Unit Testing with ILogger<T>

Some common ways to work with ILogger<T> objects in unit testing.

EDIT 11/16/2020: The code has updated to .NET 5

EDIT 04/13/2020: The code has updated to .NET Core 3.1

An effective logging system is essential for application monitoring and analysis. .NET Core supports a logging API that works with a variety of built-in and third-party logging providers, such as the Console provider, the Azure Application Insights provider, Serilog, elma.io, and so on.

In order to create logs, we need to use an ILogger<T> object, which is readily accessible in our applications. (1) In a web application or a hosted service, we can get an ILogger<T> object from the native .NET Core dependency injection (DI) system. (2) In a non-host console application, we can use the LoggerFactory to create an ILogger<T> object.

Because ILogger<T> objects are frequently used in controllers and service classes, we cannot avoid them in unit tests. In this post, we will go over some common ways to work with ILogger<T> objects in unit testing. The full project can be found in this repository.

Table of Contents

1. Replace ILogger<T> with NullLogger<T>
2. Create a Real ILogger<T> that Logs to Console
3. Mock an ILogger<T> Object

Let’s consider a service class CalculationService that has a dependency on an ILogger<CalculationService> object. We are going to test the contrived AddTwoPositiveNumbers(int a, int b) method.

1. Replace ILogger<T> with NullLogger<T>

The NullLogger<T> implements ILogger<T> and it is a minimalistic logger that does nothing. In other words, it doesn’t log messages to any place and it swallows exceptions. It is worth mentioning that both the ILogger<T> interface and the NullLogger<T> class are included in…