search arraylist by property using binarysearch

Question

Hi,

I currently have an arraylist containing classes in C#. The arraylist is filled like this:

 foreach (XmlElement Path in locaties)
                    {


                        ISoundSource track = engine.AddSoundSourceFromFile(Path.InnerXml);

                        mixarray.Add(track);

                    }

then the array has many ISoundSource classes as its items. Now the thing that sets them apart in the array is their 'name' property. Later on I want to get the ISoundSource from the array by doing a search. I looked up on how to search arraylists and it is said to use a binarysearch but I don't see a way to look up an object with a certain property. How can I get the item from the array which has the name I specify?

Answer 1

You should probably use a Dictionary<,> as it will be much easier to maintain. Also, you should use List<> instead of ArrayList. If you must use BinarySearch, you will have to pass it a custom implementation of IComparer in order to have it use the Name property. Here's an example with a dictionary:

var dictionary = new Dictionary<string, ISoundSource>();
foreach (XmlElement Path in locaties)
{
    ISoundSource track = engine.AddSoundSourceFromFile(Path.InnerXml);
    mixarray.Add(track);
    dictionary[track.Name] = track;
}

ISoundSource item = dictionary["MyTrackName"];

Answer 2

Check out the two parameter overload of BinarySearch which takes an IComparer as the second parameter - you then need to create a small class that inherits from IComparer that will compare the names of two of your Track objects, and pass an instance of this comparer into the BinarySearch.

Answer 3

There are many ways to do what you're asking for, and the right way depends on information that you haven't provided:

• Does the Name property uniquely identify items?
• Does every item have a Name?
• Does the match have to be exact?
• Is it important to know what order the items were originally added to the list in, i.e. the order that they appear in the source XML?
• Are you trying to find items given their Name, or access them in order by their Name?
• How important is it that this be efficient?

It may be that the right solution is to simply use LINQ to find an item:

ISoundSource track = mixarray
   .Cast<ISoundSource>
   .Where(x => x.Name == name)
   .FirstOrDefault();

which will set track to the first item in the list whose name matches the value you're looking for, and to null if there's no match found. (If you use a List<ISoundSource> instead of an ArrayList, you can omit the Cast<ISoundSource> - one of many, many reasons to use List<T> over ArrayList in most cases.)

Most of the time I'll use a Dictionary<TKey, TValue> for this kind of thing, but that's because most the time the answers to those questions are yes, yes, yes, no, don't care about the order, pretty important.

Answer 4

For posterity, here is an alternative way to generate a dictionary using a simple Linq expression.

var dictionary = locaties
    .Select(p->engine.AddSoundSourceFromFile(Path.InnerXml))  
    .ToDictionary(t->t.Name);

The .Select() transforms each node into an ISoundSource. When done, a collection (IEnumerable of ISoundSource) is returned. The .ToDictionary() then converts that list of ISoundSource to a Dictionary of string, ISoundSource.

This requires .NET Framework 3.5 or higher.