vs Dns.GetHostEntry Resolve 获得Aliases为空的问题

Question

IPHostEntry hostinfo = Dns.GetHostEntry("www.google.com");
            IPAddress[] ip = hostinfo.AddressList;
            String[] names = hostinfo.Aliases;
使用上面的代码获得的names数组长度为0，而把Dns.GetHostEntry改为 Dns.Resolve后得到的结果却是大于0的，在MSDN中说Resolve已经过时应用GetHostEntry来代替，那么这两个功能应该是相同的吧，为什么会出现上面的情况呢？

Answer 1

还真是，通过Reflector查看了一下public static IPHostEntry GetHostEntry(string hostNameOrAddress)
{
    IPAddress address;
    IPHostEntry hostByName;
    if (Logging.On)
    {
        Logging.Enter(Logging.Sockets, "DNS", "GetHostEntry", hostNameOrAddress);
    }
    s_DnsPermission.Demand();
    if (hostNameOrAddress == null)
    {
        throw new ArgumentNullException("hostNameOrAddress");
    }
    if (TryParseAsIP(hostNameOrAddress, out address))
    {
        if (address.Equals(IPAddress.Any) || address.Equals(IPAddress.IPv6Any))
        {
            throw new ArgumentException(SR.GetString("net_invalid_ip_addr"), "hostNameOrAddress");
        }
        hostByName = InternalGetHostByAddress(address, true, false);
    }
    else
    {
        hostByName = InternalGetHostByName(hostNameOrAddress, true);
    }
    if (Logging.On)
    {
        Logging.Exit(Logging.Sockets, "DNS", "GetHostEntry", hostByName);
    }
    return hostByName;
} 
[Obsolete("Resolve is obsoleted for this type, please use GetHostEntry instead. http://go.microsoft.com/fwlink/?linkid=14202")]
public static IPHostEntry Resolve(string hostName)
{
    IPAddress address;
    IPHostEntry hostByName;
    if (Logging.On)
    {
        Logging.Enter(Logging.Sockets, "DNS", "Resolve", hostName);
    }
    s_DnsPermission.Demand();
    if (hostName == null)
    {
        throw new ArgumentNullException("hostName");
    }
    if (TryParseAsIP(hostName, out address) && ((address.AddressFamily != AddressFamily.InterNetworkV6) || Socket.LegacySupportsIPv6))
    {
        hostByName = InternalGetHostByAddress(address, false, false);
    }
    else
    {
        hostByName = InternalGetHostByName(hostName, false);
    }
    if (Logging.On)
    {
        Logging.Exit(Logging.Sockets, "DNS", "Resolve", hostByName);
    }
    return hostByName;
} 

Answer 2

看上去问题就在于InternalGetHostByName(hostName, false);中第二个参数的false与true上，但是具体是怎么执行的呢？我没有找到InternalGetHostByName的说明，另外在获取别名的时候是用Dns.GetHostEntry还是用 Dns.Resolve呢？

Answer 3

的确是这样的。
关键是这个内部方法里面，没有渠道aliases:private static IPHostEntry NativeToHostEntry(IntPtr nativePointer)
{
    hostent hostent = (hostent) Marshal.PtrToStructure(nativePointer, typeof(hostent));
    IPHostEntry entry = new IPHostEntry();
    if (hostent.h_name != IntPtr.Zero)
    {
        entry.HostName = Marshal.PtrToStringAnsi(hostent.h_name);
    }
    ArrayList list = new ArrayList();
    IntPtr ptr = hostent.h_addr_list;
    nativePointer = Marshal.ReadIntPtr(ptr);
    while (nativePointer != IntPtr.Zero)
    {
        int newAddress = Marshal.ReadInt32(nativePointer);
        list.Add(new IPAddress(newAddress));
        ptr = IntPtrHelper.Add(ptr, IntPtr.Size);
        nativePointer = Marshal.ReadIntPtr(ptr);
    }
    entry.AddressList = new IPAddress[list.Count];
    list.CopyTo(entry.AddressList, 0);
    list.Clear();
    ptr = hostent.h_aliases;
    nativePointer = Marshal.ReadIntPtr(ptr);
    while (nativePointer != IntPtr.Zero)
    {
        string str = Marshal.PtrToStringAnsi(nativePointer);
        list.Add(str);
        ptr = IntPtrHelper.Add(ptr, IntPtr.Size);
        nativePointer = Marshal.ReadIntPtr(ptr);
    }
    entry.Aliases = new string[list.Count];
    list.CopyTo(entry.Aliases, 0);
    return entry;
} 

Answer 4

由于内部代码只能分析，不能跟踪，所以这个问题只好问微软啦，说不定是个bug呢。

Answer 5

非也非也,Dns.GetHostEntry也是可以得到"别名",不过需要点小技巧~