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
– mjwills, Commented Jun 15, 2018 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
– jamheadart, Commented Jun 15, 2018 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
– John Bustos, Commented Jun 15, 2018 at 12:43

DavidG · Accepted Answer · 2018-06-15 12:50:27Z

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, 2018 at 12:50

answered Jun 15, 2018 at 12:41

DavidG

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10 Comments

jamheadart Over a year ago

I take it this'd work with string functions too like .Select(s => s.Replace("-",""))

DavidG Over a year ago

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.

jamheadart Over a year ago

Funny when you say a keyword and something clicks. Immutable! I'm with you.

mjwills Over a year ago

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.

vc 74 Over a year ago

@mjwills If you really (unlikely) want to cover this case, you can var copy = list.Select(s => s + "-") .ToList(); list.Clear(); list.AddRange(copy)

|

GaneshJ · Accepted Answer · 2018-06-15 12:51:43Z

-2

Try this

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

answered Jun 15, 2018 at 12:51

GaneshJ

353 bronze badges

1 Comment

DavidG Over a year ago

I posted basically the same solution 10 minutes ago, how is yours any different?

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