Possible multiple enumeration of IEnumerable
2013-07-29 18:25 执着的梦想 阅读(1675) 评论(1) 编辑 收藏 举报Consider the following code snippet:
1:
2: IEnumerable<string> names = GetNames();
3: foreach (var name in names)
4: Console.WriteLine("Found " + name);
5: var allNames = new StringBuilder();
6: foreach (var name in names)
7: allNames.Append(name + " ");
Assuming that GetNames() returns an IEnumerable<string>, we are, effectively, doing extra work by enumerating this collection twice in the two foreach 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.