在foreach循环中构build的Linq查询总是从上次迭代中获取参数值

我有一个列表包含几个关键字。 我通过他们foreach构build我的linq查询与他们一样（煮沸下来删除代码噪音）：

List<string> keys = FillKeys() foreach (string key in keys){ q = q.Where(c => c.Company.Name.Contains(key)); } 

当我现在让我的键包含2键，返回结果seperatly，但永远不会一起发生（q中的每个项目是“xyz”或“123”，从来没有“123”和“xyz”），我仍然得到的结果。 结果集与最后一个string相同。

我看了一下linq查询，看起来它创build了正确的sql，但是它用相同的（最后一个被访问的）值replace@ p1和@ p2。

我究竟做错了什么？

Answer 1

你在lambdaexpression式中重用了相同的variables（ key ）。

有关更多详细信息，请参阅我的匿名方法文章，还有一些相关的SO问题：

• LINQ to SQL的错误（或非常奇怪的function）…
• 带有委托的局部variables
• C＃在一个循环中捕获variables
• C＃gotcha答案
• 以编程方式构buildLINQ查询，而不用局部variables欺骗我

简单的解决办法是先复制variables：

 List<string> keys = FillKeys() foreach (string key in keys){ string copy = key; q = q.Where(c => c.Company.Name.Contains(copy)); } 

Answer 2

可能是一个被捕获的variables问题; 尝试添加：

 List<string> keys = FillKeys() foreach (string key in keys){ string tmp = key; q = q.Where(c => c.Company.Name.Contains(tmp)); } 

Answer 3

它已经在C＃5.0中修复了，上面的例子在C＃5.0中工作，但在早期版本的C＃中失败。

但要小心，它不涉及for循环

  static void Main() { IEnumerable<char> query = "aaa bbb ccc"; string lettersToRemove = "ab"; Console.WriteLine("\nOK with foreach:"); foreach (var item in lettersToRemove) { query = query.Where(c => c != item); } foreach (char c in query) Console.Write(c); //OK: Console.WriteLine("\nOK with foreach and local temp variable:"); query = "aaa bbb ccc"; foreach (var item in lettersToRemove) { var tmp = item; query = query.Where(c => c != tmp); } foreach (char c in query) Console.Write(c); /* An IndexOutOfRangeException is thrown because: firstly compiler iterates the for loop treating i as an outsite declared variable when the query is finnaly invoked the same variable of i is captured (lettersToRemove[i] equals 3) which generates IndexOutOfRangeException The following program writes aaa ccc instead of writing ccc: Each iteration gets the same variable="C", i (last one frome abc). */ //Console.WriteLine("\nNOK with for loop and without temp variable:"); //query = "aaa bbb ccc"; //for (int i = 0; i < lettersToRemove.Length; i++) //{ // query = query.Where(c => c != lettersToRemove[i]); //} //foreach (char c in query) Console.Write(c); /* OK The solution is to assign the iteration variable to a local variable scoped inside the loop This causes the closure to capture a different variable on each iteration. */ Console.WriteLine("\nOK with for loop and with temp variable:"); query = "aaa bbb ccc"; for (int i = 0; i < lettersToRemove.Length; i++) { var tmp = lettersToRemove[i]; query = query.Where(c => c != tmp); } foreach (char c in query) Console.Write(c); }