Merging dictionaries in C#

Question

What\'s the best way to merge 2 or more dictionaries (Dictionary) in C#? (3.0 features like LINQ are fine).

I\'m thinking of a method signa

Answer 1

Option 1 : This depends on what you want to happen if you are sure that you don't have duplicate key in both dictionaries. than you could do:

var result = dictionary1.Union(dictionary2).ToDictionary(k => k.Key, v => v.Value)

Note : This will throw error if you get any duplicate keys in dictionaries.

Option 2 : If you can have duplicate key then you'll have to handle duplicate key with the using of where clause.

var result = dictionary1.Union(dictionary2.Where(k => !dictionary1.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 dictionary1's key.

Option 3 : If you want to use ToLookup. then you will get a lookup which can have multiple values per key. You could convert that lookup to a dictionary:

var result = dictionaries.SelectMany(dict => dict)
                         .ToLookup(pair => pair.Key, pair => pair.Value)
                         .ToDictionary(group => group.Key, group => group.First());

Answer 2

Merging using an extension method. It does not throw exception when there are duplicate keys, but replaces those keys with keys from the second dictionary.

internal static class DictionaryExtensions
{
    public static Dictionary<T1, T2> Merge<T1, T2>(this Dictionary<T1, T2> first, Dictionary<T1, T2> second)
    {
        if (first == null) throw new ArgumentNullException("first");
        if (second == null) throw new ArgumentNullException("second");

        var merged = new Dictionary<T1, T2>();
        first.ToList().ForEach(kv => merged[kv.Key] = kv.Value);
        second.ToList().ForEach(kv => merged[kv.Key] = kv.Value);

        return merged;
    }
}

Usage:

Dictionary<string, string> merged = first.Merge(second);