Simplest way to add all content of Dictionary to another Dictionary (equal types for key & value)?

Question

Is there a simpler way to copy/add all content of source dictionary to destination Dictionary<T1, T2> object than using one of the two options in the sample below?

Dictionary<string, int> source = new Dictionary<string, int>(),
    destination = new Dictionary<string, int>();

source.Add("Developers", 1);
source.Add("just", 2);
source.Add("wanna have", 3);
source.Add("FUN!", 4);

// Option 1 (feels like a hack):
//
source.All(delegate(KeyValuePair<string, int> p)
{
    destination.Add(p.Key, p.Value);
    return true;
});

// Option 2:
//
foreach (string k in source.Keys)
{
    destination.Add(k, source[k]);
}

What I was looking for is something like .ForEach().

You mean like `List.ForEach`? [Be careful what you wish for..](https://blogs.msdn.microsoft.com/ericlippert/2009/05/18/foreach-vs-foreach/) — stuartd, Mar 17 '16 at 12:45
Option 2 is about as simple as you can get. You have a loop condition, and one statement within the loop. Don't go complicating it by trying to shoehorn `ForEach`, and don't pass a delegate to a query method (`All`) that has side-effects. — D Stanley, Mar 17 '16 at 12:47
See [this question](http://stackoverflow.com/questions/6050633) for some arguments for and against a `ForEach` method, and some extension methods that may work. — D Stanley, Mar 17 '16 at 12:50

Tim Schmelter · Accepted Answer · 2016-03-17T12:48:59.513

5

Yes, you can use the constructor:

Dictionary<string, int> destination = new Dictionary<string, int>(source);

If destination is already filled and you don't want to lose them as commented i'd use this:

foreach (var kv in source)
    if (!destination.ContainsKey(kv.Key))
        destination.Add(kv.Key, kv.Value);

edited Mar 17 '16 at 12:48

answered Mar 17 '16 at 12:44

Tim Schmelter

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1

What if there are already key/value pairs inside `destination` you don't want to loose? – RhinoDevel Mar 17 '16 at 12:45
Here's the constructor documentation https://msdn.microsoft.com/en-us/library/et0ke8sz%28v=vs.110%29.aspx – Will Murphy Mar 17 '16 at 12:45
@RhinoDevel: that's a different question and both of your options won't work either in that case if there are duplicates. – Tim Schmelter Mar 17 '16 at 12:46
I don't want to create a copy of `source`, I want to copy/add all content of `source` to `destination`. – RhinoDevel Mar 17 '16 at 12:47
1

@RhinoDevel: look at my second approach. This is the easiest and most efficient appraoch if you want to add all "new". – Tim Schmelter Mar 17 '16 at 12:50

Alexander Derck · Answer 2 · 2016-03-17T13:02:08.470

1

You can use Concat:

Dictionary<string, int> dict1 = new Dictionary<string, int>();
dict1.Add("first", 1);
Dictionary<string, int> dict2 = new Dictionary<string, int>();
dict2.Add("second", 2);

dict2 = dict2.Concat(dict1).ToDictionary(k => k.Key, v => v.Value);

This of course doesn't really 'add' it to the dictionary, but replaces it by a new one. You can use Union to get rid of duplicates.

edited Mar 17 '16 at 13:02

answered Mar 17 '16 at 12:52

Alexander Derck

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Throws exception on duplicate keys and isn't efficient. – Tim Schmelter Mar 17 '16 at 12:57
1

@TimSchmelter Best option is just using a foreach like in OP's question but this is just an alternative. – Alexander Derck Mar 17 '16 at 12:59

score 0 · Answer 3 · answered Mar 17 '16 at 12:56

static void Main()
{
    Dictionary<string, int> source = new Dictionary<string, int>();
    Dictionary<string, int> destination = new Dictionary<string, int>();
    destination.Add("Apple", 1);
    destination.Add("Banana", 2);
    foreach (var item in destination)
    {
        source.Add(item.Key, item.Value);
    }
    foreach (var item in destination)
    {
        Console.WriteLine("Key is {0} and value is {1}", item.Key, item.Value);
    }       
}

Simplest way to add all content of Dictionary to another Dictionary (equal types for key & value)?

3 Answers3