How to determine if the string has all unique characters using C#?

To determine if a string has all unique characters, you need to check if any character appears more than once. This can be accomplished using several approaches in C#, from simple nested loops to more efficient data structures like HashSet.

Using Nested Loop Approach

The basic approach involves comparing each character with every other character in the string −

using System;

class Program {
   public static bool HasUniqueCharacters(string val) {
      for (int i = 0; i 

The output of the above code is −

'hello' has unique characters: False
'world' has unique characters: True
'unique' has unique characters: False


Using HashSet for Efficient Checking

A more efficient approach uses a HashSet to track seen characters. This method has O(n) time complexity compared to O(n²) for the nested loop −

using System;
using System.Collections.Generic;

class Program {
   public static bool HasUniqueCharactersHashSet(string val) {
      HashSet<char> seenChars = new HashSet<char>();
      
      foreach (char c in val) {
         if (seenChars.Contains(c))
            return false;
         seenChars.Add(c);
      }
      return true;
   }

   public static void Main() {
      string[] testStrings = {"programming", "alphabet", "abc", "aabbcc"};

      foreach (string str in testStrings) {
         Console.WriteLine($"'{str}' has unique characters: {HasUniqueCharactersHashSet(str)}");
      }
   }
}


The output of the above code is −

'programming' has unique characters: False
'alphabet' has unique characters: False
'abc' has unique characters: True
'aabbcc' has unique characters: False


Using LINQ Approach

LINQ provides a concise way to check for unique characters by comparing the original length with the distinct character count −

using System;
using System.Linq;

class Program {
   public static bool HasUniqueCharactersLinq(string val) {
      return val.Distinct().Count() == val.Length;
   }

   public static void Main() {
      string[] testCases = {"test", "abcd", "hello", "C#"};

      foreach (string str in testCases) {
         Console.WriteLine($"'{str}' has unique characters: {HasUniqueCharactersLinq(str)}");
      }
   }
}


The output of the above code is −

'test' has unique characters: False
'abcd' has unique characters: True
'hello' has unique characters: False
'C#' has unique characters: True


Performance Comparison

Conclusion

There are multiple ways to check if a string has unique characters in C#. The HashSet approach offers the best performance for larger strings with O(n) complexity, while the nested loop approach uses minimal memory. Choose the method based on your specific requirements for performance and readability.