Resharper报“Possible multiple enumeration of IEnumerable”

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：

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using ConsoleApplication2.EF;

 namespace ConsoleApplication2
 {
     class Program_IEnumerable
     {
         static void Main(string[] args)
         {
             // 异步访问数据库
             Task.Run(() =>
             {
                 while (true)
                 {
                     reloadDb();
                 }
             });

             // 使用死循环不停的读取数据
             int count = ;
             while (true)
             {
                 Console.WriteLine("第{0}读取", count);
                 IEnumerable<string> names = getNames();

                 var allNames = new StringBuilder();
                 foreach (var name in names)
                     allNames.Append(name + ",");
                 Thread.Sleep();

                 var allNames2 = new StringBuilder();
                 foreach (var name in names)
                     allNames2.Append(name + ",");
                 if (allNames2 != allNames)
                     Console.WriteLine("数据源发生了改变");
                 count++;

                 Thread.Sleep();
             }

             Console.ReadKey();
         }

         static void reloadDb()
         {
             using (var infosEntities = new TestEntities())
             {
                 infosEntities.Student.Add(new Student
                 {
                     EnrollmentDate = DateTime.Now,
                     FirstMidName = "han",
                     LastName = "zhu"
                 });
                 infosEntities.SaveChanges();
             }
             Thread.Sleep();

             using (var infosEntities = new TestEntities())
             {
                 var entity = infosEntities.Student.FirstOrDefault(a => a.FirstMidName == "han");
                 if (entity != null)
                 {
                     infosEntities.Student.Remove(entity);
                     infosEntities.SaveChanges();
                 }
             }
             Thread.Sleep();
         }

         static IEnumerable<string> getNames()
         {
             var infosEntities = new TestEntities();
             return infosEntities.Student.Select(a => a.FirstMidName + " " + a.LastName);
         }

     }

 }

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