In this article I will explain a simple tutorial with an example, how to use Entity Framework with Existing Database in ASP.Net Core 3 (.Net Core 3).
This article will explain how to configure Entity Framework and connect to SQL Server database and finally the fetched data is displayed in View in ASP.Net Core 3 (.Net Core 3) MVC.
Note: For beginners in ASP.Net Core (.Net Core 3), please refer my article ASP.Net Core 3: Hello World Tutorial with Sample Program example.
 
 

Installing Microsoft.EntityFrameworkCore.SqlServer package using Nuget

In order to install Microsoft.EntityFrameworkCore.SqlServer library using Nuget, please refer my article Install Microsoft.EntityFrameworkCore.SqlServer 3 Nuget Package.
 
 

Database

Here I am making use of Microsoft’s Northwind Database. You can download it from here.
Download and install Northwind Database
 
 

Model

1. Once the package is successfully installed, create a Folder named Models in your project and then a new class by right clicking the Models folder and then click on Add and then New Item option of the Context Menu.
.Net Core 3: Entity Framework with Existing Database
 
2. Create the following properties inside the Model class as shown below.
Note: In this article, only four Columns will be displayed and hence four properties are added to the class.
 
public class Customer
{
    public string CustomerID { get; set; }
    public string ContactName { get; set; }
    public string City { get; set; }
    public string Country { get; set; }
}
 
 

Database Context

1. Now you will need to add a new class to your project by right clicking the Solution Explorer and then click on Add and then New Item option of the Context Menu.
.Net Core 3: Entity Framework with Existing Database
 
2. Inside the class, first inherit the EntityFrameworkCore namespace and then inherit the DbContext class.
Then using Dependency Injection, a Constructor is created DbContextOptions are passed as parameter and also the Constructor of base class i.e. DbContext class is inherited.
Note: For more details on Dependency Injection, please refer my article .Net Core 3: Dependency Injection in ASP.Net Core.
 
Finally, a DbSet Collection property of Customer Model class is created, which will be later used for holding the Data fetched from SQL Server database.
using EF_Core_MVC.Models;
using Microsoft.EntityFrameworkCore;
 
namespace EF_Core_MVC
{
    public class DBCtx : DbContext
    {
        public DBCtx(DbContextOptions<DBCtx> options) : base(options)
        {
        }
 
        public DbSet<Customer> Customers { get; set; }
    }
}
 
 

Adding the Connection String inside AppSettings.json

The following Connection String setting has been added in the AppSettings.json file.
{
 "ConnectionStrings": {
    "MyConn": "Data Source=.\\SQL2017;Initial Catalog=Northwind;Integrated security=true"
 }
}
 
 

Configuring Database Context in Startup class

Inside the Startup class, the IConfiguration is injected in the Startup class and assigned to the private property Configuration.
Then the Connection String is read from the AppSettings.json file and is used to add the DbContext service.
Note: For more details on reading Connection String inside the Startup class, please refer my article .Net Core 3: Read Connection String inside Startup.cs from AppSettings.json file.
 
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.EntityFrameworkCore;
 
namespace EF_Core_MVC
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            this.Configuration = configuration;
        }
 
        public IConfiguration Configuration { get; }
 
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews();
            string conStr = this.Configuration.GetConnectionString("MyConn");
            services.AddDbContext<DBCtx>(options => options.UseSqlServer(conStr));
        }
 
        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            app.UseRouting();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");
            });
        }
    }
}
 
 

Controller

The Controller consists of following Action method.

Action method for handling GET operation

Inside the Index Action method, the Top 10 records are fetched from the Customers Table of the Northwind database using Entity Framework and returned to the View.
public class HomeController : Controller
{
    private DBCtx Context { get; }
    public HomeController(DBCtx _context)
    {
        this.Context = _context;
    }
 
    public IActionResult Index()
    {
        List<Customer> customers = (from customer in this.Context.Customers.Take(10)
                                    select customer).ToList();
        return View(customers);
    }
}
 
 

View

HTML Markup

Inside the View, in the very first line the Customer is declared as IEnumerable which specifies that it will be available as a Collection.
For displaying the records, an HTML Table is used. A FOR EACH loop will be executed over the Model which will generate the HTML Table rows with the Customer records.
@using EF_Core_MVC.Models;
@model IEnumerable<Customer>
 
@{
    Layout = null;
}
 
<!DOCTYPE html>
<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>Index</title>
</head>
<body>
    <h4>Customers</h4>
    <hr />
    <table cellpadding="0" cellspacing="0">
        <tr>
            <th>Customer Id</th>
            <th>ContactName</th>
            <th>City</th>
            <th>Country</th>
        </tr>
        @foreach (Customer customer in Model)
        {
            <tr>
                <td>@customer.CustomerID</td>
                <td>@customer.ContactName</td>
                <td>@customer.City</td>
                <td>@customer.Country</td>
            </tr>
        }
    </table>
</body>
</html>
 
 

Screenshot

.Net Core 3: Entity Framework with Existing Database
 
 

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