Introduction

NSubstitute.Community.Logging is a collection of helper methods that make it easy to inject substitute loggers into an IServiceProvider and verify that specific logging activity occurred.

Getting Started

First install NSubstitute.Community.Logging into your test project from Nuget.

Substitute Logger

Just call .AddSubstituteLoggers() on your ServiceCollection before buiding it into an IServiceProvider to add ILoggerFactory, ILogger, and ILogger generated using NSubstitute.

var serviceProvider = new ServiceCollection().AddSubstituteLoggers().BuildServiceProvider();

var mockLoggerFactory = serviceProvider.GetService<ILoggerFactory>();
var mockLogger = serviceProvider.GetService<ILogger>();
var mockLoggerT = serviceProvider.GetService<ILogger<MyClass>>();

Log Verification: Basic

NSubstitute.Community.Logging lets you easily assert that specific logging took place. All you need is any NSubstitute substitute ILogger. Then you can verify logs using syntax similar to how the application wrote them. See LoggerExtensionsTests for more examples.

Make sure specific LogInformation call occured

var Target = Substitute.For<ILogger>();
var exception = new KeyNotFoundException();
var message = "This is a {log} message";
var args = new object[] { "log" };
Microsoft.Extensions.Logging.LoggerExtensions.LogInformation(Target, exception, message, args);

Target.Received(1)
    .LogInformation(exception, message, args);

Target.DidNotReceive()
    .LogInformation(exception, message);
Target.DidNotReceive()
    .LogInformation(message, args);
Target.DidNotReceive()
    .LogInformation(message);

Log Verification: Advanced

Sometimes you don't want to or can't match on every nuance of a log message. For these cases there is CallToLog which behaves like Arg.Is for any provided argument and behaves like Arg.Any for omitted arguments. This allows you to verify log events by:

• log level
• OriginalFormat
• event Id
• exception
• any combination of the above in the same log!

NOTE: CallToLog methods work with any NSubstitute Recieved Calls

Make sure no warnings or errors were logged while the test was running

var Target = Substitute.For<ILogger>();
Microsoft.Extensions.Logging.LoggerExtensions.LogInformation(Target, "I am an informational log.");

Target.DidNotReceive()
    .CallToLog(LogLevel.Warning);

Target.DidNotReceive()
    .CallToLog(LogLevel.Error);

Make sure a similar message was logged

var Target = Substitute.For<ILogger>();
Microsoft.Extensions.Logging.LoggerExtensions.LogInformation(Target, "I am an informational log.");

Target.Received(1)
    .CallToLog(LogLevel.Information, "I am an informational log.");

Log Verification: Custom Logic

Somes you want to verify the value of arguments provided to a Log but cannot match all the arguments or cannot match the exact value that was used. See CallToLogPredicateTests for more examples.

Match the message and some arguments exactly

var Target = Substitute.For<ILogger>();
Microsoft.Extensions.Logging.LoggerExtensions.LogWarning(Target, "{ErrorCode} There were {ErrorCount} errors that happened on {Where}.", Guid.NewGuid(), 13, "earth");

Target.Received(1)
    .CallToLog(LogLevel.Warning,
    _ => _.OriginalFormat.Equals("{ErrorCode} There were {ErrorCount} errors that happened on {Where}.")
        && _.KeyEquals("ErrorCount", 13)
        && _.KeyEquals("Where", "earth")
    );

Match the message and arguments more generally

var Target = Substitute.For<ILogger>();
var now = DateTime.Now;
Microsoft.Extensions.Logging.LoggerExtensions.LogWarning(Target, "there were {oopies} things you might want to know about. {where} {when}", 13, "earth", now);
var planets = new[] { "mars", "earth" };

Target.Received(1)
    .CallToLog(LogLevel.Warning,
    _ => _.OriginalFormat.StartsWith("there were {oopies}")
        && _.TryGetValue("oopies", out int oopies) && oopies > 10
        && _.TryGetValue("where", out string where) && planets.Contains(where)
        && _.TryGetValue("when", out DateTime when) && when <= DateTime.Now
    );

More Examples

See the tests for more examples!!!