How to Moq Mock a LoggerFactory in C# AspNet Core

Question

I am trying to write some unit tests for controller actions. To do that, I am using XUnit and Moq. The controllers have an ILoggerFactory injected in the constructor. How does one Moq this up for testing?

I have tried mocking a Logger for the controller class, and then mocking up CreateLogger to return the mock Logger, but I keep getting various test runtime NullReferenceExceptions when the LogInformation() function is called.

        //   Logger that yields only disappointment...                   var mockLogger = new Mock<ILogger<JwtController>>();         mockLogger.Setup(ml => ml.Log(It.IsAny<LogLevel>(), It.IsAny<EventId>(), It.IsAny<object>(), It.IsAny<Exception>(), It.IsAny<Func<object, Exception, string>>()));         var mockLoggerFactory = new Mock<ILoggerFactory>();         mockLoggerFactory.Setup(mlf => mlf.CreateLogger("JwtController")).Returns(mockLogger.Object); 

I assume the problem is that LogInformation is being called, and this is an extension method, so how to moq that?

Answer 1

For what it's worth: instead of mocking an ILoggerFactory, you could also pass an instance of NullLoggerFactory. This NullLoggerFactory will return instances of NullLogger. According to the docs, this is a:

Minimalistic logger that does nothing.

Answer 2

I just mock the ILogger extension methods as below, and use a value function in the ILoggerFactory setup that returns the Mock ILogger object.

var mockLogger = new Mock<ILogger<[YOUR_CLASS_TYPE]>>(); mockLogger.Setup(     m => m.Log(         LogLevel.Information,         It.IsAny<EventId>(),         It.IsAny<object>(),         It.IsAny<Exception>(),         It.IsAny<Func<object, Exception, string>>()));  var mockLoggerFactory = new Mock<ILoggerFactory>(); mockLoggerFactory.Setup(x => x.CreateLogger(It.IsAny<string>())).Returns(() => mockLogger.Object); 

This will return your mocked Logger and you can verify any calls on it. No need to write wrappers or helpers.

You can even mock the IsEnabled, which is necessary for some code that leverages that functionality.

        mockLogger.Setup(             m => m.IsEnabled(                 Microsoft.Extensions.Logging.LogLevel.Debug)).Returns(true); 

Because there is only one method to mock, (and that you have to call), below shows the logging call you (might) use to have everything pointed to the this exact method.

 catch (ArgumentOutOfRangeException argEx) {     // this.logger.LogError(argEx, argEx.Message); /* << this is what you would like to do, BUT it is an extension method, NOT (easily) mockable */     Func<object, Exception, string> returnAnEmptyStringFunc = (a, b) => string.Empty;     this.logger.Log(LogLevel.Error, ushort.MaxValue, argEx.Message, argEx, returnAnEmptyStringFunc);     throw argEx; }