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