LINQ - Convert List to Dictionary with Value as List

Question

I have a

List<MyObject>

that I retrieve from the database. However, I would like it keyed by a property in MyObject for grouping purposes. What is the best way with LINQ to cast my list to:

Dictionary<long, List<MyObject>>

I have the following:

myObjectList.ToDictionary(x => x.KeyedProperty)

But it returns:

Dictionary<long, MyObject>

You want the dictionary keyed to a propery of WHICH MyObject? -- you have a whole list of them.... — James Curran
– James Curran, Commented Aug 23, 2010 at 15:41

JaredPar · Accepted Answer · 2010-08-23 15:40:14Z

256

It sounds like you want to group the MyObject instances by KeyedProperty and put that grouping into a Dictionary<long,List<MyObject>>. If so then try the following

List<MyObject> list = ...;
var map = list
  .GroupBy(x => x.KeyedProperty)
  .ToDictionary(x => x.Key, x => x.ToList());

answered Aug 23, 2010 at 15:40

JaredPar

759k152 gold badges1.3k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

casperOne · Accepted Answer · 2012-11-20 15:15:43Z

20

You should use the ToLookup extension method on the Enumerable class like so:

List<MyObject> list = ...;

ILookup<long, MyObject> lookup = list.ToLookup(o => o.KeyedProperty);

If you want to place that in a dictionary, then you could use the ToDictionary extension method, like so:

IDictionary<long, IEnumerable<MyObject>> dictionary = lookup.ToDictionary(
    l => l.Key);

edited Nov 20, 2012 at 15:15

answered Aug 23, 2010 at 15:40

casperOne

74.7k19 gold badges189 silver badges262 bronze badges

4 Comments

PatPeter Over a year ago

What's the difference between this and the accepted answer? Does the ILookup preserve references whereas the accepted answer does not?

casperOne Over a year ago

@PatPeter The idea is that when you have a one-to-many relationship between the key and the items, a Lookup<TKey, TValue> may be a better structure than a Dictionary<TKey, TValue>.

gbjbaanb Over a year ago

@PatPeter i would say the ILookup is the better structure to use (its like multimap to dictonary's map, if you know C++ collection algorithms) but it has one big factor to bear in mind - the lookup created is immutable. You cannot add or remove entries afterwards.

Jeff B Over a year ago

The other consideration is that even though ILookup is often the ideal in-code structure, it often isn't handled well for serialization.

Collectives™ on Stack Overflow

LINQ - Convert List to Dictionary with Value as List

2 Answers 2

Comments

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related