How can I trim all elements in a list?

Ran into the same problem. @Lee already explained that Lamda .ForEach() uses a copy.

You can write an Extension Method like this and use a for loop (foreach also not possible):

public static class StringListExtensions
{
    public static void TrimAll(this List<string> stringList)
    {
        for (int i = 0; i < stringList.Count; i++)
        {
            stringList[i] = stringList[i].Trim(); //warning: do not change this to lambda expression (.ForEach() uses a copy)
        }
    }
}

Use it like this:

var productNumbers = new List<string>(){ "11111", " 22222 " }
productNumbers.TrimAll();

should result in: List(){ "11111", "22222" }

I didn't use the split and re-join solution (chosen solution) because there can be a comma inside one of the string items. The regex version is not self explanatory. This is old-school but safer and can be easily understood...

What's going on is that you're trying to modify a collection using a foreach statement- which is a no-no. Collections cannot be modified with a foreach.

You'll need to modifiy it a for loop, or, using lambdas, you can use LukeH's solution.

The reason your approach doesn't work is that the x is a copy of the current string reference being processed in the ForEach call (i.e. local to the delegate). Therefore the assignment doesn't affect the item referenced in the list.

// you can omit the final ToArray call if you're using .NET 4
var result = string.Join(",", tl.Split(',').Select(s => s.Trim()).ToArray());

If you only need the final result string, rather than the intermediate collection, then you could use a regular expression to tidy the string. You'll need to benchmark to determine whether or not the regex outperforms the split-trim-join technique:

var result = Regex.Replace(tl, @"(?<=^|,) +| +(?=,|$)", "");