C# String ToLower

Summary: in this tutorial, you will learn how to use the C# String ToLower() method to convert a string to lowercase.

Introduction to the C# ToLower() method

The String ToLower() method returns a copy of the current string converted to lowercase. Here’s the syntax of the ToLower() method:

public string ToLower();Code language: PHP (php)

For example, the following program uses the ToLower() method to convert the message "Hello, World!" to lowercase:

using static System.Console;

var message = "Hello, World!";
var lower = message.ToLower();

WriteLine(lower);Code language: C# (cs)

Output:

hello, world!

By default, the ToLower() method considers the case rule of the current culture. If you want to convert a string to lowercase using a specific culture, you can use the following overload of the ToLower() method:

public string ToLower (
   System.Globalization.CultureInfo? culture
);Code language: CSS (css)

In this syntax, the culture is an instance of CultureInfo that specifies a culture-specific casing rule.

The following example demonstrates how using different CultureInfo objects can lead to different lowercase conversions based on specific language rules:

using System.Globalization;
using static System.Console;

var s = "Istanbul İZMİR Ankara";

// Using the default (English) culture
var englishLowercase = s.ToLower(CultureInfo.InvariantCulture);
WriteLine("Default (English) lowercase: " + englishLowercase);

// Using the Turkish culture
var turkishCulture = new CultureInfo("tr-TR");
var turkishLowercase = s.ToLower(turkishCulture);
Console.WriteLine("Turkish lowercase: " + turkishLowercase);Code language: JavaScript (javascript)

In this example, we have a string that contains Turkish characters, such as "İ" (capital i with a dot) and "İ" (small i with a dot) which are specific to the Turkish alphabet.

When we use the ToLower() to convert a string to lowercase using the default culture (English), the method uses the English rules for lowercase conversion, and we get "istanbul İZMİR ankara".

However, when we use the ToLower() method with the Turkish culture ("tr-TR"), the method uses Turkish-specific lowercase rules for conversion.

In Turkish, the lowercase equivalent of "İ" is "i" without a dot, and the lowercase equivalent of "I" is "ı" (a dotless i). As a result, the Turkish lowercase string becomes "i̇stanbul i̇zmir ankara".

C# String ToLowerInvariant()

The String ToLowerInvariant() returns a lowercase version of a string using the casing rules of the invariant culture.

Here’s the syntax of the ToLowerInvariant() method:

public string ToLowerInvariant();Code language: PHP (php)

The ToLowerInvariant() method is equivalent to the ToLower() method with the CultureInfo.InvariantCulture argument:

ToLower(CultureInfo.InvariantCulture)Code language: CSS (css)

One practical example of using the ToLowerInvariant() method is when you need to perform a case-sensitive string comparison.

Using ToLowerInvariant() ensures consistent behavior and avoids potential issues with culture-specific rules.

The following example demonstrates how to use the ToLowerInvariant() method to check if the input username is in a list:

using static System.Console;


// Get the username from input
Write("Enter you username:");
var searchUsername = ReadLine();

// Username list
var usernames = new List<string>{ "JohnDoe", "JaneSmith", "MikeJohnson", "SarahBrown" };

// Check if the username exists
var userExists = usernames.Exists(
    username => username.ToLowerInvariant() == searchUsername?.ToLowerInvariant()
);

// Set the message
var message = userExists 
    ? $"The user {searchUsername} exists in the list." 
    : $"The user {searchUsername} does not exist in the list.";

WriteLine(message);
Code language: JavaScript (javascript)

Output:

Enter you username:johndoe
The user johndoe exists in the list.Code language: CSS (css)

Summary

Use the C# String ToLower() method to convert a string to lowercase.

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