.NET Core Database The Connection String property has not been initialized

I’m trying to build a .NET 6 Core MVC application that shows a list of user records from the database.
But I’m getting the "The Connection String property has not been initialized" error when trying to open the connection.
Right now I’m trying to give the data in query string but getting this error.

**DATA ACCESS LAYER**

using ABL_USER_DebitCard_Info.Models;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;

namespace ABL_USER_DebitCard_Info.Context
{
    public class DebitCard_DAL
    {

        string connectionString = "Data Source = (localdb)\\MSSQLLocalDB; Initial Catalog = ABL_DebitCard_User_Info_DB";

        
        public IEnumerable<Users> GetUserByCNIC(string? CNIC)
        {
            var debitcardList = new List<Users>();

            using (SqlConnection conn = new SqlConnection())
            {
                SqlCommand cmd = new SqlCommand("ABL_GetUserByCNIC", conn);
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@CNIC", CNIC);
                

conn.Open(); —> GETTING ERROR HERE

                SqlDataReader reader = cmd.ExecuteReader();
                while (reader.Read())
                {
                    var user = new Users();
                    user.Id = Convert.ToInt32(reader["ID"].ToString);
                    user.CNIC = reader["CNIC"].ToString();
                    user.UserName = reader["UserName"].ToString();
                    user.CardNumber = reader["CardNumber"].ToString();
                    user.CardStatus = reader["CardStatus"].ToString();

                    debitcardList.Add(user);
                }
                conn.Close();
            }
            return debitcardList;
        }

    }
}


**Controller**

using ABL_USER_DebitCard_Info.Context;
using ABL_USER_DebitCard_Info.Models;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;

namespace ABL_USER_DebitCard_Info.Controllers
{
    public class UsersController : Controller
    {

        DebitCard_DAL dbcontext = new DebitCard_DAL();

        [HttpGet]
        public ActionResult Details(string CNIC)
        {
            if(CNIC == null)
            {
                return NotFound();
            }
            else
            {
                List<Users> debitcardList = dbcontext.GetUserByCNIC(CNIC).ToList();
                return View(debitcardList);
                if(debitcardList.Count == 0)
                {
                    return NotFound();
                }
                else
                {
                    return View(debitcardList);
                }
            }
            
        }



    }
}

>Solution :

You create the SqlConnection without providing any connection string.
Pass the connection string to the SqlConnection constructor
using (SqlConnection conn = new SqlConnection(connectionString ))