ILogger for xUnit

Posted on June 1, 2018

It is very common to have logging in your code. Unfortunately there is not a great way for asynchronous test frameworks to capture that output when running unit tests. The xUnit test package is my favourite test framework and I would like to see the logging from my classes being tested ending up in the xUnit test results.

I have published the Divergic.Logging.Xunit package on NuGet to support this. The package returns an ILogger or ILogger<T> that wraps around the ITestOutputHelper supplied by xUnit. xUnit uses this helper to write log messages to the test output of each test execution. This means that any log messages from classes being tested will end up in the xUnit test result output.

Installation

Run the following in the NuGet command line or visit the NuGet package page.

Install-Package Divergic.Logging.Xunit

Usage

The common usage of this package is to call the BuildLogger extension method on the xUnit ITestOutputHelper.

using System;
using FluentAssertions;
using Microsoft.Extensions.Logging;
using Xunit;
using Xunit.Abstractions;

public class MyClass
{
    private readonly ILogger _logger;

    public MyClass(ILogger logger)
    {
        _logger = logger;
    }

    public string DoSomething()
    {
        _logger.LogInformation("Hey, we did something");

        return Guid.NewGuid().ToString();
    }
}

public class MyClassTests
{
    private readonly ITestOutputHelper _output;
    private readonly ILogger _logger;

    public MyClassTests(ITestOutputHelper output)
    {
        _output = output;
        _logger = output.BuildLogger();
    }

    [Fact]
    public void DoSomethingReturnsValueTest()
    {
        var sut = new MyClass(_logger);

        var actual = sut.DoSomething();

        // The xUnit test output should now include the log message from MyClass.DoSomething()

        actual.Should().NotBeNullOrWhiteSpace();
    }
}

This would output the following in the test results.

Information [0]: Hey, we did something

Support for ILogger<T> is there using the BuildLoggerFor<T> extension method.

using System;
using FluentAssertions;
using Microsoft.Extensions.Logging;
using Xunit;
using Xunit.Abstractions;

public class MyClassTests
{
    private readonly ITestOutputHelper _output;
    private readonly ILogger<MyClass> _logger;

    public MyClassTests(ITestOutputHelper output)
    {
        _output = output;
        _logger = output.BuildLoggerFor<MyClass>();
    }

    [Fact]
    public void DoSomethingReturnsValueTest()
    {
        var sut = new MyClass(_logger);

        var actual = sut.DoSomething();

        // The xUnit test output should now include the log message from MyClass.DoSomething()

        actual.Should().NotBeNull();
    }
}

Inspection

Using this library makes it really easy to output log messages from your code as part of the test results. You may want to also inspect the log messages written as part of the test assertions as well.

The BuildLogger and BuildLoggerFor<T> extension methods support this by returning a ICacheLogger or ICacheLogger<T> respectively. The cache logger is a wrapper around the created logger and exposes all the log entries written by the test.

using System;
using Divergic.Logging.Xunit;
using FluentAssertions;
using Microsoft.Extensions.Logging;
using Xunit;
using Xunit.Abstractions;

public class MyClassTests
{
    private readonly ITestOutputHelper _output;
    private readonly ICacheLogger _logger;

    public MyClassTests(ITestOutputHelper output)
    {
        _output = output;
        _logger = output.BuildLogger();
    }

    [Fact]
    public void DoSomethingReturnsValueTest()
    {
        var sut = new MyClass(_logger);

        sut.DoSomething();
        
        _logger.Count.Should().Be(1);
        _logger.Entries.Should().HaveCount(1);
        _logger.Last.Message.Should().Be("Hey, we did something");
    }
}

Perhaps you don’t want to use the xUnit ITestOutputHelper but still want to use the ICacheLogger to run assertions over log messages written by the class under test. You can do this by creating a CacheLogger or CacheLogger<T> directly.

using System;
using Divergic.Logging.Xunit;
using FluentAssertions;
using Microsoft.Extensions.Logging;
using Xunit;

public class MyClassTests
{
    [Fact]
    public void DoSomethingReturnsValueTest()
    {
        var logger = new CacheLogger();

        var sut = new MyClass(_logger);

        sut.DoSomething();
        
        logger.Count.Should().Be(1);
        logger.Entries.Should().HaveCount(1);
        logger.Last.Message.Should().Be("Hey, we did something");
    }
}

Configure LoggerFactory

You may have an integration or acceptance test that requires additional configuration to the log providers on ILoggerFactory while also supporting the logging out to xUnit test results. You can do this by create a factory that is already configured with xUnit support.

using System;
using Divergic.Logging.Xunit;
using FluentAssertions;
using Microsoft.Extensions.Logging;
using Xunit;
using Xunit.Abstractions;

public class MyClassTests
{
    private readonly ILogger _logger;

    public MyClassTests(ITestOutputHelper output)
    {
        var factory = LogFactory.Create(output);

        // call factory.AddConsole or other provider extension method

        _logger = factory.CreateLogger(nameof(MyClassTests));
    }

    [Fact]
    public void DoSomethingReturnsValueTest()
    {
        var sut = new MyClass(_logger);

        // The xUnit test output should now include the log message from MyClass.DoSomething()

        var actual = sut.DoSomething();

        actual.Should().NotBeNullOrWhiteSpace();
    }
}

Existing Loggers

Already have an existing logger and want the above cache support? Got you covered there too using the WithCache() method.

using System;
using Divergic.Logging.Xunit;
using FluentAssertions;
using Microsoft.Extensions.Logging;
using NSubstitute;
using Xunit;

public class MyClassTests
{
    [Fact]
    public void DoSomethingReturnsValueTest()
    {
        var logger = Substitute.For<ILogger>();

        logger.IsEnabled(Arg.Any<LogLevel>()).Returns(true);

        var cacheLogger = logger.WithCache();

        var sut = new MyClass(cacheLogger);

        sut.DoSomething();

        cacheLogger.Count.Should().Be(1);
        cacheLogger.Entries.Should().HaveCount(1);
        cacheLogger.Last.Message.Should().Be("Hey, we did something");
    }
}

The WithCache() also supports ILogger<T>.

using System;
using Divergic.Logging.Xunit;
using FluentAssertions;
using Microsoft.Extensions.Logging;
using NSubstitute;
using Xunit;

public class MyClassTests
{
    [Fact]
    public void DoSomethingReturnsValueTest()
    {
        var logger = Substitute.For<ILogger<MyClass>>();

        logger.IsEnabled(Arg.Any<LogLevel>()).Returns(true);

        var cacheLogger = logger.WithCache();

        var sut = new MyClass(cacheLogger);

        sut.DoSomething();

        cacheLogger.Count.Should().Be(1);
        cacheLogger.Entries.Should().HaveCount(1);
        cacheLogger.Last.Message.Should().Be("Hey, we did something");
    }
}