Interop type 'ChinaPay_Net.NetPayClientClass' cannot be embedded.

NetPayClientClass npc = new NetPayClientClass(); 

using System;
using System.Web;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using ChinaPay_Net;

namespace Epay
{
   public class Chinapay
    {
       string  strUrl = HttpContext.Current.Request.PhysicalApplicationPath;
      
       public string getSign(string MerId, string OrdId, string TransAmt, string CuryId, string TransDate, string TransType)
       {
           NetPayClientClass npc = new NetPayClientClass(); 
           //npc.setMerKeyFile("Bin/MerPrK.key");         
           npc.setMerKeyFile(strUrl + "\\key\\MerPrK.key");
           string strChkValue = "";                        
           strChkValue = npc.sign(MerId, OrdId, TransAmt, CuryId, TransDate, TransType);
           return strChkValue.Trim();
       }

       public string signData(string MerId, string SignMsg)
       {
           NetPayClientClass npc = new NetPayClientClass(); 
           //npc.setMerKeyFile("Bin/MerPrK.key");         
           npc.setMerKeyFile(strUrl + "\\key\\MerPrK.key");
           string strChkValueData = "";
           strChkValueData = npc.signData(MerId, SignMsg);
           return strChkValueData.Trim();
       }

       public bool getCheck(string MerId, string OrdId, string TransAmt, string CuryId, string TransDate, string TransType, string OrderStatus, string CheckValue)
       {
           NetPayClientClass npc = new NetPayClientClass(); 
           //npc.setPubKeyFile("Bin/PgPubk.key");          //chinapay d:\\PgPubk.key
           npc.setPubKeyFile(strUrl + "\\key\\PgPubk.key");
           string strFlag = "";
           bool bolFlag = false;
           strFlag = npc.check(MerId, OrdId, TransAmt, CuryId, TransDate, TransType, OrderStatus, CheckValue); 
           if (strFlag == "0") //"0"meaning succeed
           {
               bolFlag = true;
           }
           return bolFlag;
       }

       public bool checkData(string PlainData, string CheckValue)
       {
           NetPayClientClass npc = new NetPayClientClass(); //NetPay
           //npc.setPubKeyFile("Bin/PgPubk.key");          //chinapay d:\\PgPubk.key
           npc.setPubKeyFile(strUrl + "\\key\\PgPubk.key");
           string strFlagData = "";
           bool bolFlagData = false;
           strFlagData = npc.checkData(PlainData, CheckValue);
           if (strFlagData == "true")
           {
               bolFlagData = true;
           }
           return bolFlagData;
       }
    }
}

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