How to End-to-End Test SMS Applications with C# .NET and Twilio

April 25, 2023
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End-to-End Test SMS Apps  with C# .NET and Twilio

Testing your software is an important part of the software development lifecycle. Unfortunately, it is sometimes skipped because it takes a lot of work, it is hard, and its value isn't always evident to stakeholders. There are many different types of tests you can do, and the most well known types are unit, integration, and end-to-end (E2E) testing.

E2E tests will simulate how a real user interacts with your application, so in the case of a web application, which is the most common, the E2E test will click links, buttons, fill out forms, etc. This is also considered a UI test because the UI of the application is being tested. The popularity of E2E tests have increased because tools like Cypress and Playwright are making it easier to develop E2E and UI tests for web apps.

However, not all applications are web based. What if your application doesn't have a conventional UI to test, like applications built on top of SMS or Voice? Messaging and voice calling is, in my opinion, another way of users interfacing with your application, so you should test the "UI" of these applications too, right? Unfortunately, there are no official testing tools for SMS or voice applications, but with Twilio you can build your own.

In this tutorial, you will learn how to test SMS applications by developing an intuitive virtual phone that can send and receive text messages inside your tests. Let's go! 👏


Here’s what you will need to follow along:

You can find the source code for this tutorial on GitHub. Use it if you run into any issues, or submit an issue if you run into problems.

Project Overview

The goal of this tutorial is to write tests that will follow the commonly used arrange, act, and assert (AAA) pattern:

  • Arrange: Set up the virtual phone (to be developed) that can send and receive SMS from a Twilio phone number, number A.
  • Act: Send SMS messages from number A, to a phone number which may or may not be a Twilio number, number B, and receive SMS messages from number B. Number B is the phone number that you are testing.
  • Assert: Verify that the response messages from number B are the expected body.

The virtual phone, that you will develop, will use the Twilio SDK to send SMS and the Twilio SMS webhook to receive SMS. To receive incoming messages via the SMS webhook, you'll need to spin up an ASP.NET Core web application that receives Twilio's HTTP webhook requests and then find a way to deliver those back to the running test.

Diagram showing that your tests will send and receive messages between a Twilio phone number and the phone number to test. The diagram is described in more detail below.

In the diagram above, you can see that your tests instruct the virtual phone number to send and receive messages between the Twilio phone number (number A) and the phone number to test (number B).
The virtual phone consists of two parts:

  1. The HTTP client using the Twilio SDK creates the messages in the Twilio API by sending HTTP requests, which in turn sends the SMS's out from your Twilio phone number.
  2. The HTTP server using ASP.NET Core that receives the HTTP requests sent by Twilio's SMS webhook when your Twilio phone number receives an SMS. What's not pictured here in the diagram is that you'll tunnel the locally running ASP.NET Core server to the public using ngrok.

The virtual phone, consisting of the HTTP client and server, will run inside the test project itself, but you could refactor the code to a separate application that you can communicate with from your tests.

The virtual phone in this tutorial is designed to support only one Twilio phone number. If you ran multiple tests in parallel, you wouldn't be able to determine for which test the incoming SMS is destined, hence, these tests will have to be run sequentially.

If you're up for the challenge, you could change the virtual phone to support multiple Twilio phone numbers and find a way to parallelize your tests.

The end goal of this virtual phone is to be able to write tests as intuitively as this:

// Arrange (including the virtual phone setup before this test runs)
using var conversation = virtualPhone.CreateConversation(toPhoneNumber);

// Act
var message = await conversation.WaitForMessage(timeToWait: TimeSpan.FromSeconds(10));

// Assert
Assert.Equal("Hi", message.Body);

The arrange-step sets up the virtual phone and creates a conversation, then the act-step sends a message and waits 10 seconds for a response, finally, the assert-step validates that the response body says "Ahoy!".

Set up your test project

Open a shell and run the following command to create an xUnit test project:

dotnet new xunit -n E2eSmsTest
cd .\E2eSmsTest\

This tutorial uses xUnit, but you could use NUnit, MSTest, or any other test framework with some modifications if that's what you prefer.

