Question

I have 2 collections and i have the following code to loop through one collection and see if it exists in another collection. If it does exist, then update a property of that item.

        foreach (var favorite in myFavoriteBooks)
        {
            var book = allBooks.Where(r => r.Name == favorite.Name).FirstOrDefault();

            if (book != null)
            {
                book.IsFavorite = true;
            }
        }

Is there a more elegant or faster way to achieve this code above?

Answer 1

You can use Join for this, either in extension method syntax or LINQ syntax:

Extension method:

foreach(var favorite in myFavoriteBooks.Join(allBooks, 
                                             f => f.Name, 
                                             a => a.Name, 
                                            (f, a) => a))
{
    a.IsFavorite = true;
}

LINQ:

var favorites = from f in myFavoriteBooks
                join a in allBooks on f.Name equals a.Name
                select a

foreach(var favorite in favorites)
{
    favorite.IsFavorite = true;
}

These solutions are functionally identical; they differ only by syntax and they are faster than your original solution, since LINQ will build a hashtable on both sides and use that for matching rather than scanning the other list for every item in the original list.