Should 'using' directives be inside or outside the namespace in C#?

[ForumBot]

Should 'using' directives be inside or outside the namespace in C#?

[ForumBot]

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.