Add the following NuGet packages:

dotnet add package Twilio
dotnet add package Twilio.AspNet.Core
dotnet add package Microsoft.Extensions.Configuration
dotnet add package Microsoft.Extensions.Configuration.UserSecrets

Next, create a file named appsettings.json and add the following JSON:

  "VirtualPhone": {
    // Pick an available URL, but you have to tunnel it to the internet, 
    // and then configure the Twilio message webhook to point to the public URL wiht /message as path.
    "Urls": "http://localhost:5000",
    "PhoneNumber": "[YOUR_TWILIO_PHONE_NUMBER]",
    "Twilio": {
      // See
      "Client": {
        "AccountSid": "[USE_USER_SECRETS_OR_ENV_VARS]",
        "AuthToken": "[USE_USER_SECRETS_OR_ENV_VARS]"
      // See
      "RequestValidation": {
        "AuthToken": "[USE_USER_SECRETS_OR_ENV_VARS]"
  "ToPhoneNumber": "[PHONE_NUMBER_TO_TEST]"

Update the following placeholders:

  • Replace [YOUR_TWILIO_PHONE_NUMBER] with the Twilio phone number you want to send SMS from, in E.164 formatting.
  • Replace [PHONE_NUMBER_TO_TEST] with the phone number you want to test SMS interactions with, also in E.164 formatting. You can put your own personal phone number here if you don't have a phone number to test.
  • Leave [USE_USER_SECRETS_OR_ENV_VARS] as is because you'll configure these settings using user-secrets instead.

If you're using a Twilio trial account, you can only send SMS to Verified Caller IDs. You can verify caller IDs in the Twilio console, which will verify that you own the phone number by sending you a verification code. The phone number you used to sign up for Twilio will already be verified.

The appsettings.json file needs to be placed along the binaries when the project is compiled, therefore you need to update the E2eSmsTest.csproj file by adding the following <ItemGroup> inside the <Project> node:

                <None Update="appsettings.json">

Now, enable user-secrets for your project and configure the following secrets using the .NET CLI command below:

dotnet user-secrets init
dotnet user-secrets set VirtualPhone:Twilio:Client:AccountSid [TWILIO_ACCOUNT_SID]
dotnet user-secrets set VirtualPhone:Twilio:Client:AuthToken [TWILIO_AUTH_TOKEN]
dotnet user-secrets set VirtualPhone:Twilio:RequestValidation:AuthToken [TWILIO_AUTH_TOKEN]

Before running these commands, be sure to update the placeholders as follows:

  • Replace [TWILIO_ACCOUNT_SID] with your Twilio Account SID, which you can find in the Twilio console.
  • Replace [TWILIO_AUTH_TOKEN] with your Twilio Auth Token, which you can find in the Twilio console.

Now let me explain what these settings will do:

  • VirtualPhone is a section that holds all the configuration relating to the virtual phone.
  • VirtualPhone:Urls configures the URL that ASP.NET Core will run on and listen to for HTTP requests.
  • VirtualPhone:PhoneNumber configures which Twilio phone number to send messages from and receive messages at.
  • VirtualPhone:Twilio:Client is a section that configures Twilio's API client. You can learn more about this configuration in the Twilio.AspNet README file.
  • VirtualPhone:Twilio:RequestValidation is a section that configures the ASP.NET Core application to validate that HTTP request originate from Twilio. You can learn more about this configuration in the Twilio.AspNet README file.

Create the virtual phone

Now that your app’s been configured, create a new file called VirtualPhone.cs and add the code from this C# file.

Let's break down the code step by step:

The VirtualPhone class is responsible for sending and receiving SMS, but it uses an inner class Conversation to keep track of the ongoing conversations.

A conversation represents a conversation between the Twilio phone number used by the VirtualPhone and the phone number to converse with. While the VirtualPhone class is responsible for actually sending and receiving messages, the Conversation class is the class you will interface with from your tests.

The VirtualPhone class has three fields:

  • WebApplication webApplication: The running ASP.NET Core application.
  • PhoneNumber fromPhoneNumber: The Twilio phone number to send and receive SMS, number A.
  • Dictionary<string, Conversation> conversations: A dictionary to keep track of the ongoing conversations between the virtual phone and other phone numbers. The virtual phone uses this dictionary to forward incoming messages to the right conversation.

