利用Winsock下载文件（支持断点续传）[1]

第一步，建立工程，引用winsock（visual basic最好打sp6,否则ms有一个bug）,在此省略

第二步，具体实现代码步骤1：发送请求
说明：
(1)这里简单采用了判断是否已经有同名文件表示是否要断点续传
(2)下载的地址，大小和已下载字节数也只是简单地存在ini文件中，更安全的做法本文不作讨论
有兴趣的朋友可以联系我

'--------------------------------------------------------------------------------
'   name:downloadfile
'   author:reker 2004/3/20
'   desc:连接远端主机，发送接收文件请求，等待远端主机响应
'   params:none
'   history:none
'--------------------------------------------------------------------------------
private sub downloadfile()
    on error resume next
    starttime = time()
    with winsck
        .remotehost = host '远端主机地址
        .remoteport = 80
        .connect
        '等待服务器连接相应
        do while .state <> sckconnected
            doevents: doevents: doevents: doevents
            '20秒超时
            if datediff("s", starttime, time()) > 20 then
                showinfo "连接超时"
                .close
                exit sub
            end if
        loop
        '发送下载文件请求
        '此处使用http/1.0协议
        strcommand = "get " + updateurl + " http/1.0" + vbcrlf '下载地址
        strcommand = strcommand + "accept: */*" + vbcrlf      '这句可以不要
        strcommand = strcommand + "accept: text/html" + vbcrlf '这句可以不要
        strcommand = strcommand + vbcrlf
        strcommand = strcommand & "host: " & host & vbcrlf
        if dir(savefilename) <> "" then '是否已经存在下载文件
            dim confirm
            confirm = msgbox("已经存在文件，是否断点续传?", vbyesno + vbquestion, "提示")
            if confirm = vbyes then
                downposition = ""
                if not ofilectrl.readkeyfromini("update", "downsize", apppath + "update.ini", downposition) then
                '读取上次下载的字节数
                    msgbox "读取大小错误", vbinformation, "提示"
                end if
                '发送断点续传请求
                strcommand = strcommand & "range: bytes=" & clng(downposition) & "-" & vbcrlf
            else
                kill savefilename '删除原文件
            end if
        end if
        strcommand = strcommand & "connection: keep-alive" & vbcrlf
        strcommand = strcommand & vbcrlf
        .senddata strcommand
    end with
    if err then
        lblprocessresult.caption = lblprocessresult.caption & vbcrlf & vbcrlf & "下载文件出错：" & err.description
        lblprocessresult.refresh
    end if
end sub


第二步，具体实现代码步骤2：接收数据
'--------------------------------------------------------------------------------
'   name:winsck_dataarrival
'   author:reker 2004/3/20
'   desc:略
'   params:略
'   return:none