Merging 2 dictionaries having duplicate keys with linq

Question

How to merge 2 dictionaries of IDictionary<Guid, MyObject> where MyObject is a class instance?

IDictionary<Guid, MyObject> d1 = new Dictionary<Guid, MyObject>();
d1.Add(guid1, m1);
d1.Add(guid2, m2);
d1.Add(guid3, m3);
IDictionary<Guid, MyObject> d2 = new Dictionary<Guid, MyObject>();
d2.Add(guid2, m2);
d2.Add(guid3, m3);
d2.Add(guid4, m4);
IDictionary<Guid, MyObject> d3 = d1.Union(d2) ???

That in d3 there are the following entries:

guid1,m1
guid2,m2
guid3,m3
guid4,m4

duplicate of http://stackoverflow.com/questions/294138/merging-dictionaries-in-c — hatchet - done with SOverflow, Aug 01 '11 at 19:22
possible duplicate of [Combine two Dictionaries with linq](http://stackoverflow.com/questions/4903784/combine-two-dictionaries-with-linq); more exact duplicate than the other one — Mechanical snail, Nov 21 '12 at 06:31
Duplicate of [Combine two Dictionaries with LINQ](https://stackoverflow.com/questions/4903784/combine-two-dictionaries-with-linq) — 41686d6564, Oct 05 '20 at 13:50

score 20 · Accepted Answer · edited Nov 16 '18 at 13:42

20

d1.Concat(d2.Where( x=> !d1.Keys.Contains(x.Key)));

edited Nov 16 '18 at 13:42

croxy

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answered Aug 01 '11 at 19:20

Eric H

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1

thank you very much, that is the correct approach though leading processing of the concatenated set – Chesnokov Yuriy Aug 01 '11 at 19:37
@Eric This looks like a good solution. But, if I have `IEnuemrable>` colection, how can I match items? – barteloma Aug 17 '17 at 13:10
I found union faster when d1 was small (2 records) and d2 was large (100,000 records). – DrGriff Sep 04 '19 at 09:54

agent-j · Answer 2 · 2012-11-23T21:17:38.000

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d1.Union(d2).GroupBy (kvp => kvp.Key).ToDictionary(kvp => kvp.Key, kvp => kvp.First().Value); out to do the trick.

IDictionary<Guid, MyObject> d1 = new Dictionary<Guid, MyObject>();
d1.Add(guid1, m1);
d1.Add(guid2, m2);
d1.Add(guid3, m3);
IDictionary<Guid, MyObject> d2 = new Dictionary<Guid, MyObject>();
d2.Add(guid2, m2);
d2.Add(guid3, m3);
d2.Add(guid4, m4);
IDictionary<Guid, MyObject> d3 = 
   d1.Union(d2).GroupBy (kvp => kvp.Key)
       .ToDictionary (kvp => kvp.Key, kvp => kvp.First ().Value);

edited Nov 23 '12 at 21:17

answered Aug 01 '11 at 19:20

agent-j

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thanks but I tried that and Union leads to Exception with 'An item with the same key has already been added' – Chesnokov Yuriy Aug 01 '11 at 19:32
I fixed it. ` IDictionary d3 = d1.Union(d2).GroupBy (kvp => kvp.Key) .ToDictionary (kvp => kvp.Key, kvp => kvp.First ().Value); ` – agent-j Aug 01 '11 at 22:20
thank you very much, that works, how that line avoids duplicate keys? is that faster than `d1.Concat(d2.Where(x => !d1.Keys.Contains(x.Key)))`? – Chesnokov Yuriy Aug 02 '11 at 05:27
I tested that using Stopwatch, using of Concat is 3 time faster, SO DO NOT USE Union – NET3 Nov 22 '12 at 23:00

score 1 · Answer 3 · answered May 08 '19 at 05:46

If you have duplicate key then you'll have to handle duplicate key with the using of where clause.

var result = d1.Union(d2.Where(k => !d1.ContainsKey(k.Key))).ToDictionary(k => k.Key, v => v.Value)

Note : It will not get duplicate key. if there will be any duplicate key than it will get d1's key.

score 0 · Answer 4 · answered Aug 01 '11 at 19:18

You could try something like

d1.Concat(d2).Distinct(kv => kv.Key).ToDictionary(kv => kv.Key, kv => kv.Value)

The result of concat makes use of the fact that the dictionary is an IEnumerable<KeyvaluePair<Guid,MyObject>>

Since I do not have a compiler I just checked that Distinct cannot accept just a lambda selecting the property to be compared. However it can accept an EqualityComparer. What I often have in projects is a Generic Equality Comparer that allows to pass in lambdas which define the equality operation.

score 0 · Answer 5 · answered Aug 01 '11 at 19:19

0

When no duplicates keys exist, the following works for 2 (or more) dictionaries:

var dictionaries = new [] { d1, d2 };
var result = dictionaries.SelectMany(dict => dict)
                     .ToDictionary(pair => pair.Key, pair => pair.Value);

answered Aug 01 '11 at 19:19

Chaim Zonnenberg

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score 0 · Answer 6 · answered Aug 01 '11 at 19:26

0

Union looks good: http://msdn.microsoft.com/en-us/vcsharp/aa336761.aspx#union1

answered Aug 01 '11 at 19:26

Paul Nikonowicz

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Merging 2 dictionaries having duplicate keys with linq

6 Answers6