provides it to the Twilio helper library to make the necessary HTTP requests.

        Call Twilio through the proxy server

        Now that we understand how all the components fit together, we can create our own TwilioRestClient that can connect through a proxy server. To make this reusable, here’s a class that you can use to create this TwilioRestClient whenever you need one.


              ProxiedTwilioClientCreator.cs (.NET Core)

              Notice the use of the _configuration variable (an instance of IConfiguration) to retrieve various configuration settings:

              • Your Twilio Account Sid and Auth Token (found here, in the Twilio console)
              • Your proxy server URL (including the server name or address and port number)
              • Your username and password for the proxy server

              These settings can be placed in a file like appsettings.json like so:

                "TwilioAccountSid": "ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
                "TwilioAuthToken": "your_auth_token",
                "ProxyServerUrl": "",
                "ProxyUsername": "your_username",
                "ProxyPassword": "your_password"

              Here’s a console program that sends a text message and shows how it all can work together. It loads the appsettings.json file. For a more in-depth discussion of your configuration options with ASP.NET Core, refer to the documentation.


                    Call the Twilio REST API with a custom TwilioRestClient in C# (.NET Core)

                    What else can this technique be used for?

                    Now that you know how to inject your own System.Net.HttpClient into the Twilio API request pipeline, you could use this technique to add custom HTTP headers to the requests (perhaps as required by an upstream proxy server).

                    You could also implement your own Twilio.Http.HttpClient to mock the Twilio API responses so your unit and integration tests can run quickly without the need to make a connection to Twilio. In fact, there’s already an example online showing how to do exactly that.

                    We can’t wait to see what you build!

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                    Need some help?

                    We all do sometimes; code is hard. Get help now from our support team, or lean on the wisdom of the crowd by visiting Twilio's Stack Overflow Collective or browsing the Twilio tag on Stack Overflow.


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