在.NET中using是一个多用途的关键字,除了可以引用命名空间之外,还可以用来进行异常处理。今天我们再来说说用using关键字实现的另一个功能,既定义别名,通俗的讲就是起外号!:)
using可以为命名空间或是类型定义别名,如果定义在命名空间之外,别名生效的范围就仅限于当前文件;如果定义在命名空间之内,则生效范围为当前代码块与定义别名的命名空间之交集。下面我们就来看看如何用using关键字来为命名空间定义别名:
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可以给命名空间起别名,这在一定程度上方便了我们开发,但是我们有时候会遇到一个新的问题。请看下面的例子:
//Assebly1中有如下代码:
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//然后我们有一个类同时引用了上面的两个程序集
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注意上面的代码是不能通过编译的,因为编译器不知道FooIO.Stream是指assembly1下的Foo.IO.Stream(该类是通过别名引用的)还是指assembly2下的FooIO.Stream(该类是直接引用的)。这时,我们就要请出“命名空间别名限定符::”来帮忙了,这们用“::”来将上面的代码重写如下,就可以编译通过了。
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注意,这时stream1是Assembly1中Foo.IO.Stream的一个实例。
另外还有两个比较特殊的别名符,就是全局限定符和外部限定符。分别示例如下:
全局限定符:
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外部别名:
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