Why use the yield keyword, when I could just use an ordinary IEnumerable?

c# yield
James P. Wright picture James P. Wright · Dec 27, 2012 · Viewed 10.5k times · Source

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

Robert Harvey picture Robert Harvey · Dec 27, 2012

Using yield makes the collection lazy.

Let's say you just need the first five items. Your way, I have to loop through the entire list to get the first five items. With yield, I only loop through the first five items.

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