public void Given_A_DeckThumbnail_Should_Invoke_SaveDeckThumbnail_Once() { // Arrange const int expected = 1; var deckThumbnailModel = new DeckThumbnail { DeckId = 12312321 }; _deckFileSystem.SaveDeckThumbnail(Arg.Any <DeckThumbnail>()).Returns(23423); // Act _sut.SaveDeckThumbnail(deckThumbnailModel); // Assert _deckFileSystem.Received(expected).SaveDeckThumbnail(Arg.Any <DeckThumbnail>()); }