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

I've tried to mock that Logger interface using NSubstitute (and failed because Arg.Any<T>() requeres a type parameter, which I can't provide), but ended up creating a test logger (similarly to @jehof's answer) in the following way:

    internal sealed class TestLogger<T> : ILogger<T>, IDisposable
    {
        private readonly List<LoggedMessage> _messages = new List<LoggedMessage>();

        public IReadOnlyList<LoggedMessage> Messages => _messages;

        public void Dispose()
        {
        }

        public IDisposable BeginScope<TState>(TState state)
        {
            return this;
        }

        public bool IsEnabled(LogLevel logLevel)
        {
            return true;
        }

        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
        {
            var message = formatter(state, exception);
            _messages.Add(new LoggedMessage(logLevel, eventId, exception, message));
        }

        public sealed class LoggedMessage
        {
            public LogLevel LogLevel { get; }
            public EventId EventId { get; }
            public Exception Exception { get; }
            public string Message { get; }

            public LoggedMessage(LogLevel logLevel, EventId eventId, Exception exception, string message)
            {
                LogLevel = logLevel;
                EventId = eventId;
                Exception = exception;
                Message = message;
            }
        }
    }

You can easily access all logged messages and assert all meaningful parameters provided with it.

Answer 2

Use Telerik Just Mock to create a mocked instance of the logger:

using Telerik.JustMock;
...
context = new XDbContext(Mock.Create<ILogger<XDbContext>>());