LINQ C# Book

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  4. 0383 Captured Variables
If the query's lambda expressions reference local variables and you later change their value, the query changes as well.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
int[] numbers = { 1, 2 };
int factor = 10;
IEnumerable query = numbers.Select(n => n * factor);
factor = 20;
foreach (int n in query) Console.Write(n + "|");
}
}

The output:
20|40|
Compare the following two examples:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
IEnumerable query = "aeiou";
query = query.Where(c => c != 'a');
query = query.Where(c => c != 'e');
foreach (char ch in query)
Console.WriteLine(ch);
}
}

The output:
i
o
u

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
IEnumerable query = "aeiou";
foreach (char vowel in "ae")
query = query.Where(c => c != vowel);
foreach (char c in query)
Console.Write(c);
}
}

The output:
aiou
To solve this, we can assign the loop variable to another variable:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
class Program
{
static void Main()
{
IEnumerable query = "aeiou";
foreach (char vowel in "ae")
{
char temp = vowel;
query = query.Where(c => c != temp);
}
}
}