Combine Dictionary keys

Sometimes in C# programs we want to combine the keys of 2 Dictionaries. We can even union all the keys in several Dictionaries.

Method result

HashSet yields a collection of every key in the Dictionaries that we can display. We use the HashSet class for the best results.

Example program

The solution uses HashSet's UnionWith instance method. This combines the Hash Set with another collection of the same value type.

using System;
using System.Collections.Generic;

// Part 1: create dictionaries.
Dictionary<string, int> d1 = new Dictionary<string, int>();
d1.Add("cat", 1);
d1.Add("dog", 2);

Dictionary<string, int> d2 = new Dictionary<string, int>();
d2.Add("cat", 3);
d2.Add("man", 4);

Dictionary<string, int> d3 = new Dictionary<string, int>();
d3.Add("sheep", 5);
d3.Add("fish", 6);
d3.Add("cat", 7);

// Part 2: get union of all keys.
HashSet<string> h1 = new HashSet<string>(d1.Keys);
h1.UnionWith(d2.Keys);
h1.UnionWith(d3.Keys);

// Part 3: display all the keys.
// ... You can look up the values in the loop body.
foreach (string k in h1)
{
    Console.WriteLine(k);
}
cat
dog
man
sheep
fish

A discussion

This code is fairly fast, but the performance has not been extensively tested. What the code does is clear. The UnionWith has a known effect.

Notes, built-in methods

Often we can solve a problem in C# with custom looping code. But built-in methods, like UnionWith on HashSet, tend to be more maintainable.

A summary

We use Hash Set with Dictionary keys to combine collections and find the union of the keys. HashSet has the ideal logic here.