So, as the message says, you have to give only path to the geckodriver.exe, you do not have to mention the geckodriver.exe file name.
Your code should look like,
FirefoxDriverService service = FirefoxDriverService.CreateDefaultService(@"C:\SeleniumDrivers");
service.FirefoxBinaryPath= @"C:\Program Files\Mozilla Firefox\firefox.exe";
driver = new FirefoxDriver();
baseURL = "http://oasis-test:9998/#/login";
verificationErrors = new StringBuilder();
Hope that helps!