为啥C#模块还有用不了的属性？不理解

编译失败, Compilation, (47,11): error CS1061: 'TextBox' does not contain a definition for 'PlaceholderText' and no accessible extension method 'PlaceholderText' accepting a first argument of type 'TextBox' could be found (are you missing a using directive or an assembly reference?)

用的V2