Transaction failed due to incorrectly calculated hash parame

I tried below code but got error in redirection of payumoney page kindly check , my hash value is not generated properly.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class BuyNow : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
Label1.Text = Request.QueryString["Price"].ToString();
Random random = new Random();
txnid.Value = (Convert.ToString(random.Next(10000, 20000)));
txnid.Value = "AVJFINANCE" + txnid.Value.ToString();
Response.Write(txnid.Value.ToString());
}
}
protected void Button1_Click(object sender, EventArgs e)
{
Double amount = Convert.ToDouble(Label1.Text);
// String text = Key.Value.ToString() + "|" + txnid.Value.ToString() +"|" + amount + "|" + "Wear" + "|" + TextBox1.Text + "|" + TextBox2.Text + "|" + "1" + "|" + "1" + "|" + "1" + "|" + "1" + "|" + "1" + "||||||" + salt.Value.ToString();
String text = Key.Value.ToString() + "|" + txnid.Value.ToString() +"|" + amount + "|" + "Wear" + "|" + TextBox1.Text + "|" + TextBox2.Text + "|" + "||||||" + salt.Value.ToString();
//byte[] message = Encoding.UTF8.GetBytes(text);
//UnicodeEncoding UE = new UnicodeEncoding();
//byte[] hashValue;
//SHA512Managed hashString = new SHA512Managed();
//string hex = "";
//hashValue = hashString.ComputeHash(message);
//foreach (byte x in hashValue)
//{
// hex += String.Format("{0:x2}", x);
//}
byte[] message = Encoding.UTF8.GetBytes(text);
UnicodeEncoding UE = new UnicodeEncoding();
byte[] hashValue;
SHA512Managed hashString = new SHA512Managed();
string hex = "";
hashValue = hashString.ComputeHash(message);
foreach (byte x in hashValue)
{
hex += String.Format("{0:x2}", x);
}
hash.Value = hex;
System.Collections.Hashtable data = new System.Collections.Hashtable();
data.Add("hash", hex.ToString());
data.Add("txnid", txnid.Value);
data.Add("Key", Key.Value);
data.Add("amount", amount);
data.Add("Firstname", TextBox1.Text.Trim());
data.Add("email", TextBox2.Text.Trim());
data.Add("phone", TextBox3.Text.Trim());
data.Add("productinfo", "WEarnew");
data.Add("ufd1", "1");
data.Add("ufd2", "1");
data.Add("ufd3", "1");
data.Add("ufd4", "1");
data.Add("ufd5", "1");
data.Add("surl", "http://localhost:15073/web/Successpayment.aspx");
data.Add("furl", "http://localhost:15073/web/Failar.aspx");
data.Add("service_provider", "");
string strform = PreparePOSTForm("https://test.payu.in/_payment", data);
Page.Controls.Add(new LiteralControl(strform));
}
private string PreparePOSTForm(string url, System.Collections.Hashtable data) // post form
{
//Set a name for the form
string formID = "PostForm";
//Build the form using the specified data to be posted.
StringBuilder strForm = new StringBuilder();
strForm.Append(" + formID + "\" name=\"" +
formID + "\" action=\"" + url +
"\" method=\"POST\">");
foreach (System.Collections.DictionaryEntry key in data)
{
strForm.Append(" + key.Key +
"\" value=\"" + key.Value + "\">");
}
strForm.Append("");
//Build the JavaScript which will do the Posting operation.
StringBuilder strScript = new StringBuilder();
strScript.Append("");
strScript.Append("var v" + formID + " = document." +
formID + ";");
strScript.Append("v" + formID + ".submit();");
strScript.Append("");
//Return the form and the script concatenated.
//(The order is important, Form then JavaScript)
return strForm.ToString() + strScript.ToString();
}
//private string PreparePOSTform (string url, System.Collections.Hashtable data)
//{
// string formID = "Postform";
// StringBuilder strform = new StringBuilder();
// strform.Append(" ");
// foreach (System.Collections.DictionaryEntry key in data)
// {
// strform.Append("< input type= \"hidden\" name=\"" + key.Key + "\" value = \"" + Key.Value + "\" > ");
// }
// strform.Append("/form");
// StringBuilder strScript = new StringBuilder();
// strScript.Append("
// strScript.Append("var v " + formID + " = documnet." + formID + ";" );
// strScript.Append("v" + formID + ".submit();");
// strScript.Append("");
// return strform.ToString() + strScript.ToString();
// }
}