Why is .ForEach() on IList<T> and not on IEnumerable<T>?

Question

Possible Duplicate:
Why is there not a ForEach extension method on the IEnumerable interface?

I've noticed when writing LINQ-y code that .ForEach() is a nice idiom to use. For example, here is a piece of code that takes the following inputs, and produces these outputs:

{ "One" } => "One"
{ "One", "Two" } => "One, Two"
{ "One", "Two", "Three", "Four" } => "One, Two, Three and Four";

And the code:

private string InsertCommasAttempt(IEnumerable<string> words)
{
    List<string> wordList = words.ToList();
    StringBuilder sb = new StringBuilder();
    var wordsAndSeparators = wordList.Select((string word, int pos) =>
        {
            if (pos == 0) return new { Word = word, Leading = string.Empty };
            if (pos == wordList.Count - 1) return new { Word = word, Leading = " and " };
            return new { Word = word, Leading = ", " };
        });

    wordsAndSeparators.ToList().ForEach(v => sb.Append(v.Leading).Append(v.Word));
    return sb.ToString();
}

Note the interjected .ToList() before the .ForEach() on the second to last line.

Why is it that .ForEach() isn't available as an extension method on IEnumerable<T>? With an example like this, it just seems weird.

Solution 1

According to Eric Lippert, this is mostly for philosophical reasons. You should read the whole post, but here's the gist as far as I'm concerned:

I am philosophically opposed to providing such a method, for two reasons.

The first reason is that doing so violates the functional programming principles that all the other sequence operators are based upon. Clearly the sole purpose of a call to this method is to cause side effects.

The purpose of an expression is to compute a value, not to cause a side effect. The purpose of a statement is to cause a side effect. The call site of this thing would look an awful lot like an expression (though, admittedly, since the method is void-returning, the expression could only be used in a “statement expression” context.)

It does not sit well with me to make the one and only sequence operator that is only useful for its side effects.

The second reason is that doing so adds zero new representational power to the language.

Solution 2

Because ForEach() on an IEnumerable is just a normal for each loop like this:

for each T item in MyEnumerable
{
    // Action<T> goes here
}

Solution 3

ForEach isn't on IList it's on List. You were using the concrete List in your example.