How to test asp.net core built-in Ilogger

Question

I want to verify some logs logged. I am using the asp.net core built-in ILogger, and inject it with the asp.net core built-in DI:

private readonly ILogger<InvoiceApi> _logger;

public InvoiceApi(ILogger<InvoiceApi> logger)
{
    _logger = logger;
}

then I use it like: _logger.LogError("error));

I tried to mock it (with moq) as usual by:

MockLogger = new Mock<ILogger<InvoiceApi>>();

and inject this in the service for test by:

new InvoiceApi(MockLogger.Object);

then tried verify:

MockLogger.Verify(m => m.LogError(It.Is<string>(s => s.Contains("CreateInvoiceFailed"))));

but it throw:

Invalid verify on a non-virtual (overridable in VB) member: m => m.LogError

So, how can I verify this logs logged?

Answer 1

As @Nkosi've already said, you can't mock an extension method. What you should mock, is the ILogger.Log method, which LogError calls into. It makes the verification code a bit clunky, but it should work:

MockLogger.Verify(
    m => m.Log(
        LogLevel.Error,
        It.IsAny<EventId>(),
        It.Is<FormattedLogValues>(v => v.ToString().Contains("CreateInvoiceFailed")),
        It.IsAny<Exception>(),
        It.IsAny<Func<object, Exception, string>>()
    )
);

(Not sure if this compiles, but you get the gist)