C# Program to change a character from a string

In C#, strings are immutable, meaning their characters cannot be changed directly. However, you can change characters in a string using the StringBuilder class, which provides a mutable sequence of characters.

The StringBuilder class allows you to modify individual characters using indexer notation str[index] = newChar, where the index is zero-based.

Syntax

Following is the syntax for changing a character in a StringBuilder −

StringBuilder stringBuilder = new StringBuilder("original");
stringBuilder[index] = 'newChar';

Using StringBuilder to Change Characters

Let's say our string is −

StringBuilder str = new StringBuilder();
str.Append("pre");

To change a character, set the value at that particular index. The following sets a character at the 3rd position (index 2) −

str[2] = 'o';

Example

using System;
using System.Text;

public class Demo {
   public static void Main() {
      StringBuilder str = new StringBuilder();
      str.Append("pre");
      Console.WriteLine("String : " + str);
      Console.WriteLine("Change 3rd character");
      str[2] = 'o';
      Console.WriteLine("New String : " + str);
   }
}

The output of the above code is −

String : pre
Change 3rd character
New String : pro

Changing Multiple Characters

Example

using System;
using System.Text;

public class Demo {
   public static void Main() {
      StringBuilder str = new StringBuilder("hello");
      Console.WriteLine("Original String: " + str);
      
      str[0] = 'H';  // Change first character
      str[4] = 'a';  // Change last character
      
      Console.WriteLine("Modified String: " + str);
   }
}

The output of the above code is −

Original String: hello
Modified String: Hella

Alternative Method using String Methods

For immutable strings, you can create a new string with changed characters using string manipulation methods −

Example

using System;

public class Demo {
   public static void Main() {
      string originalString = "hello";
      Console.WriteLine("Original String: " + originalString);
      
      // Change character at index 1 from 'e' to 'a'
      string newString = originalString.Substring(0, 1) + 'a' + originalString.Substring(2);
      Console.WriteLine("Modified String: " + newString);
   }
}

The output of the above code is −

Original String: hello
Modified String: hallo

Comparison of Methods

Conclusion

Use StringBuilder for efficient character modification when you need to change multiple characters. For single character changes, string manipulation methods work well, but remember they create new string objects rather than modifying the original.