Possible multiple enumeration of IEnumerable
https://www.jetbrains.com/help/resharper/2016.1/PossibleMultipleEnumeration.html
Consider the following code snippet:
IEnumerable<string> names = GetNames(); foreach (var name in names) Console.WriteLine("Found " + name); var allNames = new StringBuilder(); foreach (var name in names) allNames.Append(name + " ");
Assuming that GetNames()
returns an IEnumerable<string>
, we are, effectively, doing extra work by enumerating this collection twice in the twoforeach
statements.
If GetNames()
results in a database query, you end up doing that query twice, while both times getting the same data.
This kind of problem can be easily fixed – simply force the enumeration at the point of variable initialization by converting the sequence to an array or a list, e.g.:
IEnumerable<string> names = GetNames().ToList();
The rest of your code can stay the same, because both array and list types implement the IEnumerable
interface.
问题:如果GetNames是一个sql语句操作的话,2次的foreach会导致2次的sql查询
如果转为List,那么第二次遍历的时候,直接用第一次转换是List,就不会有2次的sql查询