Mocking and Test Doubles

Learning objectives#

• Name the types of test doubles
• Stub return values
• Verify interactions with mocks
• Avoid over-mocking

Explanation#

Dummy: passed but never used. Stub: returns canned answers. Fake: simple working implementation (in-memory repo). Mock: records and verifies calls. Spy: wraps a real object and records calls.

C#: Moq or NSubstitute. JavaScript: jest.fn(), jest.mock().

Mock at boundaries you own (interfaces to email, payment, time). Over-mocking internal details makes tests fragile and less meaningful.

Code example: Stubbing and verifying with Moq#

1// dotnet add package Moq
2public interface IPaymentGateway { Task<bool> ChargeAsync(decimal amount); }
3public interface IEmailSender { Task SendAsync(string to, string subject); }
4

5public class CheckoutService(IPaymentGateway pay, IEmailSender email)
6{
7    public async Task<bool> PlaceOrderAsync(string customer, decimal total)
8    {
9        if (!await pay.ChargeAsync(total)) return false;
10        await email.SendAsync(customer, "Order confirmed");
11        return true;
12    }
13}
14

15public class CheckoutServiceTests
16{
17    [Fact]
18    public async Task Successful_payment_sends_confirmation()
19    {
20        var pay = new Mock<IPaymentGateway>();
21        pay.Setup(p => p.ChargeAsync(500m)).ReturnsAsync(true);          // stub
22        var email = new Mock<IEmailSender>();
23

24        var ok = await new CheckoutService(pay.Object, email.Object).PlaceOrderAsync("asha@example.com", 500m);
25

26        Assert.True(ok);
27        email.Verify(e => e.SendAsync("asha@example.com", "Order confirmed"), Times.Once);   // mock
28    }
29}

Passed — no real payment or email was sent, and the test proves the email is sent exactly once.

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. The payment gateway is stubbed to succeed.
2. The email sender is a mock.
3. The service runs with both doubles.
4. Verify checks the interaction happened once.

Real-world use case#

Common mistakes#

Best practices#

• Mock at system boundaries
• Prefer stubs over strict mocks
• Use fakes for repositories
• Verify only important interactions

Remember this#

Practice exercise#

1pay.Setup(p => p.ChargeAsync(It.IsAny<decimal>())).ReturnsAsync(false);
2// ...
3Assert.False(ok);
4email.Verify(e => e.SendAsync(It.IsAny<string>(), It.IsAny<string>()), Times.Never);

Mini quiz#

Want more? Practise all Automation Testing MCQs · Automation Testing interview questions

Related topics#

My notes#

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