The static VirtualPhone.Create method:

  • Accepts a configuration object as a parameter. This configuration will be the VirtualPhone configuration section.
  • Runs a new ASP.NET Core web application configuring the Forwarded Header middleware, the Twilio API client, and Twilio request validation. The web application maps HTTP requests going to path /message to the VirtualPhone.MessageEndpoint. Later, you will configure the Twilio SMS webhook to send HTTP requests to /message.
  • Creates a new VirtualPhone object and returns it.
public static async Task<VirtualPhone> Create(IConfiguration configuration)
    var builder = WebApplication.CreateBuilder();
        options => options.ForwardedHeaders = ForwardedHeaders.All

    ChangeServiceLifetime(builder.Services, typeof(ITwilioRestClient), ServiceLifetime.Singleton);
    ChangeServiceLifetime(builder.Services, typeof(TwilioRestClient), ServiceLifetime.Singleton);

    var webApplication = builder.Build();
    var fromPhoneNumber = new PhoneNumber(
        ?? throw new Exception("PhoneNumber configuration is required")
    var virtualPhone = new VirtualPhone(webApplication, fromPhoneNumber);

    webApplication.MapPost("/message", virtualPhone.MessageEndpoint)

    await webApplication.StartAsync();

    return virtualPhone;

By default, the builder.Services.AddTwilioClient method adds the ITwilioRestClient and TwilioRestClient to the dependency injection (DI) container as a scoped service. Since the virtual phone will run from within the tests, I decided to change the lifetime to singleton, so I didn't have to deal with creating a scope every time I need the Twilio client.

The VirtualPhone.CreateConversation creates a new conversation between the virtual phone and to phone number, adds it to the conversations dictionary, and finally, returns the created conversation.

public Conversation CreateConversation(PhoneNumber to)
    var conversation = new Conversation(this, to);
    conversations.Add(to.ToString(), conversation);
    return conversation;

The VirtualPhone.RemoveConversation will remove the conversation from the conversations dictionary.
This method is private because the Conversation object will invoke this method when it is disposed.

private void RemoveConversation(PhoneNumber to) => conversations.Remove(to.ToString());

Twilio will send HTTP requests to the MessageEndpoint when your Twilio phone number receives an SMS. The endpoint extracts the sender’s phone number that is stored in the From form parameter. Using the sender’s phone number, the corresponding conversation is retrieved from the conversations dictionary. Then, finally, the message is forwarded to the Conversation.OnMessageReceived method.

private async Task MessageEndpoint(
    HttpRequest request,
    [FromServices] ITwilioRestClient twilioClient
    var form = await request.ReadFormAsync();
    var from = form["From"].ToString();
    if (string.IsNullOrEmpty(from)) return;

    if (conversations.TryGetValue(from, out var conversation))
        var message = await MessageResource.FetchAsync(pathSid: form["MessageSid"], client: twilioClient);
        await conversation.OnMessageReceived(message);

The VirtualPhone.SendMessage method retrieves the Twilio client from the DI container and uses the MessageResource.CreateAsync method to create a new message resource in Twilio's API, which will send an SMS.

private async Task<MessageResource> SendMessage(PhoneNumber to, string body)
    var twilioClient = webApplication.Services.GetRequiredService<ITwilioRestClient>();
    return await MessageResource.CreateAsync(
        to: to,
        from: fromPhoneNumber,
        body: body,
        client: twilioClient

The VirtualPhone.DisposeAsync class will stop the running ASP.NET Core application.

Next, I’ll break down the Conversation class.

The Conversation class has three fields:

  • VirtualPhone virtualPhone: The virtual phone is stored in a field so the conversation can call its methods to send messages.
  • Channel<MessageResource> incomingMessageChannel: A channel to write incoming messages to and read incoming messages from. Channels lets you pass data between producers and consumers.
  • PhoneNumber to: The phone number that messages are sent to and messages are received from, number B.

The Conversation.SendMessage method passes the body parameter and the to field to the VirtualPhone.SendMessage method.

public async Task<MessageResource> SendMessage(string body)
    var message = await virtualPhone.SendMessage(to, body);
    return message;

The Conversation.OnMessageReceived method is called by the virtual phone when a message comes in for this conversation. The Conversation.OnMessageReceived method writes the message to the incomingMessageChannel channel, so it can be read elsewhere.

internal async Task OnMessageReceived(MessageResource message)
    => await incomingMessageChannel.Writer.WriteAsync(message);

The Conversation.WaitForMessage accepts a TimeSpan timeToWait which is the amount of time the application will wait for an incoming message. The CancellationTokenSource and its Token will take care of cancelling the operation when the timeToWait has passed.

