Question

Not sure if the question is titled clearly, but here goes. I have some rather messy code in my ASP.NET MVC 3 app to wrap every 2 (but make this configurable from 1-n) items in a list in a div tag when they are printed out. I’d like to create a LINQ function like so:

// Can't think of a better name
IEnumerable<V> FormattedSubsetList<T, V>(IEnumerable<T> items, int every = 2, [delegate to a method to join the N elements])
{
}

Sorry if this is kind of complicated… I’ll try an example. Let’s say I have a list of Widget. I want to print out the names of all my widgets (let’s say I have 7) and I want every 2 widgets to be inside a div. So if I have…

var list = new List<Widget>();
list.Add(new Widget() { Name = "One" });
//... you get the picture
list.Add(new Widget() { Name = "Seven" });

IEnumerable<string> newList = FormattedSubsetList<Widget, string>(list, 2, (one, two) => (return "<div>" + string.Join(" ", one.Name, two.Name) + "</div>");
string finalString = string.Join(string.Empty, newList);
// finalString == <div>One Two</div><div>Three Four</div><div>Five Six</div><div>Seven</div>

I apologize if anything is unclear but I simply don’t know what sort of thing this is called and have no idea how to go about implementing it. I know my LINQ syntax is a little off in some places too.

Answer 1

Here are two extension methods that will do exactly what you need. Split method is reusable, it simply splits any IEnumerable into a list of enumerables so each subenumerable has not more than size elements. The second method ToFormattedList is the customer method that does what you asked for.

public static class Extenstions
{
    public static IEnumerable<TRestul> ToFormattedList<TElement, TRestul>(
        this IEnumerable<TElement> source,
        int count,
        Func<List<TElement>, TRestul> formatter)
    {
        return source.Split(count).Select(arg => formatter(arg.ToList()));
    }

    public static IEnumerable<IEnumerable<T>> Split<T>(this IEnumerable<T> source, int size)
    {
        var i = 0;
        return
            from element in source
            group element by i++ / size into splitGroups
            select splitGroups.AsEnumerable();
    }
}

How to use:

var list = new List<Widget> { new Widget { Name = "One" }, new Widget { Name = "Two" }, new Widget { Name = "Three" }, new Widget { Name = "Four" }, new Widget { Name = "Five" }, new Widget { Name = "Six" }, new Widget { Name = "Seven" }, new Widget { Name = "Eight" } };

var newList = list.ToFormattedList(2, args => "<div>" + args[0].Name + args[1].Name + "</div>");
var finalString = string.Join(string.Empty, newList);
// finalString = <div>OneTwo</div><div>ThreeFour</div><div>FiveSix</div><div>SevenEight</div>

However, there will be a problem if list has odd number of elements, because args[1] will throw exception on the last element. So you can do:

var newList = list.ToFormattedList(2, args => "<div>" + string.Join(" ", args.Select(arg => arg.Name)) + "/<div>");