FakeLoggerFactory.cs

using System.Collections.Concurrent;
using JetBrains.Annotations;
using Microsoft.Extensions.Logging;

namespace TestBuildingBlocks;

[PublicAPI]
public sealed class FakeLoggerFactory : ILoggerFactory, ILoggerProvider
{
    public FakeLogger Logger { get; }

    public FakeLoggerFactory(LogLevel minimumLevel)
    {
        Logger = new FakeLogger(minimumLevel);
    }

    public ILogger CreateLogger(string categoryName)
    {
        return Logger;
    }

    public void AddProvider(ILoggerProvider provider)
    {
    }

    public void Dispose()
    {
    }

    public sealed class FakeLogger : ILogger
    {
        private readonly LogLevel _minimumLevel;
        private readonly ConcurrentBag<FakeLogMessage> _messages = new();

        public IReadOnlyCollection<FakeLogMessage> Messages => _messages;

        public FakeLogger(LogLevel minimumLevel)
        {
            _minimumLevel = minimumLevel;
        }

        public bool IsEnabled(LogLevel logLevel)
        {
            return _minimumLevel != LogLevel.None && logLevel >= _minimumLevel;
        }

        public void Clear()
        {
            _messages.Clear();
        }

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

        public IDisposable BeginScope<TState>(TState state)
            where TState : notnull
        {
            return NullScope.Instance;
        }

        private sealed class NullScope : IDisposable
        {
            public static readonly NullScope Instance = new();

            private NullScope()
            {
            }

            public void Dispose()
            {
            }
        }
    }

    public sealed class FakeLogMessage
    {
        public LogLevel LogLevel { get; }
        public string Text { get; }

        public FakeLogMessage(LogLevel logLevel, string text)
        {
            LogLevel = logLevel;
            Text = text;
        }

        public override string ToString()
        {
            return $"[{LogLevel.ToString().ToUpperInvariant()}] {Text}";
        }
    }
}