Resharper报“Possible multiple enumeration of IEnumerable”

Posted 2020-10-01 远方V3

问题描述：在IEnumerable使用时显示警告

分析：如果对IEnumerable多次读取操作，会有因数据源改变导致前后两次枚举项不固定的风险，最突出例子是读取数据库的时候，第二次foreach时恰好数据源发生了改变，那么读取出来的数据和第一次就不一致了。

查看测试代码

几乎所有返回类型为 IEnumerable<T> 或 IOrderedEnumerable<TElement> 的标准查询运算符都以延迟方式执行。如下表我们可以看到where时，返回的IEnumerable是延迟加载的。

标准查询运算符	Return Type	立即执行	延迟流式执行	延迟非流式执行
Aggregate	TSource	√
All<TSource>	Boolean	√
Any	Boolean	√
AsEnumerable<TSource>	IEnumerable<T>		√
Average	单个数值	√
Cast<TResult>	IEnumerable<T>		√
Concat<TSource>	IEnumerable<T>		√
Contains	Boolean	√
Count	Int32	√
DefaultIfEmpty	IEnumerable<T>		√
Distinct	IEnumerable<T>		√
ElementAt<TSource>	TSource	√
ElementAtOrDefault<TSource>	TSource	√
Empty<TResult>	IEnumerable<T>	√
E√cept	IEnumerable<T>		√	√
First	TSource	√
FirstOrDefault	TSource	√
GroupBy	IEnumerable<T>			√
GroupJoin	IEnumerable<T>		√	√
Intersect	IEnumerable<T>		√	√
Join	IEnumerable<T>		√	√
Last	TSource	√
LastOrDefault	TSource	√
LongCount	Int64	√
Ma√	单个数值、TSource 或 TResult	√
Min	单个数值、TSource 或 TResult	√
OfType<TResult>	IEnumerable<T>		√
OrderBy	IOrderedEnumerable<TElement>			√
OrderByDescending	IOrderedEnumerable<TElement>			√
Range	IEnumerable<T>		√
Repeat<TResult>	IEnumerable<T>		√
Reverse<TSource>	IEnumerable<T>			√
Select	IEnumerable<T>		√
SelectMany	IEnumerable<T>		√
SequenceEqual	Boolean	√
Single	TSource	√
SingleOrDefault	TSource	√
Skip<TSource>	IEnumerable<T>		√
SkipWhile	IEnumerable<T>		√
Sum	单个数值	√
Take<TSource>	IEnumerable<T>		√
TakeWhile	IEnumerable<T>		√
ThenBy	IOrderedEnumerable<TElement>			√
ThenByDescending	IOrderedEnumerable<TElement>			√
ToArray<TSource>	TSource 数组	√
ToDictionary	Dictionary<TKey, TValue>	√
ToList<TSource>	IList<T>	√
ToLookup	ILookup<TKey, TElement>	√
Union	IEnumerable<T>		√
Where	IEnumerable<T>		√

 

解决方案：

多次使用IEnumerable时，最好转换为List或者Array

 

测试代码：

 

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Text;
 5 using System.Threading;
 6 using System.Threading.Tasks;
 7 using ConsoleApplication2.EF;
 8 
 9 namespace ConsoleApplication2
10 {
11     class Program_IEnumerable
12     {
13         static void Main(string[] args)
14         {
15             // 异步访问数据库
16             Task.Run(() =>
17             {
18                 while (true)
19                 {
20                     reloadDb();
21                 }
22             });
23 
24             // 使用死循环不停的读取数据
25             int count = 1;
26             while (true)
27             {
28                 Console.WriteLine("第{0}读取", count);
29                 IEnumerable<string> names = getNames();
30 
31                 var allNames = new StringBuilder();
32                 foreach (var name in names)
33                     allNames.Append(name + ",");
34                 Thread.Sleep(500);
35 
36                 var allNames2 = new StringBuilder();
37                 foreach (var name in names)
38                     allNames2.Append(name + ",");
39                 if (allNames2 != allNames)
40                     Console.WriteLine("数据源发生了改变");
41                 count++;
42 
43                 Thread.Sleep(1000);
44             }
45 
46             Console.ReadKey();
47         }
48 
49         static void reloadDb()
50         {
51             using (var infosEntities = new TestEntities())
52             {
53                 infosEntities.Student.Add(new Student
54                 {
55                     EnrollmentDate = DateTime.Now,
56                     FirstMidName = "han",
57                     LastName = "zhu"
58                 });
59                 infosEntities.SaveChanges();
60             }
61             Thread.Sleep(1000);
62 
63             using (var infosEntities = new TestEntities())
64             {
65                 var entity = infosEntities.Student.FirstOrDefault(a => a.FirstMidName == "han");
66                 if (entity != null)
67                 {
68                     infosEntities.Student.Remove(entity);
69                     infosEntities.SaveChanges();
70                 }
71             }
72             Thread.Sleep(1000);
73         }
74 
75         static IEnumerable<string> getNames()
76         {
77             var infosEntities = new TestEntities();
78             return infosEntities.Student.Select(a => a.FirstMidName + " " + a.LastName);
79         }
80 
81     }
82 
83 }

 

 

 

参考资料：

Resharper官方对于这个警告的描述：

https://www.jetbrains.com/help/resharper/PossibleMultipleEnumeration.html

MSDN的解释：

https://msdn.microsoft.com/zh-cn/library/vs/alm/bb882641(v=vs.90)/css