Understanding Method Invocation Verification with Moq's Verifiable Constructs

Introduction

I encountered a scenario where I needed to test if a method inside another method was actually invoked. This led me to learn about the Verifiable construct in the Moq library. 💻 It provides a concrete way to ensure that the calls configured in the mock object are indeed invoked. ℹ

Example 🚀

Suppose a CartsController has a CreateOrder method that calls the GetCartItems method within it:

public async Task<IActionResult> CreateOrder(OrderViewModel orderViewModel)
{
    var cartItems = await _cartService.GetCartItems(orderViewModel.CartId);

    // More codes...

    return View("OrderCreated");
}

A unit test for this method can be written to verify whether GetCartItems is actually invoked or not. Let's see how we can achieve this.

How to do? 🤷‍♀️

Mark the mock setup for the method as Verifiable and then call the Verify method on the mock object.

Here is a code snippet for one unit test case:

// Mock setup inside the unit test for the GetCartItems method.
_cartServiceMock.Setup(x => x.GetCartItems(It.IsAny<int>())).ReturnsAsync(() => cartItems).Verifiable();

// The following CreateOrder calls the GetCartItems inside it.
var result = await cartsController.CreateOrder(orderModel);

// Now we can verify
_cartServiceMock.Verify();

Notice that we set up a method GetCartItems to be Verifiable. Then, we called the method inside the CartsController to create an order. The CreateOrder method calls the GetCartItems method inside it.

So, _cartServiceMock.Verify(); will now be successful as it invoked the GetCartItems method.

If for some reason it does not, then the test will throw an exception like in the following screenshot.

Feedback

I hope it is clear. Let me know if you have any doubts. 💪 Share, if you care. 🙌

Reference

1. Moq GitHub Repository

2. Moq Documentation on Verifications