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C# HashSet Examples

Use HashSet, an optimized set collection. Test Overlaps, SymmetricExceptWith and benchmark HashSet.
HashSet. This is an optimized set collection. It helps eliminates duplicate strings or elements in an array. It is a set that hashes its contents.Duplicates
With HashSet, we have a simple syntax for taking the union of elements in a set. This is performed in its constructor. More complex methods can be used on the HashSet.Constructor
An example. The program calls the HashSet constructor. The HashSet constructor receives a single parameter, which must implement the IEnumerable<string> generic interface.Generic Class, Method

Part 1: We create an array that contains several duplicated strings: the string "cat" is repeated 3 times.

String Literal

Part 2: We use the HashSet constructor, which takes the union of elements. It internally calls UnionWith to eliminate duplicates.

Part 3: We invoke ToArray to convert the HashSet into a new array, which may be easier to use elsewhere.

ToArray

Part 4: The program displays string arrays onto the console or as single strings using the string.Join static method.

StaticJoin
C# program that uses HashSet on duplicates using System; using System.Collections.Generic; using System.Linq; class Program { static void Main() { // Part 1: input array that contains three duplicate strings. string[] array1 = { "cat", "dog", "cat", "leopard", "tiger", "cat" }; // Part 2: use HashSet constructor to ensure unique strings. var hash = new HashSet<string>(array1); // Part 3: convert to array of strings again. string[] array2 = hash.ToArray(); // Part 4: display the resulting array. Console.WriteLine(string.Join(",", array2)); } } Output cat,dog,leopard,tiger
Overlaps. This method returns true or false. It tests to see if any of the HashSet's elements are contained in the IEnumerable argument's elements. Only one equal element is required.IEnumerable

Next: The element 3 is in the HashSet. This means Overlaps returns true for array2, but false for array3.

C# program that uses Overlaps using System; using System.Collections.Generic; class Program { static void Main() { int[] array1 = { 1, 2, 3 }; int[] array2 = { 3, 4, 5 }; int[] array3 = { 9, 10, 11 }; HashSet<int> set = new HashSet<int>(array1); bool a = set.Overlaps(array2); bool b = set.Overlaps(array3); // Display results. Console.WriteLine(a); Console.WriteLine(b); } } Output True False
SymmetricExceptWith. HashSet has advanced set logic. SymmetricExceptWith changes HashSet so that it contains only the elements in one or the other collection—not both.

Var: This example shows the use of the var-keyword. This simplifies the syntax of the HashSet declaration statement.

Var
C# program that uses SymmetricExceptWith using System; using System.Collections.Generic; using System.Linq; class Program { static void Main() { char[] array1 = { 'a', 'b', 'c' }; char[] array2 = { 'b', 'c', 'd' }; var hash = new HashSet<char>(array1); hash.SymmetricExceptWith(array2); // Write char array. Console.WriteLine(hash.ToArray()); } } Output ad
Benchmark. Is there any performance benefit to using HashSet instead of Dictionary? In the C# language, a Dictionary with bool values can work as a set.

Version 1: We test a HashSet(string). We add strings as keys and see if those keys exist.

Version 2: We use the Dictionary generic collection instead of a Hashset, and perform the same steps otherwise.

Result: The Dictionary had slightly better performance in this test than did the HashSet. In most tests the Dictionary was faster.

Thus: Dictionary should be used instead of HashSet in places where advanced HashSet functionality is not needed.

C# program that times HashSet performance using System; using System.Collections.Generic; using System.Diagnostics; class Program { const int _max = 10000000; static void Main() { var h = new HashSet<string>(StringComparer.Ordinal); var d = new Dictionary<string, bool>(StringComparer.Ordinal); var a = new string[] { "a", "b", "c", "d", "longer", "words", "also" }; var s1 = Stopwatch.StartNew(); // Version 1: use HashSet. for (int i = 0; i < _max; i++) { foreach (string s in a) { h.Add(s); h.Contains(s); } } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: use Dictionary. for (int i = 0; i < _max; i++) { foreach (string s in a) { d[s] = true; d.ContainsKey(s); } } s2.Stop(); Console.WriteLine(h.Count); Console.WriteLine(d.Count); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); Console.Read(); } } Output 7 7 529.99 ns HashSet 517.05 ns Dictionary
Dictionary. Set logic can also be implemented by using a Dictionary instead of a HashSet. With a Dictionary you must specify a value type. This may lead to more confusing code.Dictionary
Allocations. Using Dictionary and HashSet results in allocations on the managed heap. For small source inputs, the HashSet and Dictionary will be slower than simple nested loops.

But: When the source input becomes large with thousands of elements, hashed collections are faster.

Dictionary vs. List
A summary. HashSet can be applied to elegantly eliminate duplicates in an array. Its constructor takes a union of a collection that implements the IEnumerable generic interface.
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