Why is "using System;" not considered bad practice?

Why is “using System;” not considered bad practice?

"using System;" is not universally not considered a bad practice. See for example: Why would you not use the 'using' directive in C#?

But it may be true that it is not considered quite as bad as using namespace std. Probably because:

1. C# does not have header files. It is uncommon to "include" one C# source file into another using a pre-processor.

2. std namespace is nearly flat i.e. almost all standard library functions, types and variables are in it (there are few exceptions such as the filesystem sub-namespace). It contains very, very high number of identifiers. To my understanding, System contains much fewer names, and instead has more sub-namespaces.

3. In C#, there are no global functions or variables. As such, the number of global identifiers is typically quite small in contrast to C++ which does have those: Furthermore, it is typical to use C libraries (often indirectly) which doesn't have namespaces, and therefore place all their names into the global namespace.

4. As far as I know, C# has no argument dependent lookup. ADL in conjunction with name hiding, overloading etc. can produce cases where some programs are not affected by a name conflict, while others are subtly affected, and catching all corner cases is not feasible with testing.

Because of these differences, “using System;” has lower chance of name conflict than using namespace std.

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Also, namespace "importing" is in a way, a self-perpetuating convention: If it is conventional to import a standard namespace, then programmers will conventionally try to avoid choosing names from that namespace for their own identifiers, which helps to reduce problems with such convention.

If such an import is considered a bad practice, then programmers will be less likely to even attempt such avoidance of conflicts with imported namespaces. As such, conventions tend to get polarised either for or against the practice, even if weights of arguments between the choices were originally subtle.