求翻译：　　显然，命名空间相当长，键入起来很繁琐，用这种方式指定某个特定的类也是不必要的。如本章开头所述，C#允许简写类的全名。为此，要在文件的顶部列出类的命名空间，前面加上using关键字。在文件的其他地方，就可以使用其类型名称来引用命名空间中的类型了:是什么意思？

Obviously, the namespace is quite long and cumbersome to type, it is unnecessary to specify a particular class this way. As described in the beginning of this chapter, c # to allow the full name of the abbreviated class. To this end, are listed at the top of the file namespace of the class, preceded

It is clear that namespace is quite long, it is a cumbersome typing in this manner, and specify a particular class is not necessary. As described in the beginning of this chapter, C #allows the short class name. For this reason, you want to at the top of the file lists the namespace class, preceded