namespaces in custom libraries best practices

Just wondering what the pros and cons between the two following two examples:

using MyLib;

namespace System.IO
{
    public class IoService : IIoService
    {
    ...

and

using System.IO;
using MyLib;

namespace MyLib.IO
{
    开发者_运维知识库public class IoService : IIoService
    {
    ...

Don't add stuff to library namespaces that aren't yours.