internal class DatabaseController
{
    private readonly DatabaseContext _databaseContext;

    public DatabaseController(DatabaseContext dbContext)
    {
        _databaseContext = dbContext;
    }

    public DatabaseContext GetContext() => _databaseContext;
}

public class DatabaseContext : DbContext
{
    private readonly string _connectionString;

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

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        // server=localhost;database=peach;user=root;password=
        optionsBuilder.UseMySQL(_connectionString);
    }

    public virtual DbSet<User> Users { get; set; }
}

var user = new User
{
    Id = 1,
    Username = "John",
};

using (var dbContext = Program.DatabaseController.GetContext())
{
    dbContext.Users.Add(user);
    dbContext.SaveChanges();
}