How to unit test with ILogger in ASP.NET Core

Question

This is my controller:

public class BlogController : Controller
{
    private IDAO _blogDAO;
    private readonly ILogger _         


        

Answer 1

It is easy as other answers suggest to pass mock ILogger, but it suddenly becomes much more problematic to verify that calls actually were made to logger. The reason is that most calls do not actually belong to the ILogger interface itself.

So the most calls are extension methods that call the only Log method of the interface. The reason it seems is that it's way easier to make implementation of the interface if you have just one and not many overloads that boils down to same method.

The drawback is of course that it is suddenly much harder to verify that a call has been made since the call you should verify is very different from the call that you made. There are some different approaches to work around this, and I have found that custom extension methods for mocking framework will make it easiest to write.

Here is an example of a method that I have made to work with NSubstitute:

public static class LoggerTestingExtensions
{
    public static void LogError(this ILogger logger, string message)
    {
        logger.Log(
            LogLevel.Error,
            0,
            Arg.Is(v => v.ToString() == message),
            Arg.Any(),
            Arg.Any>());
    }

}

And this is how it can be used:

_logger.Received(1).LogError("Something bad happened");   

It looks exactly as if you used the method directly, the trick here is that our extension method gets priority because it's "closer" in namespaces than the original one, so it will be used instead.

It does not give unfortunately 100% what we want, namely error messages will not be as good, since we don't check directly on a string but rather on a lambda that involves the string, but 95% is better than nothing :) Additionally this approach will make the test code

P.S. For Moq one can use the approach of writing an extension method for the Mock> that does Verify to achieve similar results.

P.P.S. This does not work in .Net Core 3 anymore, check this thread for more details: https://github.com/nsubstitute/NSubstitute/issues/597#issuecomment-573742574