There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:
// File1.cs
using System;
namespace Outer.Inner
{
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now imagine that someone adds another file (File2.cs) to the project that looks like this:
// File2.cs
namespace Outer
{
class Math
{
}
}
The compiler searches Outer
before looking at those using
directives outside the namespace, so it finds Outer.Math
instead of System.Math
. Unfortunately (or perhaps fortunately?), Outer.Math
has no PI
member, so File1 is now broken.
This changes if you put the using
inside your namespace declaration, as follows:
// File1b.cs
namespace Outer.Inner
{
using System;
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now the compiler searches System
before searching Outer
, finds System.Math
, and all is well.
Some would argue that Math
might be a bad name for a user-defined class, since there's already one in System
; the point here is just that there is a difference, and it affects the maintainability of your code.
It's also interesting to note what happens if Foo
is in namespace Outer
, rather than Outer.Inner
. In that case, adding Outer.Math
in File2 breaks File1 regardless of where the using
goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using
directive.
This is not related to performance at all. But consider this: you are using two libraries called Foo and Bar:
using namespace foo;
using namespace bar;
Everything works fine, and you can call Blah()
from Foo and Quux()
from Bar without problems. But one day you upgrade to a new version of Foo 2.0, which now offers a function called Quux()
. Now you've got a conflict: Both Foo 2.0 and Bar import Quux()
into your global namespace. This is going to take some effort to fix, especially if the function parameters happen to match.
If you had used foo::Blah()
and bar::Quux()
, then the introduction of foo::Quux()
would have been a non-event.
Best Answer
If you are using C# then you need to specify the namespace for the "anyType" element in your XPath: