C# Program to remove duplicate characters from String

Removing duplicate characters from a string is a common programming task in C#. There are several approaches to achieve this, with HashSet being one of the most efficient methods due to its automatic handling of unique elements.

Using HashSet to Remove Duplicates

The HashSet<char> collection automatically maintains unique elements, making it perfect for removing duplicate characters from a string −

using System;
using System.Linq;
using System.Collections.Generic;

class Program {
   static void Main(string[] args) {
      string myStr = "kkllmmnnoo";
      Console.WriteLine("Initial String: " + myStr);
      
      var unique = new HashSet<char>(myStr);
      string result = new string(unique.ToArray());
      
      Console.WriteLine("New String after removing duplicates: " + result);
   }
}

The output of the above code is −

Initial String: kkllmmnnoo
New String after removing duplicates: klmno

Using LINQ Distinct Method

Another elegant approach uses LINQ's Distinct() method to filter unique characters −

using System;
using System.Linq;

class Program {
   static void Main(string[] args) {
      string myStr = "programming";
      Console.WriteLine("Initial String: " + myStr);
      
      string result = new string(myStr.Distinct().ToArray());
      
      Console.WriteLine("New String after removing duplicates: " + result);
   }
}

The output of the above code is −

Initial String: programming
New String after removing duplicates: progamin

Using StringBuilder with Contains Check

For preserving the first occurrence order without using LINQ, you can use a StringBuilder with a contains check −

using System;
using System.Text;

class Program {
   static void Main(string[] args) {
      string myStr = "hello world";
      Console.WriteLine("Initial String: " + myStr);
      
      StringBuilder sb = new StringBuilder();
      
      foreach (char c in myStr) {
         if (sb.ToString().IndexOf(c) == -1) {
            sb.Append(c);
         }
      }
      
      string result = sb.ToString();
      Console.WriteLine("New String after removing duplicates: " + result);
   }
}

The output of the above code is −

Initial String: hello world
New String after removing duplicates: helo wrd

Comparison of Methods

Conclusion

The HashSet approach is most efficient for removing duplicates when order doesn't matter, while LINQ's Distinct() method is ideal when you need to preserve the original character order. Choose the method based on your specific requirements for performance and ordering.