Fakes via interfaces; test data builders
Inject dependencies via interfaces, write simple fakes to observe behavior, and build complex inputs with a tiny test data builder.
Fakes via interfaces; test data builders is a free C# Academy lesson on CoddyKit — lesson 2 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Isolation via fakes
Goal: Isolate code under test.
- Depend on interfaces
- Provide fakes in tests
- Use a tiny builder for complex inputs
- Observe calls, not the real world
Inject time via IClock
Hide time behind IClock. Tests will pass a fake clock for predictable results.
using System;
public interface IClock { DateTime NowUtc(); }
public sealed class SystemClock : IClock
{
public DateTime NowUtc() { return DateTime.UtcNow; }
}
public sealed class DiscountService
{
private readonly IClock _clock;
public DiscountService(IClock clock) { _clock = clock; }
// 50% off on Black Friday (Nov 27), else 0
public int GetDiscountPercent()
{
DateTime d = _clock.NowUtc().Date;
if (d.Month == 11 && d.Day == 27) return 50;
return 0;
}
}
public class Program
{
public static void Main(string[] args)
{
DiscountService live = new DiscountService(new SystemClock());
Console.WriteLine("Today's discount: " + live.GetDiscountPercent() + "%");
}
}
Fake implementation in tests
A fake implements the interface and returns controlled data so assertions are stable.
using System;
public interface IClock { DateTime NowUtc(); }
public sealed class FakeClock : IClock
{
private readonly DateTime _fixed;
public FakeClock(DateTime fixedUtc) { _fixed = fixedUtc; }
public DateTime NowUtc() { return _fixed; }
}
public static class Assert
{
public static void AreEqual(int expected, int actual)
{
if (expected != actual)
throw new Exception("Expected " + expected + " but got " + actual);
}
}
public sealed class DiscountService
{
private readonly IClock _clock;
public DiscountService(IClock clock) { _clock = clock; }
public int GetDiscountPercent()
{
DateTime d = _clock.NowUtc().Date;
if (d.Month == 11 && d.Day == 27) return 50;
return 0;
}
}
public class Program
{
public static void Main(string[] args)
{
try
{
// Black Friday: 2015-11-27
DiscountService s1 = new DiscountService(new FakeClock(new DateTime(2015, 11, 27, 0, 0, 0, DateTimeKind.Utc)));
Assert.AreEqual(50, s1.GetDiscountPercent());
// Normal day
DiscountService s2 = new DiscountService(new FakeClock(new DateTime(2015, 11, 26, 0, 0, 0, DateTimeKind.Utc)));
Assert.AreEqual(0, s2.GetDiscountPercent());
Console.WriteLine("PASS: discount logic with fake clock");
}
catch (Exception ex)
{
Console.WriteLine("FAIL: " + ex.Message);
}
}
}
Observe side effects
A fake can capture calls so the test verifies side effects without real email/sms.
using System;
using System.Collections.Generic;
public interface INotifier { void Send(string message); }
public sealed class OrderService
{
private readonly INotifier _notifier;
public OrderService(INotifier notifier) { _notifier = notifier; }
public void Place(string product, int qty)
{
if (qty <= 0) throw new ArgumentOutOfRangeException("qty");
// ...
_notifier.Send("Placed: " + product + " x" + qty);
}
}
public sealed class FakeNotifier : INotifier
{
public readonly List<string> Messages = new List<string>();
public void Send(string message) { Messages.Add(message); }
}
public static class Assert
{
public static void IsTrue(bool condition, string message)
{
if (!condition) throw new Exception(message);
}
}
public class Program
{
public static void Main(string[] args)
{
FakeNotifier fake = new FakeNotifier();
OrderService svc = new OrderService(fake);
try
{
svc.Place("Book", 2);
Assert.IsTrue(fake.Messages.Count == 1, "Expected exactly one notification");
Assert.IsTrue(fake.Messages[0].IndexOf("Book") >= 0, "Message should mention product");
Console.WriteLine("PASS: observed notification via fake");
}
catch (Exception ex)
{
Console.WriteLine("FAIL: " + ex.Message);
}
}
}
Test data builder
A tiny builder sets defaults and overrides only what a test needs, making setup short and clear.
using System;
public sealed class Order
{
public string Product;
public int Quantity;
public double UnitPrice;
public double Total() { return Quantity * UnitPrice; }
}
// Minimal builder for tests
public sealed class OrderBuilder
{
private string _product = "Item";
private int _qty = 1;
private double _price = 1.0;
public OrderBuilder WithProduct(string name) { _product = name; return this; }
public OrderBuilder WithQty(int qty) { _qty = qty; return this; }
public OrderBuilder WithPrice(double price) { _price = price; return this; }
public Order Build()
{
Order o = new Order();
o.Product = _product;
o.Quantity = _qty;
o.UnitPrice = _price;
return o;
}
}
public static class Assert
{
public static void AreEqual(double expected, double actual)
{
if (Math.Abs(expected - actual) > 0.0001)
throw new Exception("Expected " + expected + " but got " + actual);
}
}
public class Program
{
public static void Main(string[] args)
{
try
{
// Quick setup with readable intent
Order o = new OrderBuilder()
.WithProduct("Pen")
.WithQty(3)
.WithPrice(2.5)
.Build();
Assert.AreEqual(7.5, o.Total());
Console.WriteLine("PASS: builder created valid order");
}
catch (Exception ex)
{
Console.WriteLine("FAIL: " + ex.Message);
}
}
}
Practical tips
Tips:
- Prefer small interfaces to isolate side effects
- Keep fakes tiny; capture outputs in fields/lists
- Builders hold sane defaults and fluent setters
- Tests read like stories: setup → act → assert
Isolation approach
Recap
Recap: Inject interfaces, use fakes to control and observe behavior, and create inputs quickly with a small test data builder.
Frequently asked questions
Is the “Fakes via interfaces; test data builders” lesson free?
Yes — the full text of “Fakes via interfaces; test data builders” is free to read here on the web, and the C# Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.
What will I learn in “Fakes via interfaces; test data builders”?
Inject dependencies via interfaces, write simple fakes to observe behavior, and build complex inputs with a tiny test data builder. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start C# Academy?
No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “Fakes via interfaces; test data builders” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this C# Academy lesson?
Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- xUnit/MSTest setup, assertions
- Fakes via interfaces; test data builders
- Debugging failing tests