using references inside a namespace vs root level

Question

What's the advantage of doing this:

namespace a
{
    using System;

    class ClassA
    {

    }
}

over this

using System;

namespace a
{
     class ClassA
     {
     }
}

is there any performance advantage or best practices?

Answer 1

There's definitely no performance difference at runtime since namespace resolution is done at compile time, and I doubt there's a significant compile time difference either.

All the C# code I've seen has the using statements at the top of the file, so I'd consider that more "normal".