WebClient 指定出口 IP

当一个服务器有多个 IP 时， 在使用 webclient 对象需要指定出口 IP

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;
    }
}

KeepAlive 和 ConnectionLeaseTimeout 这两个属性非常重要， 不然 ServicePoint 会有缓存， 导致出口 IP 错乱。