Working with HashSet (Sets)

In this lesson you will learn how to hold duplicate-free data with HashSet in C#, and understand the basics of the set data structure. This is for anyone searching for "C# HashSet how to use".

HashSet<T> is a box that refuses to hold the same value twice. It is handy for stripping duplicates out of an array, or for checking membership quickly.

The sample code builds a set from the array {1, 2, 2, 3, 3, 3} with new HashSet<int>(nums); the duplicates disappear automatically and uniqueNums keeps only 1, 2, and 3. Notice how Contains() checks whether a value is present.

A common stumbling block for beginners is that the order of a HashSet is not guaranteed. Unlike an array, items are not necessarily kept in insertion order, so an array or List is a better fit when order matters.

In real-world development, HashSet's Contains() also outperforms a List's Contains() on larger data sets. SortedSet is a similar structure that does keep its contents ordered.

📖 Reference code

int[] nums = { 1, 2, 2, 3, 3, 3 };
var uniqueNums = new HashSet<int>(nums);
Console.WriteLine(string.Join(", ", uniqueNums));
Console.WriteLine(uniqueNums.Contains(2));

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