Why use the yield keyword, when I could just use a

Question

Given this code:

IEnumerable<object> FilteredList()
{
    foreach( object item in FullList )
    {
        if( IsItemInPartialList( item ) )
            yield return item;
    }
}

Why should I not just code it this way?:

IEnumerable<object> FilteredList()
{
    var list = new List<object>(); 
    foreach( object item in FullList )
    {
        if( IsItemInPartialList( item ) )
            list.Add(item);
    }
    return list;
}

I sort of understand what the yield keyword does. It tells the compiler to build a certain kind of thing (an iterator). But why use it? Apart from it being slightly less code, what's it do for me?

Answer 1

The yield return statement allows you to return only one item at a time. You are collecting all the items in a list and again returning that list, which is a memory overhead.

Answer 2

With the "list" code, you have to process the full list before you can pass it on to the next step. The "yield" version passes the processed item immediately to the next step. If that "next step" contains a ".Take(10)" then the "yield" version will only process the first 10 items and forget about the rest. The "list" code would have processed everything.

This means that you see the most difference when you need to do a lot of processing and/or have long lists of items to process.