Serilog.Sinks.InMemory

In-memory sink for Serilog to use for testing with FluentAssertions support for easy to write assertions.

Build status

Usage

To just use the sink, add the Serilog.Sinks.InMemory NuGet package:

dotnet CLI:

dotnet add package Serilog.Sinks.InMemory

PowerShell:

Install-Package Serilog.Sinks.InMemory

But it's better with assertions so you'll also want to add the Serilog.Sinks.InMemory.Assertions NuGet package:

dotnet CLI:

dotnet add package Serilog.Sinks.InMemory.Assertions

PowerShell:

Install-Package Serilog.Sinks.InMemory.Assertions

Example

Let's say you have a class with method implementing some complicated business logic:

public class ComplicatedBusinessLogic
{
    private readonly ILogger _logger;

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

    public string FirstTenCharacters(string input)
    {
        return input.Substring(0, 10);
    }
}

A request came in to log a message with the number of characters in the input. So to test that you can create a mock of ILogger and assert the method to log was called, however mock setups quickly become very messy (true: this is my opinion!) and assertions on mocks have the same problem when you start verifying values of arguments.

So instead let's use Serilog and a dedicated sink for testing:

public class WhenExecutingBusinessLogic
{
    public void GivenInputOfFiveCharacters_MessageIsLogged()
    {
        var logger = new LoggerConfiguration()
            .WriteTo.InMemory()
            .CreateLogger();

        var logic = new ComplicatedBusinessLogic(logger);

        logic.FirstTenCharacters("12345");

        // Use the static Instance property to access the in-memory sink
        InMemorySink.Instance
            .Should()
            .HaveMessage("Input is {count} characters long");
    }
}

The test will now fail with Expected a message to be logged with template \"Input is {count} characters long\" but didn't find any

Now change the implementation to:

public string FirstTenCharacters(string input)
{
    _logger.Information("Input is {count} characters long", input.Length);

    return input.Substring(0, 10);
}

Run the test again and it now passes. But how do we ensure this message is only logged once?

To do that, create a new test like so:

public void GivenInputOfFiveCharacters_MessageIsLoggedOnce()
{
    /* omitted for brevity */

    InMemorySink.Instance
        .Should()
        .HaveMessage("Input is {count} characters long")
        .Appearing().Once();
}

To verify if a message is logged multiple times use Appearing().Times(int numberOfTimes)

So now you'll want to verify that the property count has the expected value. This builds upon the previous test:

public void GivenInputOfFiveCharacters_CountPropertyValueIsFive()
{
    /* omitted for brevity */

    InMemorySink.Instance
        .Should()
        .HaveMessage("Input is {count} characters long")
        .Appearing().Once()
        .WithProperty("count")
        .WithValue(5);
}

Asserting a message appears more than once

Let's say you have a log message in a loop and you want to verify that:

public void GivenLoopWithFiveItems_MessageIsLoggedFiveTimes()
{
    /* omitted for brevity */

    InMemorySink.Instance
        .Should()
        .HaveMessage("Input is {count} characters long")
        .Appearing().Times(5);
}

Asserting a message has a certain level

Apart from a message being logged, you'll also want to verify it is of the right level. You can do that using the WithLevel() assertion:

public void GivenLoopWithFiveItems_MessageIsLoggedFiveTimes()
{
    /* omitted for brevity */

    InMemorySink.Instance
        .Should()
        .HaveMessage("Input is {count} characters long")
        .Appearing().Once()
        .WithLevel(LogEventLevel.Information);
}

This also works for multiple messages:

public void GivenLoopWithFiveItems_MessageIsLoggedFiveTimes()
{
    logger.Warning("Test message");
    logger.Warning("Test message");
    logger.Warning("Test message");

    InMemorySink.Instance
        .Should()
        .HaveMessage("Test message")
        .Appearing().Times(3)
        .WithLevel(LogEventLevel.Information);
}

This will fail with a message: Expected instances of log message "Hello, world!" to have level Information, but found 3 with level Warning

Asserting messages with a pattern

Instead of matching on the exact message you can also match on a certain pattern using the Containing() assertion:

InMemorySink.Instance
   .Should()
   .HaveMessage()
   .Containing("some pattern")
   .Appearing().Once();

which matches on log messages:

• this is some pattern
• some pattern in a message
• this is some pattern in a message

Asserting messages have been logged at all (or not!)

When you want to assert that a message has been logged but don't care about what message you can do that with HaveMessage and Appearing:

InMemorySink.Instance
    .Should()
    .HaveMessage()
    .Appearing().Times(3); // Expect three messages to be logged

and of course the inverse is also possible when expecting no messages to be logged:

InMemorySink.Instance
    .Should()
    .NotHaveMessage();

or that a specific message is not be logged

InMemorySink.Instance
    .Should()
    .NotHaveMessage("a specific message");

Asserting properties on messages

When you want to assert that a message has a property you can do that using the WithProperty assertion:

InMemorySink.Instance
    .Should()
    .HaveMessage("Message with {Property}")
    .Appearing().Once()
    .WithProperty("Property");

To then assert that it has a certain value you would use WithValue:

InMemorySink.Instance
    .Should()
    .HaveMessage("Message with {Property}")
    .Appearing().Once()
    .WithProperty("Property")
    .WithValue("property value");

