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.
Best Answer
I totally agree with GalacticJello, storing data outside of the regular sharepoint content database is at the moment (MOSS 2007) a complete and utter nightmare. MS provided a ExternalStorage provider baseclas for us to override, but there are major cons against using this:
This means that you would need to write your own code to keep the external storage and the list items in sync, and i'm not even mentioning workflows and versioning.
last but not least (and IMHO the worst), creating and using a custom ExternalStorage provider is not web app or site collection targetable, it's usage is farm wide (and there are NO workarounds). So any site collection you create in that farm will have it's doc libs use the external storage provider.