C# Program to Convert Character case

Converting character case in C# is a common string manipulation task. The string class provides built-in methods ToLower() and ToUpper() to convert strings to lowercase and uppercase respectively.

Syntax

Following is the syntax for converting string case −

string.ToLower()    // Converts to lowercase
string.ToUpper()    // Converts to uppercase

Both methods return a new string with the case converted and do not modify the original string.

Using ToLower() Method

The ToLower() method converts all uppercase characters in a string to lowercase −

using System;

class Program {
    static void Main(string[] args) {
        string str = "AMIT";
        Console.WriteLine("Original: {0}", str);
        Console.WriteLine("Converted to LowerCase: {0}", str.ToLower());
    }
}

The output of the above code is −

Original: AMIT
Converted to LowerCase: amit

Using ToUpper() Method

The ToUpper() method converts all lowercase characters in a string to uppercase −

using System;

class Program {
    static void Main(string[] args) {
        string str = "hello world";
        Console.WriteLine("Original: {0}", str);
        Console.WriteLine("Converted to UpperCase: {0}", str.ToUpper());
    }
}

The output of the above code is −

Original: hello world
Converted to UpperCase: HELLO WORLD

Converting Mixed Case Strings

Both methods work with mixed case strings, converting only the relevant characters −

using System;

class Program {
    static void Main(string[] args) {
        string mixedCase = "Hello World 123!";
        Console.WriteLine("Original: {0}", mixedCase);
        Console.WriteLine("ToLower(): {0}", mixedCase.ToLower());
        Console.WriteLine("ToUpper(): {0}", mixedCase.ToUpper());
    }
}

The output of the above code is −

Original: Hello World 123!
ToLower(): hello world 123!
ToUpper(): HELLO WORLD 123!

Converting Individual Characters

For individual characters, use char.ToLower() and char.ToUpper() methods −

using System;

class Program {
    static void Main(string[] args) {
        char upperChar = 'A';
        char lowerChar = 'z';
        
        Console.WriteLine("Original chars: {0}, {1}", upperChar, lowerChar);
        Console.WriteLine("After conversion: {0}, {1}", 
                         char.ToLower(upperChar), 
                         char.ToUpper(lowerChar));
    }
}

The output of the above code is −

Original chars: A, z
After conversion: a, Z

Key Points

• Case conversion methods return a new string and do not modify the original.

• Numbers and special characters remain unchanged during case conversion.

• These methods are culture-aware and use the current culture settings by default.

• For culture-specific conversion, use overloads like ToLower(CultureInfo).

Conclusion

C# provides simple and efficient methods for converting character case using ToLower() and ToUpper() for strings, and char.ToLower() and char.ToUpper() for individual characters. These methods return new instances and preserve non-alphabetic characters unchanged.