Creating a ToDictionaryList Extension

ToDictionary

.Net 3.5 introduced the Enumerable.ToDictionary Method. It allows you to take a list of items, and convert them into a dictionary based on some key given as a lambda expression. For example, this code snippet for Snippet Compiler will put a weeks worth of dates into a Dictionary<int,DateTime>, so you could do a dictionary lookup by day:

public static void RunSnippet()
{
   DateTime time = new DateTime(2228, 3, 22);
   List<DateTime> items = new List<DateTime>();
   items.Add(time);
   items.Add(time.AddDays(1));
   items.Add(time.AddDays(2));
   items.Add(time.AddDays(3));
   items.Add(time.AddDays(4));
   items.Add(time.AddDays(5));
   items.Add(time.AddDays(6));

   var dayByDay = items.ToDictionary(d => d.Day);
   foreach (var dayKeyValue in dayByDay)
   {
       WL(dayKeyValue.Key + " " + dayKeyValue.Value);
   }
}

Which results in this output:

22 3/22/2228 12:00:00 AM
23 3/23/2228 12:00:00 AM
24 3/24/2228 12:00:00 AM
25 3/25/2228 12:00:00 AM
26 3/26/2228 12:00:00 AM
27 3/27/2228 12:00:00 AM
28 3/28/2228 12:00:00 AM
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ToDictionaryList

But what if the key you want to use, is not unique? You’ll end up getting this warning:

System.ArgumentException: An item with the same key has already been added.
at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)

I had this same issue and decided to create a ToDictionaryList() extension method that would return a Dictionary<key,List<item>>. Here is an example using the ToDictionaryList() grouping the dateTimes by day:

public static void RunSnippet()
{
   DateTime time = new DateTime(2228, 3, 22);
   List<DateTime> items = new List<DateTime>();
   items.Add(time);
   items.Add(time.AddHours(1));
   items.Add(time.AddMinutes(1));
   items.Add(time.AddDays(1));
   items.Add(time.AddHours(26));

   var results = items.ToDictionaryList(d => d.Day);
   foreach (var result in results)
   {
       foreach (var item in result.Value)
       {
           WL(result.Key + " " + item);
       }
   }
}

Which results in this output:

22 3/22/2228 12:00:00 AM
22 3/22/2228 1:00:00 AM
22 3/22/2228 12:01:00 AM
23 3/23/2228 12:00:00 AM
23 3/23/2228 2:00:00 AM
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Basically it is two dictionary entries, one for the 22nd which is a List<DateTime> of 3 DateTimes, and one for the 23, which contains two items. You can even write a lambda expression for the type of item to be in the list. So in the last example, if you changed "var results = items.ToDictionaryList(d => d.Day);" to "var results = items.ToDictionaryList(d => d.Day, d => d.Hour); " item would in int and which would result in this output:

22 0
22 1
22 0
23 0
23 2
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The Code

Here is the code required to make the ToDictionaryList Possible:

public static class Extensions
{
   private static Func<TElement, TElement> Instance<TElement>()
   {
       return delegate(TElement x) { return x; };
   }

   public static Dictionary<TKey, List<TSource>> ToDictionaryList<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
   {
       return source.ToDictionaryList(keySelector, Instance<TSource>());
   }

   public static Dictionary<TKey, List<TElement>> ToDictionaryList<TSource, TKey, TElement>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TElement> elementSelector)
   {
       Dictionary<TKey, List<TElement>> dictionary = new Dictionary<TKey, List<TElement>>();
       List<TElement> elements;
       foreach (TSource local in source)
       {
           if (!dictionary.TryGetValue(keySelector(local), out elements))
           {
               elements = new List<TElement>();
               dictionary.Add(keySelector(local), elements);
           }
           elements.Add(elementSelector(local));
       }

       return dictionary;
   }
}

The Instance Method is used to return a delegate function that returns a single item of the given type (Basically a (c => c)). The ToDictionaryList() will loop through all items in the list, adding them to a new list if the key doesn’t already exist in the dictionary, or add the item to the already existing list in the dictionary. Pretty simple really.