When do LINQ Lambdas execute in a foreach loop

Question

So, to keep it short, I have this bit of code in which I loop through the results of a string split and add them to a list, if they have not occurred before in the loop. This is the code.

var res = new List<string>();
foreach(string s in input.Split(new[] { ", " }, 
              StringSplitOptions.RemoveEmptyEntries))
{
    if(res.All(p => p != s))
        res.Add(s);
}

But after I wrote this code, Visual Studio said I could convert part of the loop into LINQ. However, I'm a bit skeptical about whether this'll work or not.

Basically, my question is, would the lambda expressions be executed on each separate loop, or just once in the beginning?

var res = new List<string>();
foreach (string s in input.Split(new[] { ", " }, 
              StringSplitOptions.RemoveEmptyEntries)
                          .Where(s => res.All(p => p != s)))
{
    res.Add(s);
}

possible duplicate of [Does "foreach" cause repeated Linq execution?](http://stackoverflow.com/questions/16946207/does-foreach-cause-repeated-linq-execution) — xZ6a33YaYEfmv, Sep 28 '15 at 11:45
`foreach` will enumerate the IEnumerable that results after evaluation of the entire statement. The entire LINQ statement is evaluated before `foreach` even starts — Panagiotis Kanavos, Sep 28 '15 at 11:47
@AlekDepler actually, before the loop even starts. `foreach` will execute *after* the call to `Where` returns the final IEnumerable — Panagiotis Kanavos, Sep 28 '15 at 11:49

Dmitrii Dovgopolyi · Accepted Answer · 2015-09-28T11:56:09.843

9

Here is the example of working lazy evaluation of Linq queries.

List<int> vals = new List<int> {1, 1, 2, 2, 3, 4};
var res = new List<int>();
foreach (int s in vals.Where(s => 
                       { 
                           Console.WriteLine("lambda"); 
                           return s % 2 == 0; 
                       }))
{
    Console.WriteLine("loop");
}

And the output will be

lambda
lambda
lambda
loop
lambda
loop
lambda
lambda
loop

As you can see lambda will be evaluated only when the next element is required by foreach loop

edited Sep 28 '15 at 11:56

answered Sep 28 '15 at 11:54

Dmitrii Dovgopolyi

6,231
2
27
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1

This is a nice example and actually answers the question. – juharr Sep 28 '15 at 11:58
Actually, I find it doesn't explicitly answer the question *"Would the lambda expressions be executed on each separate loop, or just once in the beginning?"* because Dmitry says it will be evaluated *"only when the next element is required"* - this means that actually the lambda **is** evaluated *on each separate loop*. I would suggest changing the wording of the answer to reflect this fact better. – Marcus Mangelsdorf Oct 20 '15 at 07:15

score 7 · Answer 2 · answered Sep 28 '15 at 11:44

7

Basically you are just trying to get the distinct values:

var res = input.Split(new[] { ", " }, StringSplitOptions.RemoveEmptyEntries)
               .Distinct()
               .ToList();

answered Sep 28 '15 at 11:44

Ufuk Hacıoğulları

37,978
12
114
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1

While this is a better alternative it doesn't answer the question. – juharr Sep 28 '15 at 11:58
3

@juharr yet this answer belongs here just to show how good code can fix problems like "I don't know how this code works" – Dmitrii Dovgopolyi Sep 28 '15 at 12:00

Marcus Mangelsdorf · Answer 3 · 2023-07-07T08:57:47.093

I built on Dmitry's answer and made output a little more verbose to show you what is really going on. Also note, that as Ufuk noted, there a better ways to achieve a list of distinct values.

Feel free to test this for your own and play around with it a little in this dotnetfiddle.

Here is the example:

List<int> input = new List<int> {1, 1, 2, 2, 3, 4};
var res = new List<int>();
foreach (int s in input.Where(s => 
    { 
        Console.WriteLine("lambda: s=" + s);
        Console.WriteLine("lambda: " + s + " contained in res? " + res.Contains(s));
        return !res.Contains(s);
    }))
{
    res.Add(s);
    Console.WriteLine("loop: " + s + " added to res");
    Console.WriteLine();
}

It produces the following output:

lambda: s=1
lambda: 1 contained in res? False
loop: 1 added to res

lambda: s=1
lambda: 1 contained in res? True
lambda: s=2
lambda: 2 contained in res? False
loop: 2 added to res

lambda: s=2
lambda: 2 contained in res? True
lambda: s=3
lambda: 3 contained in res? False
loop: 3 added to res

lambda: s=4
lambda: 4 contained in res? False
loop: 4 added to res

When do LINQ Lambdas execute in a foreach loop

3 Answers3