Linq to Dictionary with key and a list

Question

My data is as under in two tables

Master Columns 
ID    MyDateTime 
1     07 Sept
2     08 Sept

MyDatetime column in above has unique index

Detail Columns
ID     Data1     Data2    Data3
1      a         b        c 
1      x         y        z
1      f         g        h
2      a         b        c

I want to populate this in a dictionary. I have tried

Dictionary<DateTime, List<Detail>> result = (
                          from master in db.master
                          from details in db.detail
                          where (master.ID == detail.ID)
                          select new
                          {
                              master.MyDateTime,
                              details
                          }).Distinct().ToDictionary(key => key.MyDateTime, value => new List<Detail> { value.details });

I expect two rows in the dictionary

1, List of 3 rows as details
2, List of 1 row as details

I get an error where it complains about the key of the dictionary entered twice. The key would be the datetime which is unique in the master

Answer 1

This is precisely what lookups are for - so use ToLookup instead of ToDictionary:

ILookup<DateTime, Detail> result =
    (from master in db.master
     join details in db.detail on master.ID equals detail.ID
     select new { master.MyDateTime, details })
    .ToLookup(pair => pair.MyDateTime, pair => pair.details);

(You shouldn't need to use Distinct, and note the use of a join instead of a second from and a where clause.)