LINQ – Convert List to Dictionary with Value as List

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());