Hi rani,

Refer below example.

For reading connection string refer below article.

.Net Core: Read Connection String inside Startup.cs from AppSettings.json file

Database

I have made use of table Customers with the schema as follows. CustomerId is an Auto-Increment (Identity) column.

I have already inserted few records in the table.

You can download the database table SQL by clicking the download link below.         

Download SQL file

Namespaces

using System.Data;
using System.Data.SqlClient;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.AspNetCore.Mvc.Rendering;

Model

using System.ComponentModel.DataAnnotations;

namespace CRUD_Modal_Partial_Core_MVC.Models
{
    public class Customer
    {
        public int Id { set; get; }
        [Required(ErrorMessage = "Name Required!")]
        public string Name { set; get; }
        [Required(ErrorMessage ="Country Required!")]
        public string Country { set; get; }
    }
}

Controller

public class HomeController : Controller
{
    private IConfiguration Configuration;
    public HomeController(IConfiguration _configuration)
    {
        Configuration = _configuration;
    }
    private string GetConnectionString()
    {
        return this.Configuration.GetConnectionString("MyConn");
    }

    public IActionResult Index()
    {
        ViewBag.Name = "Add";
        ViewBag.Countries = PopulateCountries();
        Customer customer = new Customer();
        customer.Customers = GetCustomers(null);
        return View(customer);
    }
    public IActionResult Create(Customer customer)
    {
        ViewBag.Countries = PopulateCountries();
        AddUpdateDeleteCustomer(customer, "Insert");
        return RedirectToAction("Index");
    }
    public IActionResult Update(Customer customer)
    {
        ViewBag.Countries = PopulateCountries();
        ViewBag.Name = "Add";
        AddUpdateDeleteCustomer(customer, "Update");
        return RedirectToAction("Index");
    }
    public IActionResult Delete(int id)
    {
        ViewBag.Countries = PopulateCountries();
        ViewBag.Name = "Add";
        AddUpdateDeleteCustomer(new Customer { Id = id }, "Delete");
        return RedirectToAction("Index");
    }

    [HttpPost]
    public IActionResult AddCustomer()
    {
        ViewBag.Countries = PopulateCountries();
        ViewBag.Name = "Add";
        Customer customer = new Customer();
        return PartialView("_InsertUpdatePartial", customer);
    }
    [HttpPost]
    public IActionResult EditCustomer([FromBody]Customer cust)
    {
        ViewBag.Countries = PopulateCountries();
        int id = Convert.ToInt32(cust.Id);
        ViewBag.Name = "Update";
        Customer customer = new Customer();
        customer.Id = id;
        customer.Name = GetCustomers(id).FirstOrDefault().Name;
        customer.Country = GetCustomers(id).FirstOrDefault().Country;
        customer.Customers = GetCustomers(null);
        return PartialView("_InsertUpdatePartial", customer);
    }

    public List<Customer> GetCustomers(int? id)
    {
        List<Customer> customers = new List<Customer>();
        using (SqlConnection con = new SqlConnection(GetConnectionString()))
        {
            using (SqlCommand cmd = new SqlCommand())
            {
                cmd.Connection = con;
                cmd.CommandType = CommandType.Text;
                cmd.Parameters.Clear();
                if (id != null)
                {
                    cmd.CommandText = "SELECT * FROM Customers WHERE CustomerId = @Id";
                    cmd.Parameters.AddWithValue("@Id", id);
                }
                else
                {
                    cmd.CommandText = "SELECT * FROM Customers";
                }
                con.Open();
                SqlDataReader rdr = cmd.ExecuteReader();
                while (rdr.Read())
                {
                    customers.Add(new Customer()
                    {
                        Id = Convert.ToInt32(rdr["CustomerId"]),
                        Name = rdr["Name"].ToString(),
                        Country = rdr["Country"].ToString()
                    });
                }
                con.Close();
            }
        }

        return customers;
    }
    public void AddUpdateDeleteCustomer(Customer customer, string action)
    {
        if (PopulateCountries().Find(x => x.Value == customer.Country) != null)
        {
            customer.Country = PopulateCountries().Find(x => x.Value == customer.Country).Text;
        }
        using (SqlConnection con = new SqlConnection(GetConnectionString()))
        {
            using (SqlCommand cmd = new SqlCommand())
            {
                cmd.CommandType = CommandType.Text;
                cmd.Connection = con;
                cmd.Parameters.Clear();
                if (action == "Insert")
                {
                    cmd.CommandText = "INSERT INTO Customers VALUES (@Name,@Country)";
                    cmd.Parameters.AddWithValue("@Name", customer.Name);
                    cmd.Parameters.AddWithValue("@Country", customer.Country);
                }
                else if (action == "Update")
                {
                    cmd.CommandText = "UPDATE Customers SET NAME = @Name,Country = @Country WHERE CustomerId = @Id";
                    cmd.Parameters.AddWithValue("@Id", customer.Id);
                    cmd.Parameters.AddWithValue("@Name", customer.Name);
                    cmd.Parameters.AddWithValue("@Country", customer.Country);
                }
                else if (action == "Delete")
                {
                    cmd.CommandText = "DELETE FROM Customers WHERE CustomerId = @Id";
                    cmd.Parameters.AddWithValue("@Id", customer.Id);
                }
                con.Open();
                cmd.ExecuteNonQuery();
                con.Close();
            }
        }
    }

    private List<SelectListItem> PopulateCountries()
    {
        List<SelectListItem> countries = new List<SelectListItem>();
        countries.Add(new SelectListItem { Text = "Select", Value = "" });
        countries.Add(new SelectListItem { Text = "USA", Value = "1" });
        countries.Add(new SelectListItem { Text = "India", Value = "2" });
        countries.Add(new SelectListItem { Text = "Russia", Value = "3" });
        countries.Add(new SelectListItem { Text = "France", Value = "4" });

        return countries;
    }
}

View

Index

@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@model CRUD_Modal_DropDown_Core_MVC.Models.Customer

