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
The
using System.Data.OracleClientdirective means that that namespace should be considered when trying to determine what non-full names mean.Adding a reference, means you add a reference to a given assembly, in this case System.Data.OracleClient.dll. From VisualStudio, SharpDevelop or MonoDevelop you will see a references folder in the project explorer view. Right click and "Add Reference" (VisualStudio and SharpDevelop) or "Edit References" (MonoDevelop), and add in System.Data.OracleClient.dll
If using nant you'll need to edit your nant script.
Assemblies and namespaces overlap, but aren't quite the same. The reference means you can use e.g.
System.Data.OracleClient.OracleDataReaderbecause the project now knows what assembly the code for that lives in. Theusingdirective means you need only typeOracleDataReader. There tends to be a heavy match between assemblies and namespaces because that makes life easier for everyone, but there are both times when an assembly has classes from more than one namespace, and when a namespace is split over more than one assembly. A classic example is that mscorlib has a lot of classes fromSystem,System.Collections,System.IOetc. that you couldn't really hope to build a .NET project without (including some that .NET uses itself), while System.dll has a bunch more from exactly the same namespaces that you could feasibly get by without using (but you still will 99% of the time).Unless you're writing an absolutely massive library though, with thousands of classes covering overlapping use-cases, your own assemblies should work with a single namespace - or at most a single one with some other namespaces within that one, like
JaredksGreatCodehavingJaredksGreatCode.UserInterfacewithin it - per single DLL.