Why are assignment operators (=) invalid in a foreach loop?

Question

Why are assignment operators (=) invalid in a foreach loop? I'm using C#, but I would assume that the argument is the same for other languages that support foreach (e.g. PHP). For example, if I do something like this:

string[] sArray = new string[5];

foreach (string item in sArray)
{
   item = "Some assignment.\r\n";
}

I get an error, "Cannot assign to 'item' because it is a 'foreach iteration variable'."

Answer 1

Here's your code:

foreach (string item in sArray)
{
   item = "Some assignment.\r\n";
}

Here's a rough approximation of what the compiler does with this:

using (var enumerator = sArray.GetEnumerator())
{
    string item;
    while (enumerator.MoveNext())
    {
        item = enumerator.Current;

        // Your code gets put here
    }
}

The IEnumerator<T>.Current property is read-only, but that's not actually relevant here, as you are attempting to assign the local item variable to a new value. The compile-time check preventing you from doing so is in place basically to protect you from doing something that isn't going to work like you expect (i.e., changing a local variable and having no effect on the underlying collection/sequence).

If you want to modify the internals of an indexed collection such as a string[] while enumerating, the traditional way is to use a for loop instead of a foreach:

for (int i = 0; i < sArray.Length; ++i)
{
    sArray[i] = "Some assignment.\r\n";
}