exception string : global instance type is not valid 异常字符串:全局实例类型无效。
a delegate has no knowledge of the instance type aside from the method it wraps, so a delegate can refer to any type of object as long as there is a method on that object that matches the delegate signature 除了它所包装的方法外,委托不了解实例类型,所以只要任意类型的对象中具有与委托签名相匹配的方法,委托就可以引用该对象。