The Power of Callbacks in Unit Testing with Moq
Tags: testing, beginners, programming, tutorial
Published: 2023-06-30 07:42:03

In the realm of programming, every little element we employ has a significant impact on the quality of our code. They're like individual pieces of a grand puzzle that, when assembled correctly, create a beautiful, efficient and maintainable picture. Today, we're delving into one of these elements - callbacks - and their transformative role in unit testing with Moq.

## What are Callbacks?

To truly appreciate callbacks, one must understand their role. In the simplest terms, a callback is a piece of executable code that is passed as an argument to other code. The code receiving the callback then decides the opportune time to execute it.

In a broader sense, callbacks introduce an inversion of control, allowing us to tailor the flow of program execution. Think of it as putting together a blueprint for a house but leaving some details open for customization. The architect lays out the foundational plan, while the future homeowner gets to dictate the colors, the finishes, and the embellishments. Callbacks essentially allow our software to have that level of flexibility and customization.

## Introducing Callbacks in Moq

Moq, a powerful .NET mocking library, supports callbacks, giving developers more control over their unit testing scenarios. Let's take a brief journey into the universe of Moq and learn how we can leverage the power of callbacks.

```csharp
var mock = new Mock<IFoo>();
var calls = 0;
var callArgs = new List<string>();

mock.Setup(foo => foo.DoSomething("ping"))
    .Callback(() => calls++)
    .Returns(true);
```

In the above code, we set up a mock object that imitates the behavior of `IFoo`. By defining a `Callback`, we are saying: "Every time `DoSomething` method is called with argument "ping", increment the `calls` counter." This can help us track how many times the method was invoked during our tests.

## Delving Deeper

Now let's go deeper into a more interesting scenario. What if our method has multiple parameters, or we wish to capture the arguments passed to our method? Well, in the world of callbacks, nothing is impossible!

```csharp
// access invocation arguments
mock.Setup(foo => foo.DoSomething(It.IsAny<string>()))
    .Callback((string s) => callArgs.Add(s))
    .Returns(true);

// access arguments for methods with multiple parameters
mock.Setup(foo => foo.DoSomething(It.IsAny<int>(), It.IsAny<string>()))
    .Callback<int, string>((i, s) => callArgs.Add(s))
    .Returns(true);
```

As seen above, we can use `Callback` to get the parameters passed to the method and even perform some operation with these parameters.

## The Game of 'Before' and 'After'

One of the compelling features of using callbacks in Moq is the ability to specify actions that happen before and after the invocation of a method. This offers us granular control over the execution flow and leads to more realistic test scenarios.

```csharp
mock.Setup(foo => foo.DoSomething("ping"))
    .Callback(() => Console.WriteLine("Before returns"))
    .Returns(true)
    .Callback(() => Console.WriteLine("After returns"));
```

## The ‘ref’ and ‘out’ Parameter Conundrum

If you're working with methods that use `ref` or `out` parameters, don't worry! Moq provides support for them. However, it does require a bit more setup, and you would need Moq 4.8 or later:

```csharp
delegate void SubmitCallback(ref Bar bar);

mock.Setup(foo => foo.Submit(ref It.Ref<Bar>.IsAny))
    .Callback(new SubmitCallback((ref Bar bar) =>
       Console.WriteLine("Submitting a Bar!")));
```

## The Dynamic Return Values

You can also have dynamic return values based on the number of invocations:

```csharp
var mock = new Mock<IFoo>();
var calls = 0;
mock.Setup(foo => foo.GetCount())
	.Callback(() => calls++)
	.Returns(() => calls);
```

In this scenario, every time `GetCount` is called, it returns an incremented value, simulating a dynamic environment.

## When to Use Callbacks?

The use of callbacks in unit testing brings an extra layer of customization and control to your testing scenarios. They're particularly useful when:

- You need to perform some action every time a mocked method is called.
- You want to inspect or manipulate the arguments passed to the mocked methods.
- The methods have `ref` or `out` parameters.
- You need to have dynamic return values.

## The Power of Moq Callbacks

The crux of the matter is, callbacks introduce a significant degree of flexibility and power to our unit testing scenarios. They provide us with a deeper layer of control, allowing us to simulate real-world scenarios more effectively. Whether it's the need to count method calls, modify or inspect parameters, or customize the execution flow of our tests, callbacks in Moq are ready to answer the call. Their inclusion elevates our unit testing game, ensuring that our code stands the test of time and quality. 

So the next time you sit down to write unit tests, remember that callbacks are like your magic wand, ready to infuse your tests with precision, flexibility, and power! Have a nice day!

For more details check [this](https://github.com/Moq/moq4/wiki/Quickstart#callbacks).