Example of connecting to SQL Server DB.

DBConnectExample.cs

using System;
using System.Data.Common;
using System.Configuration;

namespace TestCS
{
	class Program
	{
		static void Main(string[] args) {
			string provider = ConfigurationManager.AppSettings["provider"];
			string connectionString = ConfigurationManager.AppSettings["connectionString"];
			DbProviderFactory factory = DbProviderFactories.GetFactory(provider);

			using (DbConnection connection = factory.CreateConnection()) {
				if (connection == null) {
					Console.WriteLine("Connection Error");
					Console.ReadKey();
					return;
				}
				connection.ConnectionString = connectionString;
				connection.Open();

				DbCommand command = factory.CreateCommand();
				if (command == null) {
					Console.WriteLine("Command Error");
					Console.ReadKey();
					return;
				}
				command.Connection = connection;
				command.CommandText = "select * from Products";

				using (DbDataReader dataReader = command.ExecuteReader()) {
					while (dataReader.Read()) {
						Console.WriteLine($"{dataReader["ProdId"]} {dataReader["Product"]}");
					}
				}

				Console.ReadKey();
			}
			
		}
	}
}

/*
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <appSettings>
	<add key="provider" value="System.Data.SqlClient" />
	<add key="connectionString" value="Data Source=YOURDESKTOP;Initial Catalog=StoreDB;Integrated Security=True;Pooling=False" />
    </appSettings>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
    </startup>
</configuration>
*/