Remove all elements from a HashSet in C#

To remove all elements from a HashSet in C#, you use the Clear() method. This method removes all elements from the HashSet and resets its count to zero. The Clear() method is the most efficient way to empty a HashSet completely.

Syntax

Following is the syntax for the Clear() method −

hashSet.Clear();

Parameters

The Clear() method takes no parameters.

Return Value

The Clear() method returns void (no return value).

Using Clear() Method

Example

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main(String[] args) {
      HashSet<string> set1 = new HashSet<string>();
      set1.Add("A");
      set1.Add("B");
      set1.Add("C");
      set1.Add("D");
      set1.Add("E");
      set1.Add("F");
      set1.Add("G");
      set1.Add("H");
      
      Console.WriteLine("Elements in HashSet...");
      foreach (string res in set1) {
         Console.WriteLine(res);
      }
      
      Console.WriteLine("Count of HashSet1 = " + set1.Count);
      set1.Clear();
      Console.WriteLine("Count of HashSet1 (after Clear) = " + set1.Count);
   }
}

The output of the above code is −

Elements in HashSet...
A
B
C
D
E
F
G
H
Count of HashSet1 = 8
Count of HashSet1 (after Clear) = 0

Clearing Multiple HashSets

Example

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main(String[] args) {
      HashSet<string> set1 = new HashSet<string>();
      set1.Add("A");
      set1.Add("B");
      set1.Add("C");
      set1.Add("D");
      
      HashSet<string> set2 = new HashSet<string>();
      set2.Add("John");
      set2.Add("Jacob");
      set2.Add("Ryan");
      set2.Add("Tom");
      
      Console.WriteLine("Elements in HashSet1...");
      foreach (string res in set1) {
         Console.WriteLine(res);
      }
      
      Console.WriteLine("Elements in HashSet2...");
      foreach (string res in set2) {
         Console.WriteLine(res);
      }
      
      Console.WriteLine("Count of HashSet1 = " + set1.Count);
      Console.WriteLine("Count of HashSet2 = " + set2.Count);
      
      set1.Clear();
      set2.Clear();
      
      Console.WriteLine("Count of HashSet1 (after Clear) = " + set1.Count);
      Console.WriteLine("Count of HashSet2 (after Clear) = " + set2.Count);
   }
}

The output of the above code is −

Elements in HashSet1...
A
B
C
D
Elements in HashSet2...
John
Jacob
Ryan
Tom
Count of HashSet1 = 4
Count of HashSet2 = 4
Count of HashSet1 (after Clear) = 0
Count of HashSet2 (after Clear) = 0

Comparison with Other Collection Types

Conclusion

The Clear() method is the standard and most efficient way to remove all elements from a HashSet in C#. After calling Clear(), the HashSet becomes empty with a count of zero, and you can immediately start adding new elements if needed.