Today something about one of exceptions that can occure in Selenium PageFactory.

IllegalLocatorException was unhadled by user code.
An exception of type „OpenQA.Selenium.IllegalLocatorException” occured in WebDriver.dll but was not handled in user code.
Additional information: Compound class names are not supported. Consider searching for one class name and filtering the results.

In my case, this exception was thrown in line:

PageFactory.InitElements(webDriver, pageElement);

This is probably a bug in Selenium. Solution is easy: just do not use:

[FindsBy(How=How.ClassName, Using="name_of_class"]

Instead of ClassName, try to use Css Selectors:

[FindsBy(How=How.CssSelector, Using=".name_of_class"]

Remember about a dot!