Why are these linq outputs different? [duplicate]

Question

1st statement:

IEnumerable<char> query = "Not what you might expect";  query = query.Where (c => c != 'a'); query = query.Where (c => c != 'e'); query = query.Where (c => c != 'i'); query = query.Where (c => c != 'o'); query = query.Where (c => c != 'u'); 

Output of String.Join("", query): "Nt wht y mght xpct"

2nd statement:

query = "Not what you might expect";  foreach (char vowel in "aeiou")     query = query.Where (c => c != vowel); 

Output of String.Join("", query): "Not what yo might expect"

The outputs from these statements are different. Can any one explain why?

Answer 1

If you're using a C# version lower than 5.0 (where this was fixed), this is the reason:

The lambda in your query captures the loop variable vowel.
Because Linq likes to use deferred execution, the value of this reference is not read until the query gets executed (by iterating over it), which is after the foreach loop has finished. At that point, the most recent value of vowel is u, which is why you get the unexpected output.

You can get around this by copying the value to another temporary variable (or by upgrading to C# 5.0).

Try this:

query = "Probably what you might expect";  foreach (char vowel in "aeiou") {     char currentVowel = vowel;     query = query.Where (c => c != currentVowel ); } 

Answer 2

It's because you create a closure over the vowel variable, which changes in time. Store its value in a separate variable and it will work:

query = "Not what you might expect";  foreach (char vowel in "aeiou") {     var current = vowel;     query = query.Where (c => c != current); }