Allow duplicate keys with ToDictionary() from LINQ query

Question

I need the Key/Value stuff from a Dictionary. What I do not need is that it does not allow duplicate Keys.

Regex template = new Regex(@"\{(?<key>.+?)\}(?<value>[^{}]*)");
IDictionary<string, string> dictionary = template.Matches(MyString)
                                             .Cast<Match>()
                                             .ToDictionary(x => x.Groups["key"].Value, x => x.Groups["value"].Value);

How can I return the Dictionary allowing duplicate keys?

http://stackoverflow.com/questions/146204/duplicate-keys-in-net-dictionaries — Alex, Mar 01 '11 at 15:02
Maybe he was testing if the Stackoverflow Dictionary allowed duplicate keys? :-P — Steven Ryssaert, Mar 01 '11 at 15:03
@Alex ToLookup does not help in my case. I need the Key/Value access for later usage... — msfanboy, Mar 01 '11 at 15:06
how do you expect the key -> value lookup to work with duplicate keys? Either it's not going to work, or you want to group by the key value first. — Massif, Mar 01 '11 at 15:13
@msfanboy: you could access values by key by using an indexer; e.g. `dictionary[key]` this will get you an object of type `IEnumerable` which holds your values. — Alex, Mar 01 '11 at 15:14
Why not just groupBy(key) and store group.ToList() as Value? — Guillaume86, Mar 01 '11 at 15:28

Alex · Accepted Answer · 2011-03-01T15:54:16.317

7

Use the Lookup class:

Regex template = new Regex(@"\{(?<key>.+?)\}(?<value>[^{}]*)");
ILookup<string, string> dictionary = template.Matches(MyString)
    .Cast<Match>()
    .ToLookup(x => x.Groups["key"].Value, x => x.Groups["value"].Value);

EDIT: If you expect to get a "plain" resultset (e.g. {key1, value1}, {key1, value2}, {key2, value2} instead of {key1, {value1, value2} }, {key2, {value2} }) you could get the result of type IEnumerable<KeyValuePair<string, string>>:

Regex template = new Regex(@"\{(?<key>.+?)\}(?<value>[^{}]*)");
ILookup<string, string> dictionary = template.Matches(MyString)
    .Cast<Match>()
    .Select(x =>
        new KeyValuePair<string, string>(
            x.Groups["key"].Value,
            x.Groups["value"].Value
        )
    );

edited Mar 01 '11 at 15:54

answered Mar 01 '11 at 15:06

Alex

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yes of course I can do this, but then I have to use the for-loop AND I can not access the Value. I can only find the Keys property ? – msfanboy Mar 01 '11 at 15:22
1

@msfanboy: the `Dictionary` maps key to value when `Lookup` maps key to one or more values. the `Dictionary` doesn't allow several values to have the same key when `Lookup` does. What kind of structure do you expect to return? – Alex Mar 01 '11 at 15:27
very good solution Alex! Exactly what I meant. I tried List> but I missed your Select :) – msfanboy Mar 02 '11 at 08:45

Allow duplicate keys with ToDictionary() from LINQ query

1 Answers1