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

Question

I have been running StyleCop over some C# code, and it keeps reporting that my `using` directives should be inside the namespace. Is there a technical reason for putting the `using` directives inside instead of outside the namespace?

Accepted Answer

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.

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

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