public void Mix_FindBestPrice_ReturnsMinPrice() { // arrange var barsProvider = Substitute.For <IBarsProvider>(); barsProvider.FindCocktailId("mojito").Returns(cocktailId); barsProvider.FindPrices(cocktailId).Returns(new[] { 100m, 200m }); var service = new CocktailService(barsProvider); // act decimal?result = service.FindBestPrice("mojito", null); // assert Assert.Equal(100m, result.Value); }