c# change the string in a foreach string loop

Question

I'm trying to cycle through strings in a list with a foreach loop, but I don't know how to change the item that's being referenced - How can I change s in the list... e.g. below I want to add "-" to the end of every string, s in myStringList

foreach(string s in myStringList)
{
    s = s + "-"; 
}

I can't do things to s because it's a "foreach iteration variable" - so can I not change it as I cycle through?

Do I have to use an int to count through?

I'm sure there's a simpler way...

Have you considered using a `for` loop instead? `myStringList[i] += "-";` — mjwills, Jun 15 '18 at 12:41
I have but I honestly thought there'd be a really simple "ref" keyword to chuck in and alter the `s` directly as I cycle through, with `s.replace` and `s=s + x` things like that. — jamheadart, Jun 15 '18 at 12:42
You can't change values while iterating through them in a `foreach` - You'd have to either use linq to change them or copy the (updated) values to a new collection — John Bustos, Jun 15 '18 at 12:43
I mention int or any counter to do a for loop e.g. [pseudocode] `for int i = 1 to list.Count; list[i] = list[i] + "-";` — jamheadart, Jun 15 '18 at 12:44
Possible duplicate of [c# string.replace in foreach loop](https://stackoverflow.com/questions/2711999/c-sharp-string-replace-in-foreach-loop) — xdtTransform, Jun 15 '18 at 12:46
2500 views, 13+ upvotes on the answer and not a single positive vote for the question... I'm not bitter, I swear! — jamheadart, Nov 11 '20 at 08:28

DavidG · Accepted Answer · 2018-06-15T12:50:27.693

15

You can do this with Linq quite easily:

var newStringList = myStringList
    .Select(s => s + "-")
    .ToList();

If you really want to modify the existing list, you can use a classic for loop, for example:

for (var i = 0; i < myStringList.Count; i++)
{
    myStringList[i] = myStringList[i] + "-";
}

edited Jun 15 '18 at 12:50

answered Jun 15 '18 at 12:41

DavidG

113,891
12
217
223

I take it this'd work with string functions too like `.Select(s => s.Replace("-",""))` – jamheadart Jun 15 '18 at 12:43
2

Yes, you're creating new strings and assigning the result to a completely new list. Remember that `string` is immutable in C# so you can never change it, only create new ones. – DavidG Jun 15 '18 at 12:44
Funny when you say a keyword and something clicks. Immutable! I'm with you. – jamheadart Jun 15 '18 at 12:46
A potential issue with the LINQ solution, since a completely new list has been created, is that any references to the old list won't see the change. _This may not be an issue if this is the only reference to the list._ – mjwills Jun 15 '18 at 12:47
1

@mjwills If you really (unlikely) want to cover this case, you can `var copy = list.Select(s => s + "-") .ToList(); list.Clear(); list.AddRange(copy)` – vc 74 Jun 15 '18 at 12:52
Actually I'm just wondering - I create the list by splitting a large string - is there a way to append or replace things immediately to the split list?! – jamheadart Jun 15 '18 at 12:55
@jamheadart Something like `stringVariable.Split("...").Select(s => s + "-")...` – DavidG Jun 15 '18 at 12:57
Just adding, remember also that `.ToList()` is not absolutely required with the Linq example, it is just a matter of if one desires deferred execution or not and the possible implications. – blins Jun 15 '18 at 16:30
@vc74 Or, easier, `myStringList = myStringList.Select(s => s + "-").ToList();` Technically different, but does the exact same thing faster in practice, because you are making 3 lists instead of 2. – PRMan Nov 23 '21 at 17:13
@PRMan No, it won't be faster at all. There are still the same number of lists being created. Just because one of them doesn't have a named variable doesn't mean it doesn't exist. – DavidG Nov 23 '21 at 17:30

score -2 · Answer 2 · answered Jun 15 '18 at 12:51

-2

Try this

List<string> listString = new List<string>() { "1","2","3"};
listString=listString.Select(x => x + "-").ToList();

answered Jun 15 '18 at 12:51

GaneshJ

35
3

1

I posted basically the same solution 10 minutes ago, how is yours any different? – DavidG Jun 15 '18 at 12:52

c# change the string in a foreach string loop

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