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
    
     
      > GetOrdersAsync()        {            var orders = new List
      
       ();            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
       
        
         > EnumerateOrdersAsync()        {            var offset = 0;            while (true)            {                var list = (await QueryOrdersAsync(offset)).ToList();                yield return list;                offset++;                if (list.Count < 100)                {                    break;                }            }        }        public async Task
         
          
           > 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
           
            (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); } }}