C# - String IsUpper, IsLower

IsUpper, IsLower. Some C# strings contain only lowercase letters. These strings do not need to be lowercased. By testing first, we can avoid excess work.

C# method notes. We must implement our own custom IsLower and IsUpper methods. We add them with extension methods in a static class.

static

Example. This program contains a static class of extension methods. The IsUpper and IsLower methods are public static methods written with the extension method syntax.

Detail Here we test the IsUpper and IsLower extension methods on various string literals.

String Literal

Info A string is defined as uppercase if it contains no lowercase letters. It is lowercase if it contains no uppercase letters.

Note This is not the same as a string containing only uppercase or only lowercase letters.

Tip You could change the implementation of these methods to fit this requirement if necessary.

using System; static class Extensions { public static bool IsUpper(this string value) { // Consider string to be uppercase if it has no lowercase letters. for (int i = 0; i < value.Length; i++) { if (char.IsLower(value[i])) { return false; } } return true; } public static bool IsLower(this string value) { // Consider string to be lowercase if it has no uppercase letters. for (int i = 0; i < value.Length; i++) { if (char.IsUpper(value[i])) { return false; } } return true; } } class Program { static void Main() { Console.WriteLine("test".IsLower()); // True Console.WriteLine("test".IsUpper()); Console.WriteLine("Test".IsLower()); Console.WriteLine("Test".IsUpper()); Console.WriteLine("TEST3".IsLower()); Console.WriteLine("TEST3".IsUpper()); // True } }

True False False False False True

Syntax. We used extension method syntax. In large projects, using a static class of extension methods is often simpler than trying to remember different static methods in different classes.

Extension

We implemented IsUpper and IsLower for strings. These methods perform a fast scanning of the source string, rather than requiring another allocation and conversion.

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This page was last updated on Jun 6, 2021 (rewrite).

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