I have got it resolved. Yes CCAvenue provides good support. But the person who uses asp.net forum will always look for asp.net codes and direct answers. :) 
I hope this will help someone. I have created two properties in code behind. One is to return checksum value and another one is to return details about the checkout items.
enter code here
public string CCAvenueItemList
 {
     get
     {
         StringBuilder CCAvenueItems = new StringBuilder();
         DataTable dt = new DataTable();
         DataTable dtClientInfo = new DataTable();
         dt = (DataTable)Session["CheckedItems"];
         dtClientInfo = (DataTable)Session["ClientInfo"];
         for (int i = 0; i <= dt.Rows.Count - 1; i++)
         {
             string amountTemplate = "<input type=\"hidden\" name=\"Amount\" value=\"$Amount$\" />\n";
             string orderTemplate = "<input type=\"hidden\" name=\"Order_Id\" value=\"$Order_Id$\" />\n";
            // BILLING INFO
             string billingNameTemplate = "<input type=\"hidden\" name=\"billing_cust_name\" value=\"$billing_cust_name$\" />\n";
             string billingCustAddressTemplate = "<input type=\"hidden\" name=\"billing_cust_address\" value=\"$billing_cust_address$\" />\n";
             string billingCountryTemplate = "<input type=\"hidden\" name=\"billing_cust_country\" value=\"$billing_cust_country$\" />\n";
             string billingEmailTemplate = "<input type=\"hidden\" name=\"billing_cust_email\" value=\"$billing_cust_email$\" />\n";
             string billingTelTemplate = "<input type=\"hidden\" name=\"billing_cust_tel\" value=\"$billing_cust_tel$\" />\n";
             string billingStateTemplate = "<input type=\"hidden\" name=\"billing_cust_state\" value=\"$billing_cust_state$\" />\n";
             string billingCityTemplate = "<input type=\"hidden\" name=\"billing_cust_city\" value=\"$billing_cust_city$\" />\n";
             string billingZipTemplate = "<input type=\"hidden\" name=\"billing_zip_code\" value=\"$billing_zip_code$\" />\n";
            billingCustAddressTemplate = billingCustAddressTemplate.Replace("$billing_cust_address$", dtClientInfo.Rows[0]["Address"].ToString());
             billingCountryTemplate = billingCountryTemplate.Replace("$billing_cust_country$", dtClientInfo.Rows[0]["Country"].ToString());
             billingEmailTemplate = billingEmailTemplate.Replace("$billing_cust_email$", dtClientInfo.Rows[0]["Email_ID"].ToString());
             billingTelTemplate = billingTelTemplate.Replace("$billing_cust_tel$", dtClientInfo.Rows[0]["Phone_no"].ToString());
             billingStateTemplate = billingStateTemplate.Replace("$billing_cust_state$", dtClientInfo.Rows[0]["State"].ToString());
             billingCityTemplate = billingCityTemplate.Replace("$billing_cust_city$", dtClientInfo.Rows[0]["City"].ToString());
             billingZipTemplate = billingZipTemplate.Replace("$billing_zip_code$", dtClientInfo.Rows[0]["ZipCode"].ToString());
            strAmount = dt.Rows[i]["INR"].ToString();
             amountTemplate = amountTemplate.Replace("$Amount$", dt.Rows[i]["INR"].ToString());
             orderTemplate = orderTemplate.Replace("$Order_Id$", dt.Rows[i]["ClientID"].ToString());
             billingNameTemplate = billingNameTemplate.Replace("$billing_cust_name$", dtClientInfo.Rows[0]["Name"].ToString());
            CCAvenueItems.Append(amountTemplate)
                 .Append(orderTemplate)
                 .Append(billingNameTemplate)
                 .Append(billingCustAddressTemplate)
                 .Append(billingCountryTemplate)
                 .Append(billingEmailTemplate)
                 .Append(billingTelTemplate)
                 .Append(billingStateTemplate)
                 .Append(billingCityTemplate)
                 .Append(billingZipTemplate)
                 .Append(deliveryNameTemplate)
                 .Append(deliveryCustAddressTemplate)
                 .Append(deliveryCountryTemplate)
           }
         return CCAvenueItems.ToString();
     }
 }
public string propcheckSum
 {
     get {
         libfuncs objLib = new libfuncs();
         string strCheckSum = objLib.getchecksum("YourMerchantID", Session["ClientID"].ToString(), strAmount, "UrReturnUrl", "your working key");
         return strCheckSum;
     }
 }
<div>
     <%=CCAvenueItemList%>
     <input type="hidden" name="Merchant_Id" value="yourmerchantID" />
     <input type="hidden" name="Checksum" value="<%=propcheckSum%>" />
     <input type="hidden" name="Redirect_Url" value="YourWebsite'sThankyoupage.aspx" />
     <input type="submit" value="Submit" runat="server" />
 </div>