As logger.LogError() or logger.LogInformation() are extended method, we cannot directly mock these methods.

According to this stackoverflow, we can mock it by using underline method with little trick.

Unit Test logger.LogError

This is how I write unit test for logger.LogError to be called only once.

The Log method takes FormattedLogValues class instance in third argument, which is internal class, thus I couldn't instantiate it. Therefore I use It.IsAny<It.IsAnyType>().

Mock<MyService> mockedMyService = new();
mockedILogger.Verify(x => x.Log(
  LogLevel.Error,
  It.IsAny<EventId>(),
  It.IsAny<It.IsAnyType>(),
  It.IsAny<Exception>(),
  It.IsAny<Func<It.IsAnyType, Exception?, string>>()),
  Times.Once);

Other extension methods such as LogWarning, LogInformation shall work in the same manner.