LINQ - Convert List to Dictionary with Value as List

Solution 1:

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());

Solution 2:

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);