Adding a dictionary to another

Question

Possible Duplicates:
Merging dictionaries in C#
What's the fastest way to copy the values and keys from one dictionary into another in C#?

I have a dictionary that has some values in it, say:

Animals <string, string>

I now receive another similar dictionary, say:

NewAnimals <string,string>

How can I append the entire NewAnimals dictionary to Animals?

score 125 · Accepted Answer · edited Jun 01 '17 at 14:11

125

foreach(var newAnimal in NewAnimals)
    Animals.Add(newAnimal.Key,newAnimal.Value)

Note: this throws an exception on a duplicate key.

Or if you really want to go the extension method route(I wouldn't), then you could define a general AddRange extension method that works on any ICollection<T>, and not just on Dictionary<TKey,TValue>.

public static void AddRange<T>(this ICollection<T> target, IEnumerable<T> source)
{
    if(target==null)
      throw new ArgumentNullException(nameof(target));
    if(source==null)
      throw new ArgumentNullException(nameof(source));
    foreach(var element in source)
        target.Add(element);
}

(throws on duplicate keys for dictionaries)

edited Jun 01 '17 at 14:11

Henningsson

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answered Oct 20 '10 at 21:28

CodesInChaos

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No problem. Duplicate keys are not a worry. – Ayush Oct 20 '10 at 21:42
I suggest you take a look at my solution and implement `AddRange`. I did all the work for you. – Gabe Oct 20 '10 at 21:46
@gmcalab: I do appreciate it, however this solution works just fine for me and is a lot smaller. I was really looking for something tiny that does the job. – Ayush Oct 21 '10 at 12:07
what kind of exception does it throw? just a plain Exception? – Netorica Jun 03 '15 at 14:52
1

@Mahan MSDN for the [`Dictionary.Add` Method](https://msdn.microsoft.com/en-us/library/k7z0zy8k.aspx) says `ArgumentException` - An element with the same key already exists in the `Dictionary`. – CodesInChaos Jun 03 '15 at 16:02
1

Why don't we do this to avoid ArguementException? `foreach ( var property in anotherProperties ) properties[property.Key] = property.Value;` – Brian Hong May 19 '21 at 19:20
Why wouldn't you want to write an extention method? – Enrico Oct 06 '22 at 09:02

Gabe · Answer 2 · 2012-02-12T18:20:22.243

42

Create an Extension Method most likely you will want to use this more than once and this prevents duplicate code.

Implementation:

 public static void AddRange<T, S>(this Dictionary<T, S> source, Dictionary<T, S> collection)
 {
        if (collection == null)
        {
            throw new ArgumentNullException("Collection is null");
        }

        foreach (var item in collection)
        {
            if(!source.ContainsKey(item.Key)){ 
               source.Add(item.Key, item.Value);
            }
            else
            {
               // handle duplicate key issue here
            }  
        } 
 }

Usage:

Dictionary<string,string> animals = new Dictionary<string,string>();
Dictionary<string,string> newanimals = new Dictionary<string,string>();

animals.AddRange(newanimals);

edited Feb 12 '12 at 18:20

answered Oct 20 '10 at 21:40

Gabe

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Why don't you type the right parameter as `IEnumerable>`? And why do you use only one generic parameter for key and value? – CodesInChaos Oct 20 '10 at 21:52
@CodesInChaos, what the point to have second argument as IEnumerable>? Particular use cases? – vkelman Oct 25 '17 at 15:20
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I added public enum OnDuplicateKey { Throw, Skip, Replace } – vkelman Oct 25 '17 at 15:22
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@vkelman 1) It's satisfied by any dictionary 2) I sometimes produce sequences via Linq that I then want to add to a dictionary. – CodesInChaos Oct 25 '17 at 15:24

Ani · Answer 3 · 2010-10-20T21:39:29.750

13

The most obvious way is:

foreach(var kvp in NewAnimals)
   Animals.Add(kvp.Key, kvp.Value); 
  //use Animals[kvp.Key] = kvp.Value instead if duplicate keys are an issue

Since Dictionary<TKey, TValue>explicitly implements theICollection<KeyValuePair<TKey, TValue>>.Addmethod, you can also do this:

var animalsAsCollection = (ICollection<KeyValuePair<string, string>>) Animals;

foreach(var kvp in NewAnimals)
   animalsAsCollection.Add(kvp);

It's a pity the class doesn't have anAddRangemethod likeList<T> does.

edited Oct 20 '10 at 21:39

answered Oct 20 '10 at 21:28

Ani

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agreed. AddRange is convenient. – Ayush Oct 20 '10 at 21:41
That's why you write an extension method, which is the best solution to this question but not chosen by the OP. Spaghetti code ahoy! – Gabe Oct 20 '10 at 21:46

score 3 · Answer 4 · edited May 23 '17 at 12:18

3

The short answer is, you have to loop.

Adding a dictionary to another

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