@{
    Layout = null;
}

<!DOCTYPE html>

<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>Index</title>
    <script src="~/lib/jquery/dist/jquery.js"></script>
    <script src="~/lib/bootstrap/dist/js/bootstrap.js"></script>
    <link href="~/lib/bootstrap/dist/css/bootstrap.css" rel="stylesheet" />
    <script type="text/javascript">
        $(function () {
            $("body").on('click', '#btnEdit', function () {
                $("#MyPopup").modal("hide");
                var obj = {};
                obj.Id = $(this).attr('data-id');
                $.ajax({
                    url: 'Home/EditCustomer',
                    data: JSON.stringify(obj),
                    type: 'POST',
                    dataType: 'html',
                    contentType: "application/json; charset=utf-8",
                    success: function (response) {
                        $("#dvPartial").html(response);
                        $("#MyPopup").modal("show");
                        $("#ddlCountries option:contains(" + $(response).find('#hfCountry').val() + ")").attr('selected', 'selected');
                    }
                });
            });

            $("body").on('click', '#btnAdd', function () {
                $.ajax({
                    url: 'Home/AddCustomer',
                    data: {},
                    type: 'POST',
                    dataType: 'html',
                    contentType: "application/json; charset=utf-8",
                    success: function (response) {
                        $("#dvPartial").html(response);
                        $("#MyPopup").modal("show");
                    }
                });
            });
        });
    </script>
</head>
<body>
    <div class="container">
        <div class="row">
            <div class="col-lg-12">
                <h4>Customers</h4>
                <table class="table table-responsive">
                    <thead>
                        <tr>
                            <th>@Html.DisplayNameFor(model => model.Name)</th>
                            <th>@Html.DisplayNameFor(model => model.Country)</th>
                            <th></th>
                            <th></th>
                        </tr>
                    </thead>
                    <tbody>
                        @foreach (var customer in Model.Customers)
                        {
                            <tr>
                                <td>@Html.DisplayFor(x => customer.Name)</td>
                                <td>@Html.DisplayFor(x => customer.Country)</td>
                                <td>
                                    <a href="#" id="btnEdit" class="btn btn-primary btn-sm" data-id="@customer.Id">Edit</a>
                                </td>
                                <td>@Html.ActionLink("Delete", "Delete", new { id = customer.Id }, new { onclick = "return confirm('Are you sure you wish to delete this Customer?');", @class = "btn btn-danger btn-sm" })</td>
                            </tr>
                        }
                    </tbody>
                </table>
            </div>
        </div>
        <p>
            <a id="btnAdd" class="btn btn-outline-success btn-sm">Add Customer</a>
        </p>
    </div>
    <!-- Modal Popup -->
    <div id="MyPopup" class="modal fade" role="dialog">
        <div class="modal-dialog">
            <div class="modal-content">
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal">
                        &times;
                    </button>
                    <h4 class="modal-title"></h4>
                </div>
                <div class="modal-body">
                    <div id="dvPartial"></div>
                </div>
                <div class="modal-footer">
                    <input type="button" id="btnClosePopup" value="Close" data-dismiss="modal" class="btn btn-danger" />
                </div>
            </div>
        </div>
    </div>
</body>
</html>

PartialView

@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@model CRUD_Modal_DropDown_Core_MVC.Models.Customer

<script src="~/lib/jquery/dist/jquery.js"></script>
<script src="~/lib/jquery-validation/dist/jquery.validate.js"></script>
<script src="~/lib/jquery-validation-unobtrusive/jquery.validate.unobtrusive.js"></script>
<script src="~/lib/bootstrap/dist/js/bootstrap.js"></script>
<link href="~/lib/bootstrap/dist/css/bootstrap.css" rel="stylesheet" />
<script type="text/javascript">
    $(function () {
        $("body").on('change', '#ddlCountries', function () {
            $('#hfCountry').val($(this).find('option:selected').text());
        });
        $("body").on('click', '#btnCreate', function () {
            if (!$('form').valid()) {
                $("#MyPopup").modal("show");
                return false;
            }
        });
    });
</script>

<div class="container">
    <h4>Add/Update Customer</h4>
    <hr />
    <div class="row">
        <div class="col-md-4">
            <form asp-action="Create" asp-controller="Home">
                @if (ViewBag.Name != "Add")
                {
                    <div class="form-group">
                        <label asp-for="Id" class="control-label"></label>
                        <input asp-for="Id" class="form-control" readonly="readonly" />
                    </div>
                }
                <div class="form-group">
                    <label asp-for="Name" class="control-label"></label>
                    <input asp-for="Name" class="form-control" />
                    <span asp-validation-for="Name" class="text-danger"></span>
                </div>
                <div class="form-group">
                    <input type="hidden" id="hfCountry" asp-for="Country" />
                    <label asp-for="Country" class="control-label"></label>
                    <select name="Country" asp-items="@ViewBag.Countries" id="ddlCountries" class="form-control"></select>
                    <span asp-validation-for="Country" class="text-danger"></span>
                </div>
                <div class="form-group">
                    @if (ViewBag.Name == "Add")
                    {
                        <input id="btnCreate" type="submit" value="Add" asp-action="Create" class="btn btn-success btn-sm" />
                    }
                    else
                    {
                        <input type="submit" value="Update" asp-action="Update" class="btn btn-success btn-sm" />
                        <input type="submit" value="Cancel" asp-action="Index" class="btn btn-defaul btn-sm" />
                    }
                </div>
            </form>
        </div>
    </div>
</div>

Screenshot