扩展方法使您能够向现有类型“添加”方法,而无需创建新的派生类型、重新编译或以其他方式修改原始类型。
MSDN
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type.
传统的模式下如果想为一个类型(class)添加一个额外的自定义的特殊的逻辑上,或者业务上的新方法时,你必须重新定义的一个类型来继承原有的的方法,用继承类或者接口,但是有些用sealed修饰的,这时候就无法被继承,例如String,值类型,sealed修饰的类。
扩展方法是C#3.0这个版本提出来的。解决了必须由继承才能扩展的某个类的弊端,最重要的一点就是很好用。
注意:
扩展方法必须在非嵌套、非泛型的静态类中定义。
Note that it is defined inside a non-nested<非嵌套>, non-generic<非泛型的> static<静态> class:
扩展方法的规则有以下几点:
- 扩展方法必须是扩展方法必须是非嵌套、非泛型的静态类中定义的;
- 扩展方法的第一个参数要用this关键字修饰;
- 第一个方法参数不能有ref 或则out关键字修饰的参数;
- 引用项目的命名空间;
- 参数调用两种;
- 和传统的调用方法一样使用<ExtendClass>.<ExtendClassMethod>(参数,参数+?)
- <参数类型>.<ExtendClassMethod>(参数+?)
// Define an interface named IMyInterface.
namespace DefineIMyInterface
{
using System; publicinterface IMyInterface
{
// Any class that implements IMyInterface must define a method
// that matches the following signature.
void MethodB();
}
} // Define extension methods for IMyInterface.
namespace Extensions
{
using System;
using DefineIMyInterface; // The following extension methods can be accessed by instances of any
// class that implements IMyInterface.
publicstaticclass Extension
{
publicstaticvoid MethodA(this IMyInterface myInterface, int i)
{
Console.WriteLine
("Extension.MethodA(this IMyInterface myInterface, int i)");
} publicstaticvoid MethodA(this IMyInterface myInterface, string s)
{
Console.WriteLine
("Extension.MethodA(this IMyInterface myInterface, string s)");
} // This method is never called in ExtensionMethodsDemo1, because each
// of the three classes A, B, and C implements a method named MethodB
// that has a matching signature.
publicstaticvoid MethodB(this IMyInterface myInterface)
{
Console.WriteLine
("Extension.MethodB(this IMyInterface myInterface)");
}
}
} // Define three classes that implement IMyInterface, and then use them to test
// the extension methods.
namespace ExtensionMethodsDemo1
{
using System;
using Extensions;
using DefineIMyInterface; class A : IMyInterface
{
publicvoid MethodB() { Console.WriteLine("A.MethodB()"); }
} class B : IMyInterface
{
publicvoid MethodB() { Console.WriteLine("B.MethodB()"); }
publicvoid MethodA(int i) { Console.WriteLine("B.MethodA(int i)"); }
} class C : IMyInterface
{
publicvoid MethodB() { Console.WriteLine("C.MethodB()"); }
publicvoid MethodA(object obj)
{
Console.WriteLine("C.MethodA(object obj)");
}
} class ExtMethodDemo
{
staticvoid Main(string[] args)
{
// Declare an instance of class A, class B, and class C.
A a = new A();
B b = new B();
C c = new C(); // For a, b, and c, call the following methods:// -- MethodA with an int argument// -- MethodA with a string argument// -- MethodB with no argument.// A contains no MethodA, so each call to MethodA resolves to // the extension method that has a matching signature.
a.MethodA(); // Extension.MethodA(object, int)
a.MethodA("hello"); // Extension.MethodA(object, string)// A has a method that matches the signature of the following call// to MethodB.
a.MethodB(); // A.MethodB()// B has methods that match the signatures of the following// method calls.
b.MethodA(); // B.MethodA(int)
b.MethodB(); // B.MethodB()// B has no matching method for the following call, but // class Extension does.
b.MethodA("hello"); // Extension.MethodA(object, string)// C contains an instance method that matches each of the following// method calls.
c.MethodA(); // C.MethodA(object)
c.MethodA("hello"); // C.MethodA(object)
c.MethodB(); // C.MethodB()
}
}
}
/* Output:
Extension.MethodA(this IMyInterface myInterface, int i)
Extension.MethodA(this IMyInterface myInterface, string s)
A.MethodB()
B.MethodA(int i)
B.MethodB()
Extension.MethodA(this IMyInterface myInterface, string s)
C.MethodA(object obj)
C.MethodA(object obj)
C.MethodB()
*/
总结的结果
方法的调用次序 类型的实例方法--->当前命名空间下的扩展方法--->导入的其他命名空间扩展方法。
引发疑问
在空引用上调用实例方法或静态方法时会抛出NullReferenceException异常?那么在类调用扩展方法时会出现异常吗?(不使用类中的一些属性或方法,强调调用)
扩展方法 | 静态方法 |
TestExtend sExtend=null;
sExtend.NullUse();
|
TestExtend sExtend=null;
NullUse.(sExtend);
|
可能引发的子类污染的问题例如
你本来只想扩展TestExtend 这个类的方法,由于你要使用iSNull这个类型的扩展结果传入参数(object),结果导致所有的object 类型都扩展了这个方法,这会带来可怕的后果。
所以在扩展一个类型的方法时,要从确定类型扩展。尽量避免从父类去扩展。
有什么不对的或者错误的地方希望大家给予指正,谢谢
我的开发环境VS2015
DEMO的下载 【http://pan.baidu.com/s/1bY51P8】