Deferred Execution - Dictionary Output

I have a Linq query that returns about half a million rows. However, since I did not call ToList (), execution is still pending. So far so good.

I need the output Dictionary<int, List<DateTime>>

from this query that will return about 100,000 entries (Column Integer Values) in the dictionary.

I've tried something like this ...

var myDictionary = new Dictionary<int, List<DateTime>>();

var myDictionaryOutput = MyQuery.Select(p => AddOrUpdateDictionary(myDictionary, p));


Where MyQuery is my linq query and AddOrUpdateDictionary is a custom function I wrote.

In this project every dictionary entry which is not what I want.

I only need the output of the dictionary.

Sample data looks like this:

1234 | 2015-08-24 | 2015-08-25 | null       |
1234 | null       | 2015-08-26 | null       |
2345 | null       | null       | 2015-08-23 |


I want the output to be a Dictionary with two keys

(2 keys of 3 lines <=> 100,000 keys of 500,000 lines),

1234 -> List<DateTime>{2015-08-24, 2015-08-25, 2015-08-26}
2345 -> List<DateTime>{2015-08-23}


My thoughts are evaluating MyQuery.ToList () and passing it to my custom function would not be very efficient as then I would have to iterate over half a million rows using ForEach for example.

Am I wrong or is there a better way to achieve what I want?


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1 answer

It sounds like you want to make a group with null

s filtering and then converting to a dictionary:

var dict = MyQuery
    .Select(row => new {row.Id, Dates = new[] {row.Date1, row.Date2, row.Date3}})
    .SelectMany(row => row.Dates.Select(d => new {row.Id, Date=d}))
    .GroupBy(row => row.Id)
    .ToDictionary(g => g.Key, g => g.Select(r => r.Date).ToList());




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