为什么“使用命名空间”指令是C＃中接受的编码实践？

Question

I am just curious to know why "using namespace" directive is acceptable in C#, though in C++ it is not . I am aware that C++ and C# are different, but my guess is C++ and C# come almost from same family, and should be using the same ideas for namespace resolution. C++ and C# both have an alias keyword to get around the namespace clash.

Can anybody point me, what I am not reading between lines in C# that makes it acceptable to use "using namespace" directive, and avoid the problems that C++ cannot.

Answer 1

In C++, if you write using namespace in a header, then it will be in effect for anyone who includes that header. This makes it pretty much unusable in headers. At that point, you might as well avoid it (at global scope) in .cpp files as well, if only for the sake of consistency, and to make moving implementations between .h and .cpp easier.

(note that locally scoped using namespace - ie within a function - are generally considered fine; it's just that they don't help with verbosity much)

In C#, there is nothing like #include , and the scope of a using directive will never span beyond a single .cs file. So it's pretty much safe to use everywhere.

The other reason is the design of the standard library. In C++, you just have std (well, now also a few more underneath it, but they are rarely used). In C#, you have gems such as System.Collections.Generic , which is extremely verbose for something that's used very commonly. It's just much more painful to avoid using in C# than it is in C++.

To sum it up, while C# and C++ do share some common design, on the matter of code modularity (I'd assign headers, modules, namespaces all to that group), their design is very different.

Answer 2

For me, it comes down to the support tools. Intellisense, the quick class lookup (F1 key), and refactoring options of Visual Studio give the needed reference lookup functionality.

Also, C# has every method within a class--there are no namespace level functions.

Answer 3

In general a difference of C# and C++ is how compilation units are handled and specified.

C++ uses header files to publish class declarations, and needs a compilation unit where this class is implemented (defined). A using namespace <xxx> statement in header files is strongly discouraged practice for C++, because this can easily lead to namespace clashes and ambiguities when included from some client code. In your class declaration you should explicitly state what you want from other namespaces (including std ).

C# has single compilation units which eases use of using namespace <xxx> statements a bit. Nevertheless I'd prefer aliasing imported namespaces, if you want to avoid tedious typing. Placing using statements in a .cs file might cause ambiguous definitions as well.

Answer 4

While I completely agree with others that using namespaces should not be done in headers, I think banning them in cpp files is shortsighted. If you try to adhere to organizing declarations into namespaces 'nicely', but then ban the usage of 'using namespace' altogether, the path of least resistance for coders becomes the under use of namespaces.

For instance, in the above post by Pavel Minaev, he rightfully points out the namespace difference between a common C++ namespace, 'std', and a C# namespace, 'System.Collections.Generic'. If you stop to think of why they are this way, a clear answer IMO is that C++ culture frowns on using namespace, while C# does not, so in C# you accept more verbose namespaces because they are not inherently painful to use. I believe namespace organization in C# is much better than C++ largely because of this cultural difference, and better class organization and general readability are not trivial things.

To put a different way, think about what would happen to people's file organization habits if an application required them to type fully qualified paths to load a file. They'd more likely just shove everything into a root folder to avoid the typing, not a good way to promote quality organization.

While certainly not as clean as C#'s using directive, using namespace in cpp files is an overall win.

Answer 5

Unless you're a language purist, it saves time and makes coding easier. Unless you're dealing with complicated systems of namespaces, it's perfectly acceptable.