Combine multiple dictionaries into a single dictionary

Question

Possible Duplicate:
Merging dictionaries in C#

dictionary 1

"a", "1"
"b", "2"

dictionary 2

"c", "3"
"d", "4"

dictionary 3

"e", "5"
"f", "6"

Combined dictionary

"a", "1"
"b", "2"
"c", "3"
"d", "4"
"e", "5"
"f", "6"

How do I combine the above 3 dictionaries into a single combined dictionary?

Sani..I created a class level private variable and then call another private method 1 dictionary at a time to add them to combined method. It does not look clean so I am asking for a better method here. — dotnet-practitioner, May 11 '12 at 22:27
Can you show how your dictionaries are currently represented in the code? — EtherDragon, May 11 '12 at 22:30

Magnus · Accepted Answer · 2012-05-12T07:29:18.583

152

var d1 = new Dictionary<string, int>();
var d2 = new Dictionary<string, int>();
var d3 = new Dictionary<string, int>();

var result = d1.Union(d2).Union(d3).ToDictionary (k => k.Key, v => v.Value);

EDIT
To ensure no duplicate keys use:

var result = d1.Concat(d2).Concat(d3).GroupBy(d => d.Key)
             .ToDictionary (d => d.Key, d => d.First().Value);

edited May 12 '12 at 07:29

answered May 11 '12 at 22:31

Magnus

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Note that `ToDictionary` will throw exception on duplicates – dtb May 11 '12 at 22:53
@dtb there wont be any duplicates when used with `union` as in my answer. – Magnus May 11 '12 at 22:56
26

@Magnus: `Union` removes duplicates only if both key and value of the KeyValuePair are equal. If you have e.g. `var d1 = new Dictionary() { { "a", 5 } }; var d2 = new Dictionary() { { "a", 6 } };` then you get duplicate keys. – dtb May 12 '12 at 00:30
What would the solution be if you had an unknown number of Dictionaries you were trying to merge? Is there a way to take a List of dictionaries and merge them in the same way? thank you – Paul W. May 05 '17 at 23:28
3

@PaulW. You could do: `dictionaries.SelectMany(x => x).ToDictionary(x => x.Key, x => x.Value)` – Magnus May 08 '17 at 07:04

Ry- · Answer 2 · 2018-06-17T13:39:14.750

15

Just loop through them:

var result = new Dictionary<string, string>();

foreach (var dict in dictionariesToCombine) {
    foreach (var item in dict) {
        result.Add(item.Key, item.Value);
    }
}

(Assumes dictionariesToCombine is some IEnumerable of your dictionaries to combine, say, an array.)

edited Jun 17 '18 at 13:39

answered May 11 '12 at 22:27

Ry-

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thanks minitech, how do I createe IEnumerable of dictionaries? – dotnet-practitioner May 11 '12 at 22:29
`new Dictionary[] { your, dictionaries, here }` – Ry- May 11 '12 at 22:31
7

Note that `Add` will throw exception on duplicates – Magnus May 11 '12 at 22:47
if (dict.ContainsKey(item.Key)) dict[item.Key] = item.Value; else dict.Add(item.Key, item.Value); – Deantwo Jan 17 '18 at 08:47
@abatishchev That doesn’t do the same thing on duplicates. – Ry- Jun 17 '18 at 13:39
@Ry-: this would overwrite duplicates, indeed. But you're right, it depends on what behavior author wants. – abatishchev Jun 17 '18 at 23:48
This is much more computationally efficient than the accepted answer. – Mike Mar 06 '21 at 00:30

Combine multiple dictionaries into a single dictionary

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