public class MyWebClient : WebClient
{
    private IPAddress ipAddress;

    public MyWebClient(IPAddress ipAddress)
    {
        this.ipAddress = ipAddress;
    }

    protected override WebRequest GetWebRequest(Uri address)
    {
        var request = (HttpWebRequest)base.GetWebRequest(address);

        // 把 KeepAlive 设置为 false 表示连接用完就关
        request.KeepAlive = false;
        request.ServicePoint.ConnectionLeaseTimeout = 0;
        request.ServicePoint.BindIPEndPointDelegate += (servicePoint, remoteEndPoint, retryCount) =>
        {
            return new IPEndPoint(ipAddress, 0);
        };

        return request;
    }
}