Entity Framework Core Using data layer in application

Author : MD TAREQ HASSAN

ASP .Net Core App

Data layer => EF Core data project (i.e. AdventureWorks.Data, AdventureWorks is the sample database provided by Microsoft)
.AddDbContextPool<TDbContext>(<lambda>) => lambda will be called with DbContextOptions<TDbContext>
DbContextOptions<TDbContext> is used to set connection string
See more (advanced): using-dbcontext-with-dependency-injection in ASP.Net core app

appsettings.json

{

  "ConnectionStrings": {
    "DefaultDatabase": "Data Source=(localdb)\\MSSQLLocalDB;Initial Catalog=FooDB;Integrated Security=True"
  }
}

public class Startup
{
	public IConfiguration Configuration { get; }

	public Startup(IConfiguration configuration)
	{
		Configuration = configuration;
	}


	public void ConfigureServices(IServiceCollection services)
	{
		var dbConnectionString = Configuration.GetConnectionString("DefaultDatabase");
		services.AddDbContextPool<MasterMaintenanceContext>(options => options.UseSqlServer(dbConnectionString));

		// ... ... ...
	}
}

Points to be noted

We need to modify the DbContext class so that connection string can be configured in Startup.ConfigureServices(...) of ASP.Net core project
DbContext class can not have “parameterless constructor” (delete is there is any)

EmployeeDbContext.cs

public class EmployeeDbContext : DbContext
{

	public EmployeeDbContext(DbContextOptions<EmployeeDbContext> options) : base(options) 
	{
		// connection string is already set by configuring options in `Startup.ConfigureServices(...)`
	}

	public DbSet<Employee> Employees { get; set; }
}

Console App

Code and Commands (screenshoots below)

Install-Package Microsoft.EntityFrameworkCore
Install-Package Microsoft.Extensions.Configuration.Json

appsettings.json

{
  "connectionStrings": {
    "AdventureWorks": "Data Source=DESKTOP-OJLNQ8G\\SQLSERVERDEV;Initial Catalog=AdventureWorks2017;Integrated Security=True"
  }
}

DbContext constructor

	private readonly string _connectionString;

	public AdventureworksDbContext(string connectionString)
	{
		_connectionString = connectionString;
	}

	protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
	{
		if (!optionsBuilder.IsConfigured)
		{
			optionsBuilder.UseSqlServer(_connectionString);
		}
	}

Console app: Program.cs

using System;
using System.Diagnostics;
using System.Linq;
using System.IO;
using AdventureWorks.Data;
using Microsoft.Extensions.Configuration;

namespace AdventureWorks.ConsoleApp
{
    class Program
    {
        private static IConfiguration _configuration = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", true, true)
            .Build();

        private static readonly string _connectionString = _configuration.GetConnectionString("AdventureWorks");

        static void Main(string[] args)
        {

            var awDbContext = new AdventureworksDbContext(_connectionString);

            foreach (var person in awDbContext.People.Take(10))
            {
                Debug.WriteLine($"{person.FirstName}");
            }
        }
    }
}

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Without DI injection

Courtesy: https://www.koskila.net/how-to-instantiate-your-dbcontext-outside-your-data-project/

using Microsoft.EntityFrameworkCore;
using System;
 
namespace YourNamespace
{
    public class YourClass
    {
        private string _connectionString = "This comes from your configuration file";
 
        public YourClass()
        {
           // In this example, "ApplicationDbContext" is my DbContext class
           var options = new DbContextOptionsBuilder<ApplicationDbContext>()
                   .UseSqlServer(_connectionString)
                   .Options;
 
            // With the options generated above, we can then just construct a new DbContext class
            using (var ctx = new ApplicationDbContext(options))
            {
               // Your code here
            }
         }
    }
}