public interface IUnitOfWork<TDbConnection>
{
    TDbConnection Connection { get; }
    DbTransaction Transaction { get ; }
    void Begin();
    void Rollback();
    void Commit();
}

public void Start()
{
    if (transaction != null)
    {
        throw new InvalidOperationException("Cannot start a new transaction while the existing other one is still open.");
    }
    var connection = EnsureConnection();
    transaction = connection.BeginTransaction();
}

private SqlConnection EnsureConnection() =>
    connection ?? connection = new SqlConnection(settings.ConnectionString);

private SqlConnection EnsureConnection() =>
    connection ??= (SqlConnection) new SqlConnection(settings.ConnectionString).EnsureOpen();