异步流是C#8.0出来的特性，可以让流式数据以尽快分批的方式返回，而不像之前那样，当全部获取到数据后，再返回。

下面的例子的场景有点特殊，查询一个较多的数据，为了查询不给数据带来很大压力，做成一个分批从数据查询回数据，组装成一个整理List返回。

如果是之前的做法，就需要等所有的数据返回后，累加到一个集合中，然后返回；如果用异步流，可以返回一批，就返回，以便更快的让使用者展示或使用。

异步流也有问题，多次返回，虽然用户体验相对好一些，但用时更长，通过下面的例子可以了解到。

using System;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.Linq;
using System.Net.WebSockets;
using System.Text;
using System.Threading.Tasks;
using System.Diagnostics;
using System.Security.Cryptography;
using System.IO;
using Dapper;namespace KeyWordsDemo
{class AsyncStreamDemo : IDemoAsync{public async Task RunAsync(){Console.WriteLine("1、异常流  2、同步获取")var no = Console.ReadLine();var producer = new Producer();if(no == "1"){var watch = new Stopwatch();watch.Start();await foreach (var orders in producer.EnumerateOrdersAsync()){foreach (var order in orders){Console.WriteLine(order.ToString());}}watch.Stop();Console.Title = $"时长：{ watch.Elapsed.TotalSeconds}";}else{var watch = new Stopwatch();watch.Start();var orders = await producer.GetOrdersAsync();foreach (var order in orders){Console.WriteLine(order.ToString());}watch.Stop();Console.Title = $"获取到数据：{orders.Count()} 时长：{ watch.Elapsed.TotalSeconds}";}Console.ReadLine();}}class Producer{public async Task<List<SalesOrderDetail>> GetOrdersAsync(){var orders = new List<SalesOrderDetail>();var offset = 0;while (true){var list = (await QueryOrdersAsync(offset)).ToList();orders.AddRange(list);offset++;if (list.Count < 100){break;}}return orders;}public async IAsyncEnumerable<List<SalesOrderDetail>> EnumerateOrdersAsync(){var offset = 0;while (true){var list = (await QueryOrdersAsync(offset)).ToList();yield return list;offset++;if (list.Count < 100){break;}}}public async Task<IEnumerable<SalesOrderDetail>> QueryOrdersAsync(int offset){using var con = new SqlConnection("server=.;database=AdventureWorks2016;uid=sa;pwd=sa;");var sql = @$"select * from Sales.SalesOrderDetail order by SalesOrderID,SalesOrderDetailID  offset {offset * 100} row fetch next 100 row only";return await con.QueryAsync<SalesOrderDetail>(sql);}}class SalesOrderDetail{public int SalesOrderID { get; set; }public string CarrierTrackingNumber { get; set; }public short OrderQty { get; set; }public int ProductID { get; set; }public int SpecialOfferID { get; set; }public decimal UnitPrice { get; set; }public decimal UnitPriceDiscount { get; set; }public Guid rowguid { get; set; }public DateTime ModifiedDate { get; set; }public override string ToString(){return System.Text.Json.JsonSerializer.Serialize(this);}}
}