ContactsContext.cs

public class ContactsContext : DbContext
{
    private DbContextTransaction _currentTransaction;

    ...

    public void BeginTransaction()
    {
        if (_currentTransaction != null)
            return;

        _currentTransaction = Database.BeginTransaction(IsolationLevel.ReadCommitted);
    }

    public void CloseTransaction()
    {
        CloseTransaction(exception: null);
    }

    public void CloseTransaction(Exception exception)
    {
        try
        {
            if (_currentTransaction != null && exception != null)
            {
                _currentTransaction.Rollback();
                return;
            }

            SaveChanges();

            _currentTransaction?.Commit();
        }
        catch (Exception)
        {
            if (_currentTransaction?.UnderlyingTransaction.Connection != null)
                _currentTransaction.Rollback();

            throw;
        }
        finally
        {
            _currentTransaction?.Dispose();
            _currentTransaction = null;
        }
    }
}