Changing element value in List<T>.ForEach ForEach method

Question

I have the following code:

newsplit.ToList().ForEach(x => x = "WW");

I would expect that all elements in the list are now "WW" but they are still the original value. How come? What do I have to do different?

Answer 1

Assuming that newsplit is an IEnumerable<string>, you want:

newsplit = newsplit.Select(x => "WW");

The code that you currently have is equivalent to the following:

foreach(string x in newsplit.ToList()) {
    AssignmentAction(x);
}

...

public static void AssignmentAction(string x) {
    x = "WW";
}

This method won't modify x because of the pass-by-value semantics of C# and the immutability of strings.

Answer 2

Isn't it because the LINQ expression is creating Anonymous Types, and these are readonly? They can't be assigned to. Also, in a standard for each loop you cannot assign to a collection that is being enumerated. Try:

    foreach (var item in newsplit)
{
 item = "this won't work";
}

Answer 3

The ForEach will allow you to manipulate the elements of the IEnumerable, but not change the reference of the element.

ie, this would set a Foo property of each element in the IEnumerable to the string "WW":

newsplit.ToList().ForEach(x => x.Foo = "WW");

However, you won't be able to modify the values inside the IEnumerable itself.

Answer 4

Other answers have explained why your current code doesn't work. Here's an extension method which would fix it:

// Must be in a static non-nested class
public static void ModifyEach<T>(this IList<T> source,
                                 Func<T,T> projection)
{
    for (int i = 0; i < source.Count; i++)
    {
        source[i] = projection(source[i]);
    }
}

Then use like this:

newsplit.ModifyEach(x => "WW");

That will work with any implementation of IList<T> such as arrays and List<T>. If you need it to work with an arbitrary IEnumerable<T> then you've got a problem, as the sequence itself may not be mutable.

Using Select() is a more functional approach of course, but sometimes mutating an existing collection is worth doing...