pinvoke.net: Edit: iphlpapi.getbestinterface

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Summary: The GetBestInterface function retrieves  index of the interface that has the best route to the specified IP address.
!!!!C# Signature:
 [DllImport("iphlpapi.dll", SetLastError=true)]
 static extern int GetBestInterface(UInt32 DestAddr, out UInt32 BestIfIndex);
!!!!VB Signature:
 Declare Function GetBestInterface Lib "iphlpapi.dll" (TODO) As TODO
!!!!User-Defined Types:
None.

!!!!Notes:
DestAddr is a 32 bit representation of the four IP bytes.  For .NET 1.0, you can get this from IPAddress.Address.  In .NET 1.1, this is obsolete.  You can use BitConverter.ToUInt32(IPAddress.GetAddressBytes()) instead.

!!!!Tips & Tricks:
Use GetAdaptersInfo to determine which adapter the index corresponds to.

!!!!Sample Code:

public static class Win32ApiCalls
    {
        [DllImport("iphlpapi.dll", CharSet = CharSet.Auto)]
        public static extern int GetBestInterface(UInt32 destAddr, out UInt32 bestIfIndex);
    }

class Program
    {
        static void Main(string[] args)
        {
            try
            {
                IPHostEntry hostInfo = Dns.GetHostEntry("www.yahoo.com");

Console.WriteLine("Host name : " + hostInfo.HostName);
                Console.WriteLine("\r\nAliases : ");

foreach (var thisAlias in hostInfo.Aliases)
                {
                    Console.WriteLine(thisAlias);
                }
                Console.WriteLine("\r\nIP Address list :");
                foreach (var thisAddress in hostInfo.AddressList)
                {
                    Console.WriteLine(thisAddress);
                }
                IPAddress ipv4Address = (from thisAddress in hostInfo.AddressList
                                         where thisAddress.AddressFamily == AddressFamily.InterNetwork
                                         select thisAddress).FirstOrDefault();
                if (ipv4Address == null)
                {
                    Console.WriteLine("No Ipv4 address found for www.yahoo.com.");
                    return;
                }

Console.WriteLine("\r\nGetting best interface to reach {0}...", ipv4Address);

UInt32 ipv4AddressAsUInt32 = BitConverter.ToUInt32(ipv4Address.GetAddressBytes(), 0);
                UInt32 interfaceindex;
                int result = Win32ApiCalls.GetBestInterface(ipv4AddressAsUInt32, out interfaceindex);
                if (result != 0)
                {
                    throw new Win32Exception(result);
                }
                NetworkInterface interfaceInfo = GetNetworkInterfaceByIndex(interfaceindex);
                Console.WriteLine("Best interface:\r\n" +
                                  " Index {0}\r\n" +
                                  " Description: {1}\r\n" +
                                  " Id: {2}", interfaceindex, interfaceInfo.Description, interfaceInfo.Id);
            }
            catch (SocketException e)
            {
                Console.WriteLine("SocketException caught!!!");
                Console.WriteLine("Source : " + e.Source);
                Console.WriteLine("Message : " + e.Message);
            }
            catch (ArgumentNullException e)
            {
                Console.WriteLine("ArgumentNullException caught!!!");
                Console.WriteLine("Source : " + e.Source);
                Console.WriteLine("Message : " + e.Message);
            }
            catch (NullReferenceException e)
            {
                Console.WriteLine("NullReferenceException caught!!!");
                Console.WriteLine("Source : " + e.Source);
                Console.WriteLine("Message : " + e.Message);
            }
            catch (Win32Exception e)
            {
                Console.WriteLine("Win32Exception caught!!!");
                Console.WriteLine("Source : " + e.Source);
                Console.WriteLine("Message : " + e.Message);
            }
            catch (Exception e)
            {
                Console.WriteLine("Exception caught!!!");
                Console.WriteLine("Source : " + e.Source);
                Console.WriteLine("Message : " + e.Message);
            }
        }

private static NetworkInterface GetNetworkInterfaceByIndex(uint index)
        {
            // Search in all network interfaces that support IPv4.
            NetworkInterface ipv4Interface = (from thisInterface in NetworkInterface.GetAllNetworkInterfaces()
                                              where thisInterface.Supports(NetworkInterfaceComponent.IPv4)
                                              let ipv4Properties = thisInterface.GetIPProperties().GetIPv4Properties()
                                              where ipv4Properties != null && ipv4Properties.Index == index
                                              select thisInterface).SingleOrDefault();
            if (ipv4Interface != null)
                return ipv4Interface;

// Search in all network interfaces that support IPv6.
            NetworkInterface ipv6Interface = (from thisInterface in NetworkInterface.GetAllNetworkInterfaces()
                                              where thisInterface.Supports(NetworkInterfaceComponent.IPv6)
                                              let ipv6Properties = thisInterface.GetIPProperties().GetIPv6Properties()
                                              where ipv6Properties != null && ipv6Properties.Index == index
                                              select thisInterface).SingleOrDefault();

return ipv6Interface;
        }
    }

!!!!Alternative Managed API:
Do you know one?  Please contribute it!

Documentation: GetBestInterface@msdn on MSDN