CosmosDriver.cs

public async Task AddCustomer(Customer customer)
{
    try
    {
      ItemResponse<Customer> customerResponse = await this._container.CreateItemAsync<Customer>(customer, new PartitionKey(customer.CustomerId));
    }
    catch (CosmosException ex) 
    {
      Console.WriteLine("Exception occured in AddCustomer: Message body is {0}.\n", ex.Message);
      throw;
    }

}

public async Task AddOrders(List<Order> orders, string CustomerId)
{
    try
    {
      foreach (var order in orders)
      {
        ItemResponse<Order> orderResponse = await this._container.CreateItemAsync<Order>(order, new PartitionKey(CustomerId));
      }

    }
    catch (CosmosException ex) 
    {
      Console.WriteLine("Exception occured in AddCustomer: Message body is {0}.\n", ex.Message);
      throw;
    }
}