Char.ToLower, ToUpper. ToLower converts only uppercase letters. It changes uppercase letters to lowercase letters. It leaves all other characters unchanged.

C# method info. ToLower and ToUpper are ideal for character conversions. These methods are included in .NET, and act on all characters in a reliable way.

char

String ToLower, ToUpper

Input and output. Consider the uppercase letter "A." This has the ASCII value 65. When lowercased, it equals "a" which is ASCII 97.

A (65)
a (97)

An example. Here we see char.ToLower. This converts uppercase characters to lowercase characters, while not changing characters that are already lowercase or digits.

Console.WriteLine

using System;

char c1 = 'a'; // Lowercase a
char c2 = 'b'; // Lowercase b
char c3 = 'C'; // Uppercase C
char c4 = '3'; // Digit 3

// Part 1: use char.ToLower.
char lower1 = char.ToLower(c1); // a
char lower2 = char.ToLower(c2); // b
char lower3 = char.ToLower(c3); // c [changed]
char lower4 = char.ToLower(c4); // 3

// Part 2: use char.ToUpper.
char upper1 = char.ToUpper(c1); // A [changed]
char upper2 = char.ToUpper(c2); // B [changed]
char upper3 = char.ToUpper(c3); // C
char upper4 = char.ToUpper(c4); // 3

// Part 3: write results.
Console.WriteLine(c1 + "," + c2 + "," + c3 + "," + c4);
Console.WriteLine(lower1 + "," + lower2 + "," + lower3 + "," + lower4);
Console.WriteLine(upper1 + "," + upper2 + "," + upper3 + "," + upper4);
a,b,C,3
a,b,c,3
A,B,C,3

IsLower, IsUpper. Sometimes we need to first test if a character is lowercase or uppercase. The char.IsLower and IsUpper methods in C# help here.

using System;

string test = "AxZ";
// Call IsLower and IsUpper in loop.
foreach (char value in test)
{
    if (char.IsLower(value))
    {
        Console.WriteLine("LOWER");
    }
    else if (char.IsUpper(value))
    {
        Console.WriteLine("UPPER");
    }
}
UPPER
LOWER
UPPER

InvariantCulture. These 2 methods use the CultureInfo.InvariantCulture parameter internally. The letters are lowercased and uppercased the same in all globalization settings.

Performance notes. Finally, the char.ToLower and char.ToUpper methods have worse performance than a custom method that tests ASCII values. It can be worthwhile to use a custom method.

Summary. Programs that handle characters can benefit from char.ToLower and char.ToUpper. These are simple but useful methods, ready to call with no extra code.