string.ToUpper() / ToLower() / Trim()

// Returns a new string with all characters converted to uppercase.
string.ToUpper()

// Returns a new string with all characters converted to lowercase.
string.ToLower()

// Returns a new string with all leading and trailing whitespace (spaces, tabs, newlines) removed.
string.Trim()

// Removes leading whitespace only.
string.TrimStart()

// Removes trailing whitespace only.
string.TrimEnd()

using System;

string langName = "csharp";

// Convert to uppercase using ToUpper().
Console.WriteLine(langName.ToUpper()); // CSHARP

// Convert to lowercase using ToLower().
string input = "HELLO World";
Console.WriteLine(input.ToLower()); // hello world

// Case-insensitive comparison.
string userInput = "Yes";
if (userInput.ToLower() == "yes")
{
    Console.WriteLine("You selected yes."); // This branch executes.
}

// Remove surrounding whitespace using Trim().
string formInput = "   akiba   ";
Console.WriteLine(formInput.Trim());      // akiba
Console.WriteLine(formInput.TrimStart()); // akiba   (trailing spaces remain)
Console.WriteLine(formInput.TrimEnd());   //    akiba (leading spaces remain)

// A common pattern for normalizing user input.
string rawInput = "  User@Example.COM  ";
string normalized = rawInput.Trim().ToLower();
Console.WriteLine(normalized); // user@example.com

All of these methods leave the original string unchanged and return a new string. When processing user input from a form, a very common pattern is to call Trim() to remove whitespace and then ToLower() to normalize the case.

Trim() removes not only spaces but also tabs (\t) and newlines (\n). If you need to remove only specific characters, you can use the Trim(char[] trimChars) overload to specify which characters to strip.

For string replacement and containment checks, see Replace() / Contains().