public void AsCssData_Test() { var prop1CssData = new CssData(); prop1CssData.AddAttribute(".test", "color", "red"); prop1CssData.AddAttribute(".test", "", "red"); var prop1 = Substitute.For <ICssProperty>(); prop1 .When(x => x.InsertCss(Arg.Any <CssPropertiesSet>(), Arg.Any <CssData>())) .Do(x => x.ArgAt <CssData>(1).AddRange(prop1CssData)); _instance.Add(prop1); _instance.Selector = ".test"; var r = new CssData(); _instance.InsertCss(r); Assert.AreEqual(prop1CssData, r); prop1.Received(1).Selector = ".test"; }