LINQ Where clause misunderstanding

Question

I am getting some behavior from a LINQ Where clause that I can't quite grok.

For instance, I am trying to search a list for terms using a AND semantic.

Here is some code:

var items = new List<string>()
{
    "beef",
    "beef soup",
    "beef ribs",
    "chicken soup",
    "soup",
    "pork ribs",
    "ribs"
};

var term = "soup beef";
var terms = term.Split(' ');
var result = items.AsQueryable();
foreach(var t in terms)
{
    result = result.Where( i => i.Contains(t) );
}
result.ToList().Dump();

The result are:

beef beef
soup
beef ribs

However, I was looking for it to an AND result, returning just:

beef soup

Now, I CAN GET THE RESULT I WANT by simply adding

.ToList().AsQueryable()

to the end of the Where clause.

Of course, this is not what I want to do if the back end is a database.

What is really odd here is that when I check 'result' in the loop, I get the following:

Before first where clause => full list -> makes sense.
After first where (for 'soup') => ['beef soup', 'chicken soup', 'soup'] -> still looks good
Before second loop (for 'beef') => ['beef', 'beef soup', 'beef ribs'] -> WHOA! Whats up here
After second loop => stays the same

Can someone explain to me what is going on (and how to fix this correctly)?

Your question is often asked, and you've already found it's due to closing over the loop variable. See: http://blogs.msdn.com/b/ericlippert/archive/2009/11/12/closing-over-the-loop-variable-considered-harmful.aspx Note that they're considering [changing the behavior in C# 5](http://stackoverflow.com/questions/7123898/lambda-capture-problem-with-iterators/7124236#7124236). — Anthony Pegram, Aug 23 '11 at 15:41
Confirmed. They are making the breaking change in C#5. This makes `foreach` loops uniquely different from `for` loops. — Ishmael Smyrnow, Jan 28 '12 at 00:17

score 3 · Accepted Answer · answered Aug 23 '11 at 15:29

Review Access To Modified Closure

http://weblogs.asp.net/fbouma/archive/2009/06/25/linq-beware-of-the-access-to-modified-closure-demon.aspx

Essentially the problem is that t is being modified in your foreach loop, but you are using it within your closure (the Where lambda method), which is accessing t by reference.

score 0 · Answer 2 · answered Aug 23 '11 at 15:29

0

CLOSURES!!! Arg.... Got it.

    var cTerm = t;

    result = result.Where( i => i.Contains(cTerm) );

Sorry for the randomness...

answered Aug 23 '11 at 15:29

teleball

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LINQ Where clause misunderstanding

2 Answers2