SQL Server Installation

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

public class WorldDatabase : IDisposable
{
    private readonly string _connectionString;
    private SqlConnection _connection;
    
    public WorldDatabase(string connectionString)
    {
        _connectionString = connectionString;
        _connection = new SqlConnection(_connectionString);
        _connection.Open();
    }
    
    public List<Country> GetCountries(string region = null, int? limit = null)
    {
        var countries = new List<Country>();
        var query = "SELECT id, name, iso2, iso3, region FROM countries";
        var parameters = new List<SqlParameter>();
        
        if (!string.IsNullOrEmpty(region))
        {
            query += " WHERE region = @region";
            parameters.Add(new SqlParameter("@region", region));
        }
        
        if (limit.HasValue)
        {
            query += $" ORDER BY name OFFSET 0 ROWS FETCH NEXT {limit} ROWS ONLY";
        }
        
        using var command = new SqlCommand(query, _connection);
        command.Parameters.AddRange(parameters.ToArray());
        
        using var reader = command.ExecuteReader();
        while (reader.Read())
        {
            countries.Add(new Country
            {
                Id = reader.GetInt32("id"),
                Name = reader.GetString("name"),
                Iso2 = reader.GetString("iso2"),
                Iso3 = reader.GetString("iso3"),
                Region = reader.IsDBNull("region") ? null : reader.GetString("region")
            });
        }
        
        return countries;
    }
    
    public List<City> GetCitiesByCountry(string countryName, int? limit = null)
    {
        var cities = new List<City>();
        var query = @"
            SELECT name, latitude, longitude, state_name, country_name
            FROM cities 
            WHERE country_name = @countryName";
            
        if (limit.HasValue)
        {
            query += $" ORDER BY name OFFSET 0 ROWS FETCH NEXT {limit} ROWS ONLY";
        }
        
        using var command = new SqlCommand(query, _connection);
        command.Parameters.AddWithValue("@countryName", countryName);
        
        using var reader = command.ExecuteReader();
        while (reader.Read())
        {
            cities.Add(new City
            {
                Name = reader.GetString("name"),
                Latitude = reader.IsDBNull("latitude") ? null : reader.GetDouble("latitude"),
                Longitude = reader.IsDBNull("longitude") ? null : reader.GetDouble("longitude"),
                StateName = reader.IsDBNull("state_name") ? null : reader.GetString("state_name"),
                CountryName = reader.GetString("country_name")
            });
        }
        
        return cities;
    }
    
    public void Dispose()
    {
        _connection?.Dispose();
    }
}

// Usage
string connectionString = "Server=localhost;Database=world;Trusted_Connection=true;";
using var db = new WorldDatabase(connectionString);

var countries = db.GetCountries("Europe", 5);
foreach (var country in countries)
{
    Console.WriteLine($"{country.Name} ({country.Iso2})");
}