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Test Extension Method Received Call with NSubstitute
How can I verify that an extension method was called when using NSubstitute for unit testing? For a normal method I'd do something like:
substitutedClass.Recieved().CheckedMethod(...)
But extension methods are not actually substituted so I can't use them with Received() in this way. For example if you use an ILogger with Microsoft.Extensions.Logging and try to check a call to LogWarning() like so:
logger.Received().LogWarning(Arg.Is<string>(s => s.Contains("special text")), Arg.Any<object?[]>());
you will get the following error since LogWarning() is an extension method:
NSubstitute.Exceptions.RedundantArgumentMatcherException : Some argument specifications (e.g. Arg.Is, Arg.Any) were left over after the last call.
This is often caused by using an argument spec with a call to a member NSubstitute does not handle (such as a non-virtual member or a call to an instance which is not a substitute), or for a purpose other than specifying a call (such as using an arg spec as a return value).
How can you test a call was received when an extension method was used?
1 answer
Extension methods can't be tested for received calls as they are not directly on the substituted class, plus they are static.
However, extension methods are ultimately calling into a real method on the class that is substituted. You can still test that method is called.
This takes extra digging to find the actual method called. For instance Microsoft.Extensions.Logging extension methods eventually call into Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception, Func<TState, Exception?, string> formatter)
We can test for calls to this with:
logger.Received().Log(
LogLevel.Warning,
Arg.Any<EventId>(),
Arg.Is<object>(x => (x.ToString() ?? string.Empty).Contains("special text")),
Arg.Any<Exception?>(),
Arg.Any<Func<object, Exception?, string>>());
The logging extensions shows an example of an extra challenge. They pass a parameter that is interal, FormattedLogValues. We can't specify this as the type passed with Arg.Is<FormattedLogValues>() as we don't have access to it. But as you can see in the example above we can check it against object, and, having discovered FormattedLogValues is just a wrapper for the logged string, get its string value and test against that.
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