Remove the specified element from a HashSet in C#

A HashSet in C# is a collection that stores unique elements without any specific order. To remove a specified element from a HashSet, you use the Remove() method, which returns true if the element was found and removed, or false if the element was not present.

Syntax

The Remove() method follows this syntax −

public bool Remove(T item)

Parameters

• item − The element to remove from the HashSet.

Return Value

The method returns true if the element was successfully found and removed; otherwise, false.

Using Remove() with String HashSet

The following example demonstrates removing a string element from a HashSet −

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main() {
      HashSet<string> set1 = new HashSet<string>();
      set1.Add("AB");
      set1.Add("CD");
      set1.Add("EF");
      set1.Add("AB");
      set1.Add("IJ");
      set1.Add("KL");
      set1.Add("EF");
      set1.Add("OP");
      
      Console.WriteLine("Elements in HashSet1");
      foreach(string val in set1) {
         Console.WriteLine(val);
      }
      
      HashSet<string> set2 = new HashSet<string>();
      set2.Add("EF");
      set2.Add("KL");
      
      Console.WriteLine("Elements in HashSet2");
      foreach(string val in set2) {
         Console.WriteLine(val);
      }
      
      Console.WriteLine("Count of elements in HashSet2 = " + set2.Count);
      
      bool removed = set2.Remove("KL");
      Console.WriteLine("Was 'KL' removed? " + removed);
      
      Console.WriteLine("\nElements in HashSet2... (UPDATED)");
      foreach (string res in set2) {
         Console.WriteLine(res);
      }
      
      Console.WriteLine("Count of elements in HashSet2 (Updated) = " + set2.Count);
      Console.WriteLine("\nIs set1 a superset of set2? " + set1.IsSupersetOf(set2));
   }
}

The output of the above code is −

Elements in HashSet1
AB
CD
EF
IJ
KL
OP
Elements in HashSet2
EF
KL
Count of elements in HashSet2 = 2
Was 'KL' removed? True

Elements in HashSet2... (UPDATED)
EF
Count of elements in HashSet2 (Updated) = 1

Is set1 a superset of set2? True

Using Remove() with Integer HashSet

Here's an example of removing integer elements from a HashSet −

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main() {
      HashSet<int> set1 = new HashSet<int>();
      set1.Add(100);
      set1.Add(200);
      set1.Add(300);
      set1.Add(400);
      set1.Add(500);
      set1.Add(600);
      
      Console.WriteLine("Elements in HashSet");
      foreach(int val in set1) {
         Console.WriteLine(val);
      }
      
      Console.WriteLine("Count of elements in HashSet = " + set1.Count);
      
      bool removed = set1.Remove(300);
      Console.WriteLine("Was 300 removed? " + removed);
      
      Console.WriteLine("\nElements in HashSet... (UPDATED)");
      foreach (int res in set1) {
         Console.WriteLine(res);
      }
      
      Console.WriteLine("Count of elements in HashSet (Updated) = " + set1.Count);
   }
}

The output of the above code is −

Elements in HashSet
100
200
300
400
500
600
Count of elements in HashSet = 6
Was 300 removed? True

Elements in HashSet... (UPDATED)
100
200
400
500
600
Count of elements in HashSet (Updated) = 5

Removing Non-Existent Elements

When attempting to remove an element that doesn't exist in the HashSet, the method returns false −

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main() {
      HashSet<string> fruits = new HashSet<string>();
      fruits.Add("Apple");
      fruits.Add("Banana");
      fruits.Add("Orange");
      
      Console.WriteLine("Original HashSet:");
      foreach(string fruit in fruits) {
         Console.WriteLine(fruit);
      }
      
      bool removed1 = fruits.Remove("Banana");
      Console.WriteLine("\nRemoved 'Banana': " + removed1);
      
      bool removed2 = fruits.Remove("Grape");
      Console.WriteLine("Removed 'Grape': " + removed2);
      
      Console.WriteLine("\nFinal HashSet:");
      foreach(string fruit in fruits) {
         Console.WriteLine(fruit);
      }
   }
}

The output of the above code is −

Original HashSet:
Apple
Banana
Orange

Removed 'Banana': True
Removed 'Grape': False

Final HashSet:
Apple
Orange

Conclusion

The Remove() method in C# HashSet efficiently removes a specified element and returns a boolean indicating success. It provides O(1) average time complexity for removal operations, making HashSet ideal for scenarios requiring fast element lookups and removals.