主题：api接口请求，报签名错误，怎么回事？

Dim jo As New JObject

jo("appkey") = "APPKEy"

jo("sessionkey") = "会话密钥"

jo("method") = "gy.erp.trade.get"

jo("page_no")="1"

jo("page_size")="10"

jo("start_date") = "2017-08-16 00:00:00"

jo("end_date") = "2017-08-16 24:00:00"

jo("date_type") =2

'jo("order_state") = 2

'jo("shop_code") = 2

Dim postjsonstring As String = jo.ToString

Dim secret As String = "安全密钥"

Dim sign As String = secret &  postjsonstring  &  secret 

sign = MD5Encrypt(sign)

jo("sign") = sign.ToUpper

postjsonstring= jo.ToString

    '定义接口url

    Dim url As String = String.Format("http://v2.api.guanyierp.com/rest/erp_open/")

    'Json序列化到内存流

    Using ms As New System.IO.MemoryStream()

    Dim bytes = ConvertHelper.EncodingToBytes(postjsonstring , System.Text.Encoding.UTF8)

    ms.Write(bytes, 0, bytes.Length)

    ms.Seek(0, System.IO.SeekOrigin.Begin)

    Dim jsonString = Functions.Execute("HttpPost",url, ms) 

    Dim mresult = ConvertHelper.FromJson(Of MmsgResult)(jsonString)

e.Form.Controls("TextBox1").Text = jsonString