How to store and lookup data, based on multiple xml attributes?

Question

I am querying an xml file and returning 3 attributes per selection (each entry that meets my criteria will return 3 attributes details). I need to store these values, and then later look up the first attribute, and return the 2 other stored attributes related to it.

var items = from item in doc.Descendants("SUM")
                        select new
                        {                                    
                            id = (string)item.Attribute("id"),
                            category = (string)item.Attribute("cat"),
                            selection = (string)item.Attribute("sel")
                        };

The above code returns 3 attributes per item found. I need to store those 3 entries so they are associated together, then later perform a lookup on the stored entries. So example, I need to be able to look up a stored value of id=1, and return the corresponding category and selection entry.

I was researching the Lookup method of C# but don't understand how to use it. List's seem like they might work, but I don't know how to store multiple pieces of data into an entry in a list (maybe concatenate into a single entry, but then I'm not sure about performing the lookup on it). Any suggestions regarding how to do this with a LIST or LOOKUP (or other unmentioned way) are appreciated.

Answer 1

You can use Where (or other options, such as FirstOrDefault, etc)to filter this further:

var items = from item in doc.Descendants("SUM")
                    select new
                    {                                    
                        Id = (string)item.Attribute("id"),
                        Category = (string)item.Attribute("cat"),
                        Selection = (string)item.Attribute("sel")
                    };

var filtered = items.Where(i => i.Id == 1);

// Then use these as needed
foreach(var item in filtered)
{
    Console.WriteLine("Cat: {0}, Sel: {1}", item.Category, item.Selection);
}

The ToLookup method actually serves a very different purpose. It builds a data structure which implements ILookup<T,U>, which is a lookup table where you can easily return all of the items that match a specific key. This is useful if you're going to perform many lookups from your data, but not if you just want to "lookup" items matching a single value.

Answer 2

First step is to create a class to store the data:

public class Item // come up with a better name...
{
    public string ID {get; set;}
    public string Catagory {get; set;}
    public string Selection {get; set;}
}

Second, if your lookups are always by ID, you could store the collection in a Dictionary<string, Item> and to lookups with the indexer property:

// add
var dict = (from item in doc.Descendants("SUM")
            select new  Item
            {                                    
                ID = (string)item.Attribute("id"),
                Category = (string)item.Attribute("cat"),
                Selection = (string)item.Attribute("sel")
            })
            .ToDictionary(i=>i.ID, i=>i);
// lookup
Item foundItem = dict[lookupID];

If your lookups need to be more generic then just store them in a List<Item> and do lookups with Linq and lambda functions:

List<Item> myList = new List<Item>();

// add items
List.Add(item);

// lookup one
Item item = myList.Single(i => i.ID == lookupID);
// lookup many
var items = myList.Where(i => i.Category == lookupCategory);