c#+asp.net实现paypal接口支付

 <form id="form1" name="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
          相关的单据信息如商品名称，总金额，订单号等。
<input name="Paypal" type="button" style="height: 25px; margin-right: 20px; cursor: pointer;
                            margin-left: 40px; width: 80px;" value="Pay Now" onclick="javaScript:if(submitorder()==true){this.form.submit();}" />
<div style="display: none">
<!-- 支付方式:-->
<input type="text" name="cmd" value="_xclick" /><br />
<!--这里填写你的paypal账户email如你的qq邮箱注册的paypay帐号  邮箱： -->
<input type="hidden" name="business" value="youremail@qq.com" /><br />
<!--订单信息： 这里填写客户订单的一些相关信息，当客户连到paypal网站付款的时候将看到这些信息 【写备注】-->
<input type="hidden" name="item_name" value=" Confirm Order information" /><br />
<!--订单编号-->
<input type="hidden" name="item_number" value="<%=_ordernum %>" /><br />
<!--订单总金额 -->
<input type="hidden" name="amount" value="<%=_amount %>" /><br />
<!-- 订单数量： <%=_Number %> -->
<input type="hidden" name="quantity" value="1" /><br />
<!-- 订单币种: 订单总金额对应的货币类型 ,客户可以用其他币种来付款,比如这里订单币种是美元USD,客户可以用欧元EUR来付款,由paypal根据当前汇率自动实现币种之间的换算-->
<input type="hidden" name="currency_code" value="USD" /><br />
<!--编码格式 -->
<input type="hidden" name="charset" value="utf-8" /><br />
<%-- <input type="hidden" name="noshipping" value="1"><br />--%>
<!--运费 -->
<input type="hidden" name="shipping" value="<%=ReceivedPostFee.ToString("f2") %>" /><br />
<!--验证加密 password_validate  可额外设置加密方式-->
<input type="hidden" name="custom" value="<%=itemname %> " /><br />
<!-- 返回的路径：付款后提示给客户如支付成功。单号是多少。金额是多少，大概什么时候收到货等-->
<input type="hidden" name="return" value="backurl_info" /><br />
<!-- 取消后返回的页面：如用户不想支付可以跳到指定的页面-->
<input type="hidden" name="cancel_return" value="cancelbackurl_info" />
<!--这里显示商户的logo图片在提交的时候显示，如果没有则显示邮箱的账户-->
<input type="hidden" name="image_url" value="url/pay.png" />
<!--这里告诉paypal付款的通信url,即当客户付款后调用这个url通知系统-->
<input type="text" name="notify_url" style="width: 500px" value="responsepost_url" /><br />
</div>

</form>

 private DataContext db = new DataContext();
    protected void Page_Load(object sender, EventArgs e)
    {

        // CUSTOMIZE THIS: This is the seller‘s Payment Data Transfer authorization token.
        // Replace this with the PDT token in "Website Payment Preferences" under your account.

        string authToken = "商户号【authtoken】一系列长串的字符，注册paypal商户号上会有";
        string txToken = Request.QueryString["tx"];
        string query = "cmd=_notify-synch&tx=" + txToken + "&at=" + authToken;

        //Post back to either sandbox or live
        string strSandbox = "https://www.sandbox.paypal.com/cgi-bin/webscr";
        //string strLive = "https://www.paypal.com/cgi-bin/webscr";
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(strSandbox);

        //Set values for the request back
        req.Method = "POST";
        req.ContentType = "application/x-www-form-urlencoded";
        req.ContentLength = query.Length;


        //Send the request to PayPal and get the response
        StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
        streamOut.Write(query);
        streamOut.Close();
        StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
        string strResponse = streamIn.ReadToEnd();
        streamIn.Close();

        Dictionary<string, string> results = new Dictionary<string, string>();
        if (strResponse != "")
        {
            StringReader reader = new StringReader(strResponse);
            string line = reader.ReadLine();

            if (line == "SUCCESS")
            {
               while ((line = reader.ReadLine()) != null)
                {
                    results.Add(line.Split(‘=‘)[0], line.Split(‘=‘)[1]);
                }
              }
            else if (line == "FAIL")
            {
                  //退款后会返回信息。
                  Response.Write("Unable to retrive transaction detail");
            }
        }
        else
        {
           Response.Write("ERROR");
        }

    }

private DataContext db=new DataContext();
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            string st = "Completed";
            string item_numberget = "";
            string tx = "";
            if (StringHandler.isCheckQueryString(Request["tx"].ToString().Trim()))
            {
                tx = Request["tx"].ToString().Trim();  //税编号
            }
            string cm = "";
            if (StringHandler.isCheckQueryString(Request["cm"].ToString().Trim()))
            {
                cm = Request["cm"].ToString().Trim();  ///加密
            }


            if (StringHandler.isCheckQueryString(Request["st"].ToString().Trim()))
            {
                st = Request["st"].ToString().Trim();  ///状态
            }

            string amt = "";
            if (StringHandler.isCheckQueryString(Request["amt"].ToString().Trim()))
            {
                amt = Request["amt"].ToString().Trim();  ///购买总额
            }
            string cc = "";
            if (StringHandler.isCheckQueryString(Request["cc"].ToString().Trim()))
            {
                cc = Request["cc"].ToString().Trim();  ///币种USD(美元)
            }

            if (StringHandler.isCheckQueryString(Request["item_number"].ToString().Trim()))
            {
                item_numberget = Request["item_number"].ToString().Trim();  ///get的返回值订单号
            }
            if (st == "Completed" && item_numberget != "")
            {
                //修改对应的单据状态如已支付。并提示支付成功给用户知道
                this.lblstatus.InnerText = "Paid already.";
            }
            else
            {
                 //支付失败
                this.lblstatus.InnerText = "Error";
            }
        }
    }

<script type="text/javascript">
        var num = 5;
        function countDown() {
            if (num >= 0) {
                var str = ‘‘;
                str += ‘Will return to the home page in ‘ + num + ‘ seconds.‘;
                document.getElementById(‘daojishi‘).innerHTML = str;
                num--;
                setTimeout(countDown, 1000);
            }
            else {
                window.location.href = "home.aspx";
            }
        }
        window.onload = countDown;
    </script>