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C# Combine Dictionary KeysCombine, or union, all the keys from several Dictionary instances. The keys must be of the same type.
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.
Dictionary
Example program. The solution uses HashSet's UnionWith instance method. This combines the Hash Set with another collection of the same value type.
HashSet
Strings We populate dictionaries with strings. The approach in this article would work on Dictionaries with any value type.
UnionWith A new HashSet is created from the Keys. This HashSet is unioned on the other 2 Dictionary Keys properties.
C# program that uses HashSet
using System; using System.Collections.Generic; class Program { static void Main() { // First dictionary. Dictionary<string, int> d1 = new Dictionary<string, int>(); d1.Add("cat", 1); d1.Add("dog", 2); // Second dictionary. Dictionary<string, int> d2 = new Dictionary<string, int>(); d2.Add("cat", 3); d2.Add("man", 4); // Third dictionary. Dictionary<string, int> d3 = new Dictionary<string, int>(); d3.Add("sheep", 5); d3.Add("fish", 6); d3.Add("cat", 7); // Get union of all keys. HashSet<string> h1 = new HashSet<string>(d1.Keys); h1.UnionWith(d2.Keys); h1.UnionWith(d3.Keys); // 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.
And If you didn't use HashSet, you would have to write more code. You could refactor the HashSet code into a method.
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.
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