public void ThrowWhenUserIsNull()
{
var mockedUnitOfWork = new Mock <IUnitOfWork>();
var mockedRepository = new Mock <IRepository <Discussion> >();
var mockedDiscussion = new Mock <Discussion>();
var discussionsService = new DiscussionsService(mockedRepository.Object, mockedUnitOfWork.Object);
Assert.That(() => discussionsService.RemoveUserFromDiscussion(mockedDiscussion.Object, null),
Throws.ArgumentNullException.With.Message.Contains("user"));
}