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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.