How to verify that ILogger logged an error?

Introduction

For a recent project, I wanted to create a test that verified that LogError was called. Consider for example this piece of code below. The catch operation was added to swallow the exception of the delete action. We want to test this behavior but still would like to know if the LogError is being called.

The system under test

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public async Task Delete(long sequenceNumber)
{
  _logger.LogInformation("Deleting `{sequenceNumber}`.", sequenceNumber);
  try
  {
    await _client.Delete(..);
    _logger.LogInformation("Delete completed `{sequenceNumber}`.", sequenceNumber);
  }
  catch (InvalidOperationException ex)
    when (ex.Message.Equals($"The scheduled message with SequenceNumber = {sequenceNumber} is already being cancelled."))
  {
    _logger.LogError(ex, "Already cancelled {sequenceNumber}.", sequenceNumber);
  }
}

Verify that `LogError` is called

Have you ever tried to verify your LogError using xUnit? It does not seem to work out of the box as other FakeItEasy. I tried this code for example, but it just would not work. The mock that throws the exception has been left out to keep the code sample small.

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//Arrange
var logger = A.Fake<ILogger<SystemUnderTest>>();
var sut = new SystemUnderTest(logger);

//Act
await sut.Delete(1);

//Assert
A.CallTo(() => logger.LogError(A<string>.Ignored, A<object[]>.Ignored))
    .MustHaveHappenedOnceExactly();

The LoggerExtensions class

The solution was right at hand because my coworker had already figured it out. Thanks, Marnix. Chekout his blog: Marnix’ blog. Use the extension class as described below.

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//Arrange
var logger = A.Fake<ILogger<SystemUnderTest>>();
var sut = new SystemUnderTest(logger);

//Act
await sut.Delete(1);

//Assert
logger.VerifyLogged(LogLevel.Information, "Deleting 1");
logger.VerifyLogged(LogLevel.Error, "Already cancelled 1");

LoggerExtensions.cs

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using FakeItEasy;
using Microsoft.Extensions.Logging;
using System;
using System.Linq;
using Xunit.Sdk;

namespace SomeCoolNamespace;
public static class LoggerExtensions
{
    public static void VerifyLogged<T>(this ILogger<T> logger, LogLevel level, string logMessage)
    {
        var (found, actualLevel, actualMessage) = logger.VerifyLog(logMessage);
        if (!found)
        {
            throw new XunitException($"No log message found containing '{logMessage}' at any loglevel");
        }

        if (actualLevel != level)
        {
            throw new AssertActualExpectedException(
                $"[{level}] {logMessage}", $"[{actualLevel}] {actualMessage}",
                $"Unexpected log level for log message");
        }
    }

    public static void VerifyNotLogged<T>(this ILogger<T> logger, LogLevel level, string logMessage)
    {
        var (found, actualLevel, actualMessage) = logger.VerifyLog(logMessage);
        if (found && actualLevel == level)
        {
            throw new XunitException(
                @$"Log message found containing '{logMessage}'
                    at level {level}{Environment.NewLine}Message: {actualMessage}");
        }
    }

    public static void VerifyNotLoggedAtLevel<T>(this ILogger<T> logger, LogLevel level)
    {
        var call = Fake.GetCalls(logger)
            .FirstOrDefault(call => (LogLevel?)call.Arguments[0] == level);

        if (call != null)
        {
            throw new XunitException(
                @$"Log message found at level {level}{Environment.NewLine}
                    Message: {call.Arguments[2]}");
        }
    }
    public static void VerifyLoggedAtLevel<T>(this ILogger<T> logger, LogLevel level)
    {
        var found = Fake.GetCalls(logger)
            .Any(call => (LogLevel?)call.Arguments[0] == level);

        if (found)
        {
            throw new XunitException($"No log message found at level {level}");
        }
    }

    private static (bool found, LogLevel? level, string? message) VerifyLog<T>(this ILogger<T> logger, string message)
    {
        var call = Fake.GetCalls(logger)
            .FirstOrDefault(call => call.Arguments[2].ToString()
            .Contains(message, StringComparison.OrdinalIgnoreCase));

        return (call != null, (LogLevel?)call?.Arguments[0], call?.Arguments[2].ToString());
    }
}

Conclusion

Using this class you will be able to test your logging with Xunit and FakeItEasy.

Introduction#

The system under test#

Verify that LogError is called#

The LoggerExtensions class#

LoggerExtensions.cs#

Conclusion#