Next, the method will wait for a message and read the message from the incomingMessageChannel.

public async ValueTask<MessageResource> WaitForMessage(TimeSpan timeToWait)
    using var cts = new CancellationTokenSource(timeToWait);
    return await incomingMessageChannel.Reader.ReadAsync(cts.Token);

By using a Channel to write and read messages, you are able to synchronize data from one part of the application, the endpoint handling the webhook, to the other part of the application, your tests waiting for a message. Alternatively, you could use the TaskCompletionSource API to create a Task and set the result when a message comes in.

The WaitForMessages method is just like the WaitForMessage method, but instead you can wait for multiple messages, which will read the specified amount of messages from the incomingMessageChannel.

public async Task<MessageResource[]> WaitForMessages(int amountOfMessages, TimeSpan timeToWait)
    var messages = new MessageResource[amountOfMessages];
    using var cts = new CancellationTokenSource(timeToWait);
    for (int i = 0; i < amountOfMessages; i++)
        messages[i] = await incomingMessageChannel.Reader.ReadAsync(cts.Token);

    return messages;

Lastly, the Conversation.Dispose method will tell the virtual phone to remove the conversation.

public void Dispose() => virtualPhone.RemoveConversation(to);

Add xUnit fixtures

Every testing framework has different APIs for running setup and cleanup code before and after tests are run. In xUnit, you can add setup and cleanup code for a collection of tests by using Collection Fixtures. You'll use Collection Fixtures to set up the virtual phone before all tests and clean it up when all tests have run.

Create a new file Fixtures.cs and add the following code:

using System.Reflection;
using Microsoft.Extensions.Configuration;
using Xunit;

namespace E2eSmsTest;

/// <summary>
/// Fixture that loads configuration from appsettings.json, user secrets, and environment variables.
/// </summary>
public class ConfigurationFixture
    public IConfigurationRoot Configuration { get; set; }

    public ConfigurationFixture()
        Configuration = new ConfigurationBuilder()
            .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
            .AddUserSecrets(Assembly.GetExecutingAssembly(), optional: true, reloadOnChange: true)

/// <summary>
/// Fixture that creates the VirtualPhone using configuration section "VirtualPhone" from appsettings.json, user secrets, and environment variables.
/// </summary>
public class VirtualPhoneFixture : IAsyncLifetime
    public VirtualPhone? VirtualPhone { get; set; }

    public async Task InitializeAsync()
        var configuration = new ConfigurationBuilder()
            .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
            .AddUserSecrets(Assembly.GetExecutingAssembly(), optional: true, reloadOnChange: true)
            .AddEnvironmentVariables(prefix: "VIRTUALPHONE:")

        VirtualPhone = await VirtualPhone.Create(configuration.GetSection("VirtualPhone"));

    public async Task DisposeAsync()
        if (VirtualPhone != null)
            await VirtualPhone.DisposeAsync();

[CollectionDefinition("VirtualPhoneTests", DisableParallelization = true)]
public class TestCollectionFixture :

The ConfigurationFixture will load configuration from different sources, and store the configuration in a property to make it accessible to the test collection.

Typically, you'd configure tests using RunSettings files, but frankly, they're painful to use and .NET's configuration builder is a lot more powerful.

The VirtualPhoneFixture will also load configuration, but only pass the VirtualPhone configuration section to the VirtualPhone.Create method. The newly created virtual phone is then stored in a property to make it accessible to the test collection.

The TestCollectionFixture class tells xUnit which fixtures will be part of the "VirtualPhoneTests" collection, and by setting the DisableParallelization property to true, all tests within the collection will run without parallelization.

First, rename the file UnitTest1.cs to E2eTest.cs and rename the class inside to E2eTest too.

