Linq query built in foreach loop always takes parameter value from last iteration

Question

I have a List containing several keywords. I foreach through them building my linq query with them like so (boiled down to remove the code noise):

List

        

Answer 1

it’s been fixed in C# 5.0, and the example above in C# 5.0 works but fails in earlier versions of C#.

But be careful, it does not concern a for loop

  static void Main()
        {
            IEnumerable 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);
        }