Asserting that a message has multiple properties can be accomplished using the And constraint:

InMemorySink.Instance
    .Should()
    .HaveMessage("Message with {Property1} and {Property2}")
    .Appearing().Once()
    .WithProperty("Property1")
    .WithValue("value 1")
    .And
    .WithProperty("Property2")
    .WithValue("value 2");

When you have a log message that appears a number of times and you want to assert that the value of the log property has the expected values you can do that using the WithValues assertion:

InMemorySink.Instance
    .Should()
    .HaveMessage("Message with {Property1} and {Property2}")
    .Appearing().Times(3)
    .WithProperty("Property1")
    .WithValue("value 1", "value 2", "value 3")

Note: WithValue takes an array of values.

Sometimes you might want to use assertions like BeLessThanOrEqual() or HaveLength() and in those cases WithValue is not very helpful. Instead you can use WhichValue<T>() to access the value of the log property:

InMemorySink.Instance
    .Should()
    .HaveMessage()
    .Appearing().Once()
    .WithProperty("PropertyOne")
    .WhichValue<string>()
    .Should()
    .HaveLength(3);

If the type of the value of the log property does not match the generic type parameter the WhichValue<T> method will throw an exception.

Note: This only works for scalar values. When you pass an object as the property value when logging a message Serilog converts that into a string.

Asserting a property with a destructured object

If you use object destructuring:

var someObject = new { Foo = "bar", Baz = "quux" };
logger.Information("Hello {@SomeObject}", someObject);

and want to assert on properties of the destructured object you can use the HavingADestructuredObject() assertion like so:

InMemorySink.Instance
    .Should()
    .HaveMessage("Hello {@SomeObject}")
    .Appearing().Once()
    .WithProperty("SomeObject")
    .HavingADestructuredObject()
    .WithProperty("Foo")
    .WithValue("bar");

When the property SomeObject doesn't hold a destructured object the assertion will fail with the message: "Expected message "Hello {NotDestructured}" to have a property "NotDestructured" that holds a destructured object but found a scalar value"

Clearing log events between tests

Depending on your test framework and test setup you may want to ensure that the log events captured by the InMemorySink are cleared so tests are not interfering with eachother. To enable this, the InMemorySink implements the IDisposable interface. When Dispose() is called the LogEvents collection is cleared.

It will depend on the test framework or your test if you need this feature. With xUnit this feature is not necessary as it isolates each test in its own instance of the test class which means that they all have their own instance of the InMemorySink. MSTest however has a different approach and there you may want to use this feature as follows:

[TestClass]
public class WhenDemonstratingDisposableFeature
{
    private Logger _logger;

    [TestInitialize]
    public void Initialize()
    {
        _logger?.Dispose();

        _logger = new LoggerConfiguration()
            .WriteTo.InMemory()
            .CreateLogger();
    }

    [TestMethod]
    public void GivenAFoo_BarIsBlah()
    {
        _logger.Information("Foo");

        InMemorySink.Instance
            .Should()
            .HaveMessage("Foo");
    }

    [TestMethod]
    public void GivenABar_BazIsQuux()
    {
        _logger.Information("Bar");

        InMemorySink.Instance
            .Should()
            .HaveMessage("Bar");
    }
}

this approach ensures that the GivenABar_BazIsQuux does not see any messages logged in a previous test.

Creating a logger

Loggers are created using a LoggerConfiguration object. A default initiation would be as follows:

var logger = new LoggerConfiguration()
    .WriteTo.InMemory()
    .CreateLogger();

Output templates

Text-based sinks use output templates to control formatting. this can be modified through the outputTemplate parameter:

var logger = new LoggerConfiguration()
    .WriteTo.InMemory(outputTemplate: "{Timestamp:HH:mm:ss} {Level:u3}] {Message:lj}{NewLine}{Exception}")
    .CreateLogger();

The default template, shown in the example above, uses built-in properties like Timestamp and Level. Refer to the offcial documentation for further configuration and explanation of these properties.

Minimum level

In this example only Information level logs and higher will be written to the InMemorySink.

var logger = new LoggerConfiguration()
    .WriteTo.InMemory(restrictedToMinimumLevel: Events.LogEventLevel.Information)
    .CreateLogger();

Default Level - if no MinimumLevel is specified, then Verbose level events and higher will be processed.

Dynamic levels

If an app needs dynamic level switching, the first step is to create an instance of LoggingLevelSwitch when the logger is being configured:

var levelSwitch = new LoggingLevelSwitch();

This object defaults the current minimum level to Information, so to make logging more restricted, set its minimum level up-front:

levelSwitch.MinimumLevel = LogEventLevel.Warning;

When configuring the logger, provide the switch using MinimumLevel.ControlledBy():

var log = new LoggerConfiguration()
    .MinimumLevel.ControlledBy(levelSwitch)
    .WriteTo.InMemory()
    .CreateLogger();

Now, events written to the logger will be filtered according to the switch’s MinimumLevel property.

To turn the level up or down at runtime, perhaps in response to a command sent over the network, change the property:

levelSwitch.MinimumLevel = LogEventLevel.Verbose;
log.Verbose("This will now be logged");