C# Combine Dictionary Keys

Combine dictionary keys. The keys of two Dictionaries can be combined. We union all the keys in several Dictionaries. This yields a collection of every key in the Dictionaries that we can display. We use the HashSet class for the best results.

Key collections:

cat, dog         [1]
cat, man         [2]
sheep, fish, cat [3]


cat, dog, man, sheep, fish

Example. To start, we combine Dictionaries with the above keys using HashSet. The solution uses HashSet's UnionWith instance method. This combines the HashSet with another collection of the same value type.

Note: This is a complete console application that you can run in Visual Studio.

C# program that uses HashSet

using System;
using System.Collections.Generic;

class Program
    static void Main()
	// A. First dictionary.
	Dictionary<string, int> d1 = new Dictionary<string, int>();
	d1.Add("cat", 1);
	d1.Add("dog", 2);

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

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

	// D. Get union of all keys.
	HashSet<string> h1 = new HashSet<string>(d1.Keys);

	// E. Display all the keys.
	// You can look up the values in the loop body.
	foreach (string k in h1)



In this example, we populate dictionaries with the strings. The approach in this article would work on Dictionaries with any value type. And in part D, a new HashSet is created from the Keys collection of type string.

Then: This HashSet collection is unioned on the other two Dictionary Keys properties.

Discussion. MSDN provides some examples of HashSet use. Here is the description of UnionWith: "Modifies the current HashSet(T) object to contain all elements that are present in both itself and in the specified collection."

UnionWith Method: MSDN

This code is fairly fast, but the performance has not been extensively tested. What the code does is clear. If you didn't use HashSet, you would have to write more code. You could refactor the HashSet code into a method.

In my application, I had three Dictionaries, which were built from three different sources. I needed them to be separate. But I wanted to display them all together in an ASP.NET webpage displayed as HTML. The HashSet code was ideal.

Summary. We use HashSet with Dictionary keys to combine collections and find the union of the keys. HashSet contains the ideal logic for combining Dictionary keys. In school, professors teach us about unions and sets. Most of us forget.

But: The HashSet type comes to our rescue with its excellent and fast UnionWith method.