Effective C# 声明式编程优于命令式编程
和命令式编程相比,声明式编程可能是一种更简单、更精炼的描述软件程序行为的方式。声明式(declarative)编程意味着使用声明、而非指令的方式来定义程序的行为。和许多其他程序语言一样,C# 中绝大多数编程都是命令式(imperative)编程:通过编写方法来定义程序的行为。通过使用特性(attribute),我们也可以在C#中实现声明式编程。我们可以将特性应用在类、属性、数据成员或者方法上,.NET运行时则会为我们添加适当的行为。声明式编程更易于实现、阅读和维护。
让我们从一个大家已经使用过的典型示例开始。当编写第1个ASP.NET Web服务时,向导程序会产生如下的代码:
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VS.NET Web服务向导程序会为HelloWorld()方法添加一个[WebMethod]特性。这会将HelloWorld()方法声明为一个Web方法。由于对该特性的使用,ASP.NET运行时会为我们创建一些相关的代码。首先,它会为我们创建Web服务描述语言(Web Service Descrīption Language,简称WSDL)文档,其中包含一个对“调用HelloWorld()方法的SOAP文档”的描述。其次,ASP.NET运行时还会添加对HelloWorld()方法SOAP请求的路由支持,并且会动态创建HTML页面来支持我们在IE中测试新的Web服务。这些都有赖于对 WebMethod特性的应用。该特性声明了我们的意图,ASP.NET运行时则确保对这种意图给予适当的支持。使用这样的特性可以节省许多开发时间,也可以避免许多错误。
这并没有什么神奇的。ASP.NET运行时在后台使用反射来确定类中的哪些方法为Web方法。在找到Web方法之后,ASP.NET运行时会添加所有必要的框架代码,从而将我们编写的函数转换为Web方法。
[WebMethod] 特性只是.NET类库中定义的许多特性中的一个,这些特性可以帮助我们更快捷地创建正确的应用程序。例如,有的特性可以帮助我们创建支持序列化的类型。有的特性可以控制条件编译。通过使用特性所支持的声明式编程,我们可以更快地创建代码,并降低犯错的几率。我们应该使用. NET框架中的特性来声明我们的意图,而不是自己编写代码。这样的做法花费的时间较少,也更容易,且编译器不会犯错。
如果预定义特性不能满足我们的需要,我们可以通过定义自己的特性并使用反射来创建声明式的编程构造。作为示例,我们可以创建一个特性及相关的代码,从而允许用户创建定义有默认排序顺序的类型。下面的代码示例演示了如何通过添加特性来定义对Customer集合排序的规则。
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DefaultSort 特性为Customer类定义了默认的排序属性:Name。其隐含意思是任何Customer的集合都要使用Customer的Name进行排序。 DefaultSort特性并不是.NET框架的一部分。若要实现它,我们需要自己创建DefaultSortAttribute类:
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随后编写的代码必须根据DefaultSoft特性来对Customer集合进行排序。首先要使用反射找到正确的属性,然后比较两个不同对象上该属性的值。好在,我们只需要编写一次这样的代码就可以了。
接下来,我们需要创建一个实现了IComparer的类。IComparer有一个CompareTo()方法,用于比较给定类型的两个对象,从而允许目标类(即实现了IComparable接口的类型)定义排序顺序。GenericComparer类的构造器会根据被比较的类型,查找默认的排序属性描述符。Compare()方法则使用默认的排序属性来比较任意类型的两个对象。
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GenericComparer 的实现代码使用了一些高级技巧,比如反射。但是我们只需要编写一次就可以了。自此之后,我们需要做的就是在类上添加 DefaultSort特性,然后便可以使用GenericComparer对这些对象的集合进行排序了。如果我们更改了DefaultSort特性上的参数,也就更改了类的行为。我们不需要在代码中更改任何算法。
当一个简单的声明便可以表明我们的意图时,采用这种声明式的做法能够有效地避免重复性代码的编写。再来看一下GenericComparer类。我们可以为创建的每一个类型编写不同版本(也更为简单)的排序算法。使用声明式编程的好处在于,我们可以编写一个通用的类,然后使用一个简单的声明为每个类型创建不同的行为。这里的关键在于行为的改变是基于一个声明,而不是基于任何算法的改变。GenericComparer类适用于任何应用了DefaultSort特性的类型。如果在应用程序中只需要一两次排序功能,那么编写一些简单的函数就可以了。但是,如果我们的程序中有许多不同的类型都需要相同的行为,那么从长远来看,通用的算法加声明式的解决方案将会为我们节省大量的时间和精力。例如,我们永远也不用编写由WebMethod特性产生的所有代码。我们应该利用此技术来为我们的算法服务。也讨论了一个例子:如何使用特性来构建附加的命令处理器。其他的一些例子包括从定义附加包(add-on package)到构建动态的网页UI。
综上所述,声明式编程是一个强大的工具。当可以使用特性来声明我们的意图时,实际上也就避免了在多个类似的手工编写(hand-coded)的算法中,犯逻辑错误的可能。声明式编程会创建更为可读和清晰的代码。这意味着更少的错误。如果可以使用.NET框架中定义的特性,那么我们就应该积极地使用。如果不能,则可以考虑选择创建我们自己的特性类,然后在将来使用它创建相同的行为。
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