Now, add the tests in the E2eTest class to the "VirtualPhoneTests" collection by applying the Collection attribute to the class, inside E2eTest.cs:

using Xunit;

namespace E2eSmsTest;

public class E2eTest

Now you can add a constructor which accepts fixtures objects that are part of the collection.
Add a constructor for E2eTest and accept the VirtualPhoneFixture and ConfigurationFixture as parameters, and store the virtual phone and the phone number to test in fields, so the tests can use them.

using Twilio.Types;
using Xunit;

namespace E2eSmsTest;

public class E2eTest
    private readonly VirtualPhone virtualPhone;
    private readonly PhoneNumber toPhoneNumber;

    public E2eTest(VirtualPhoneFixture virtualPhoneFixture, ConfigurationFixture configurationFixture)
        virtualPhone = virtualPhoneFixture.VirtualPhone!;
        var configuration = configurationFixture.Configuration;
        toPhoneNumber = new PhoneNumber(
            ?? throw new Exception("ToPhoneNumber is not configured")

    public void Test1()


Fantastic, after all that setup, you can finally start testing SMS applications!

Write an end-to-end test

Remove the test method Test1 and add the following test inside the E2eTest class:

public async Task Send_Ahoy_Response_Should_Be_Hi()
    // Arrange (including the virtual phone setup before this test runs)
    using var conversation = virtualPhone.CreateConversation(toPhoneNumber);

    // Act
    _ = conversation.SendMessage("Ahoy!");
    var message = await conversation.WaitForMessage(timeToWait: TimeSpan.FromSeconds(10));

    // Assert

Before this test runs, the virtual phone will be created and reused for all tests in this test collection, then the test will:

  • Create a new conversation between the virtual phone using the Twilio phone number, number A, and the phone number to test, number B.
  • Send a message saying "Ahoy!".
  • Wait for a response message for 10 seconds.
  • Verify that the response body says "Hi".

Since I don't know what the phone number that you're testing does, you may need to change the messages to send and receive.

Configure the Twilio SMS webhook

While you can already send text messages using the virtual phone, to receive messages, you need to configure the webhook on your Twilio phone number.

In a separate shell, create a ngrok tunnel for http://localhost:5000 using the following command:

ngrok http http://localhost:5000

ngrok will print the Forwarding URL, which you'll need to publicly access your locally running ASP.NET Core web server.

Now, go to the Twilio Console in your browser, use the left navigation to navigate to Phone Numbers > Manage > Active Numbers, and then select the Twilio phone number you want to test with. (If Phone Numbers isn't on the left pane, click Explore Products and then on Phone Numbers.)

Then, on the phone number configuration page, locate the "A message comes in" section. Underneath that:

  • set the first dropdown to Webhook,
  • set the text box next to it to the ngrok forwarding URL — adding on the /message path,
  • and set the dropdown after the text box to "HTTP POST".

Then, click Save.

Run the end-to-end test

Now you're finally ready to run the test. If you're testing your own personal phone number, simply respond with the expected response, or not if you want to make the test fail.

Run the tests using the .NET CLI:

dotnet test

Feel free to add more tests. Here's an example of a test where the phone number should respond with multiple messages:

public async Task Response_Should_Be_Rick_Roll()
    using var conversation = virtualPhone.CreateConversation(toPhoneNumber);

    _ = conversation.SendMessage("What is the chorus of the Rick Roll song?");

    var messages = await conversation.WaitForMessages(
        amountOfMessages: 6

And here is a test verifying the flow of an SMS survey:

public async Task Verify_Survey_Pineapple_And_Cake()
    using var conversation = virtualPhone.CreateConversation(toPhoneNumber);

    // start conversation
    await conversation.SendMessage("Hi");

    // pineapple on pizza question
    var message = await conversation.WaitForMessage(TimeSpan.FromSeconds(10));
    Assert.Equal("On a scale of 1-10

Next steps

With the virtual phone in your tests, you can now intuitively write E2E tests for SMS applications. There are a couple of ways you could improve this solution:

Curious how you could build your own SMS applications with Twilio? Check out this tutorial on responding to incoming SMS from ASP.NET Core.

We can't wait to see what you build. Let us know!

Niels Swimberghe is a Belgian American software engineer and technical content creator at Twilio. Get in touch with Niels on Twitter @RealSwimburger and follow Niels’ personal blog on .NET, Azure, and web development at