Isolating Dependencies with Test Doubles: Stubs

Welcome to the second lesson in our exploration of Test-Driven Development (TDD) using C#, xUnit, and Moq. In the previous lesson, we covered how to utilize dummies to isolate dependencies. In this lesson, our focus will shift to another type of test double — Stubs.

By the end of this lesson, you will understand what stubs are and how to implement them in your tests, specifically for isolating dependencies like external services within the C# ecosystem.

In testing, test doubles help us isolate parts of our application. We've previously discussed dummies. Now, we will explore stubs, a more useful type of test double. Stubs provide predefined answers to method calls during testing. Unlike other test doubles, stubs do not track their usage, making them simpler yet powerful for certain scenarios.

Stubs are particularly useful when testing functions that rely on external services or complex dependencies. By simulating function outputs, stubs make tests faster and more predictable. Keep in mind that stubs focus on ensuring your application's logic functions as expected without verifying the correctness of external dependencies.

In C#, Moq is a popular library used to create stubs. It allows us to set up return values that simulate how dependencies should behave in a controlled environment. This predictability isolates and tests your application's logic without relying on the behavior of external systems, which might be complex or introduce variability.

To illustrate the concept of stubs, we will create a WeatherAlertService using stubs in a test-driven development process.

We will build a WeatherAlertService that fetches data from a WeatherService. This service will issue alerts based on specific conditions. The external data source is impractical for testing, so we'll use stubbed data for our tests instead.

Create a new test class named WeatherAlertServiceTests.cs with the following test setup:

using Xunit;

class WeatherServiceStub : IWeatherService
{
    private int _temperature = 20;
    private string _conditions = "sunny";

    public void SetWeather(int temperature, string conditions)
    {
        _temperature = temperature;
        _conditions = conditions;
    }

    public Task<WeatherData> GetCurrentWeather(string location)
    {
        return Task.FromResult(new WeatherData
        {
            Temperature = _temperature,
            Conditions = _conditions
        });
    }
}

public class WeatherAlertServiceTests
{
    private WeatherServiceStub weatherService;
    private WeatherAlertService alertService;
    
    public WeatherAlertServiceTests()
    {
        weatherService = new WeatherServiceStub();
        alertService = new WeatherAlertService(weatherService);
    }
    
    [Fact]
    public async Task ShouldReturnHeatWarningWhenTemperatureIsAbove35()
    {
        // Arrange
        weatherService.SetWeather(36, "sunny");
        
        // Act
        var alert = await alertService.ShouldSendAlert("London");

        // Assert
        Assert.Equal("Extreme heat warning. Stay hydrated!", alert);
    }
}

In this test:

• We create a hand-crafted stub WeatherServiceStub that implements the IWeatherService interface. This stub allows us to customize the weather data by setting predefined values for Temperature and Conditions via the SetWeather method.
• The stub’s GetCurrentWeather method returns a WeatherData object with these predefined values, simulating expected weather conditions for the test scenario without using Moq.
• The xUnit test validates whether the WeatherAlertService correctly interprets the weather data and returns the appropriate alert, specifically checking if it generates a heat warning when the temperature exceeds 35 degrees.

Run this test, and expect it to fail initially, as WeatherAlertService is not yet implemented to handle the specified conditions.