How do I provide ILogger<T> in my unit tests of .NET Core code?

Starting from dotnet core 2.0 there's a generic NullLogger<T> class available:

var foo = new Foo(NullLogger<Foo>.Instance);

https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.logging.abstractions.nulllogger-1?view=aspnetcore-2.1 (docs) https://github.com/aspnet/Logging/blob/master/src/Microsoft.Extensions.Logging.Abstractions/NullLoggerOfT.cs (source)

Or if you need it as part of your services:

services.AddSingleton<ILoggerFactory, NullLoggerFactory>();

https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.logging.abstractions.nullloggerfactory?view=aspnetcore-2.1 (docs)

You can create an instance of ILogger<Foo> using NullLoggerFactory as the factory.

Consider the following controller:

public abstract class Foo: Controller
{
    public Foo(ILogger<Foo> logger) {
        Logger = logger;
    }

    public ILogger Logger { get; private set; }
}

A sample unit test could be:

[TestMethod]
public void FooConstructorUnitTest()
{
    // Arrange
    ILogger<FooController> logger = new Logger<FooController>(new NullLoggerFactory());

    // Act
    FooController target = new FooController(logger);

    // Assert
    Assert.AreSame(logger, target.Logger);
}

You have two options:

1. Create empty implementation of ILogger<Foo> by hand and pass an instance of it to ctor.
2. Create same empty implementation on the fly using some mocking framework like Moq, NSubstitute, etc.