C# String Methods and Manipulation

What is a String?

A string is a sequence of characters used to represent text. Strings are one of the most commonly used data types in programming.

string sName = "John";
string sMessage = "Hello World!";
string sEmpty = "";

String Properties

Length Property

Returns the number of characters in a string.

string sName = "Alice";
int iLength = sName.Length;
Console.WriteLine(iLength);  // 5

string sMessage = "Hello World!";
Console.WriteLine(sMessage.Length);  // 12

Accessing Characters by Index

Strings use zero-based indexing like arrays.

string sName = "Hello";
char cFirst = sName[0];   // 'H'
char cSecond = sName[1];  // 'e'
char cLast = sName[4];    // 'o'

Console.WriteLine(cFirst);   // H
Console.WriteLine(cLast);    // o

String Methods

ToUpper() and ToLower()

Convert string to uppercase or lowercase.

string sName = "Alice";

string sUpper = sName.ToUpper();  // "ALICE"
string sLower = sName.ToLower();  // "alice"

Console.WriteLine(sUpper);  // ALICE
Console.WriteLine(sLower);  // alice

Practical Use:

Console.Write("Are you employed? (Y/N): ");
string sInput = Console.ReadLine().ToUpper();

if (sInput == "Y")
{
    Console.WriteLine("You are employed");
}

Trim(), TrimStart(), TrimEnd()

Remove whitespace from strings.

string sText = "   Hello World   ";

string sTrimmed = sText.Trim();           // "Hello World"
string sTrimStart = sText.TrimStart();    // "Hello World   "
string sTrimEnd = sText.TrimEnd();        // "   Hello World"

Console.WriteLine($"'{sTrimmed}'");      // 'Hello World'

Substring()

Extract a portion of a string.

string sText = "Hello World";

// Substring(startIndex)
string sPart1 = sText.Substring(6);      // "World"

// Substring(startIndex, length)
string sPart2 = sText.Substring(0, 5);   // "Hello"
string sPart3 = sText.Substring(6, 3);   // "Wor"

Console.WriteLine(sPart1);  // World
Console.WriteLine(sPart2);  // Hello

Contains()

Check if a string contains a substring.

string sEmail = "john@example.com";

bool bHasAt = sEmail.Contains("@");       // true
bool bHasCom = sEmail.Contains(".com");   // true
bool bHasXyz = sEmail.Contains("xyz");    // false

Console.WriteLine(bHasAt);   // True
Console.WriteLine(bHasXyz);  // False

StartsWith() and EndsWith()

Check if string starts or ends with specific text.

string sFileName = "document.pdf";

bool bStartsWithDoc = sFileName.StartsWith("doc");   // true
bool bEndsWithPdf = sFileName.EndsWith(".pdf");      // true
bool bEndsWithTxt = sFileName.EndsWith(".txt");      // false

Console.WriteLine(bEndsWithPdf);  // True

IndexOf() and LastIndexOf()

Find the position of a character or substring.

string sText = "Hello World";

int iPos1 = sText.IndexOf('o');        // 4 (first 'o')
int iPos2 = sText.LastIndexOf('o');    // 7 (last 'o')
int iPos3 = sText.IndexOf("World");    // 6
int iPos4 = sText.IndexOf('z');        // -1 (not found)

Console.WriteLine(iPos1);  // 4
Console.WriteLine(iPos2);  // 7
Console.WriteLine(iPos4);  // -1

Replace()

Replace characters or substrings.

string sText = "Hello World";

string sReplaced1 = sText.Replace('o', 'a');           // "Hella Warld"
string sReplaced2 = sText.Replace("World", "C#");      // "Hello C#"
string sReplaced3 = sText.Replace(" ", "");            // "HelloWorld"

Console.WriteLine(sReplaced1);  // Hella Warld
Console.WriteLine(sReplaced2);  // Hello C#

Split()

Split a string into an array of strings.

string sData = "John,25,Engineer";
string[] sParts = sData.Split(',');

Console.WriteLine(sParts[0]);  // John
Console.WriteLine(sParts[1]);  // 25
Console.WriteLine(sParts[2]);  // Engineer

// Split by space
string sSentence = "Hello World Programming";
string[] sWords = sSentence.Split(' ');

foreach (string sWord in sWords)
{
    Console.WriteLine(sWord);
}
// Output:
// Hello
// World
// Programming

Insert()

Insert text at a specific position.

string sText = "Hello World";
string sInserted = sText.Insert(6, "Beautiful ");

Console.WriteLine(sInserted);  // Hello Beautiful World

Remove()

Remove characters from a string.

string sText = "Hello World";

string sRemoved1 = sText.Remove(5);        // "Hello" (removes from index 5 to end)
string sRemoved2 = sText.Remove(5, 6);     // "Hello" (removes 6 chars from index 5)

Console.WriteLine(sRemoved1);  // Hello
Console.WriteLine(sRemoved2);  // Hello

String Concatenation

Using + Operator

string sFirst = "Hello";
string sLast = "World";
string sFull = sFirst + " " + sLast;

Console.WriteLine(sFull);  // Hello World

Using String.Concat()

string sResult = String.Concat("Hello", " ", "World");
Console.WriteLine(sResult);  // Hello World

Using String Interpolation (Recommended)

string sName = "Alice";
int iAge = 25;
string sMessage = $"My name is {sName} and I am {iAge} years old";

Console.WriteLine(sMessage);
// Output: My name is Alice and I am 25 years old

Using StringBuilder (For Multiple Concatenations)

using System.Text;

StringBuilder sb = new StringBuilder();
sb.Append("Hello");
sb.Append(" ");
sb.Append("World");

string sResult = sb.ToString();
Console.WriteLine(sResult);  // Hello World

String Comparison

Using == Operator

string sName1 = "Alice";
string sName2 = "Alice";
string sName3 = "Bob";

bool bEqual1 = (sName1 == sName2);  // true
bool bEqual2 = (sName1 == sName3);  // false

Console.WriteLine(bEqual1);  // True
Console.WriteLine(bEqual2);  // False

Case-Insensitive Comparison

string sStr1 = "Hello";
string sStr2 = "hello";

// Case-sensitive (different)
bool bEqual1 = (sStr1 == sStr2);  // false

// Case-insensitive (same)
bool bEqual2 = sStr1.ToLower() == sStr2.ToLower();  // true
bool bEqual3 = String.Equals(sStr1, sStr2, StringComparison.OrdinalIgnoreCase);  // true

Console.WriteLine(bEqual1);  // False
Console.WriteLine(bEqual2);  // True

CompareTo()

Compare strings alphabetically.

string sStr1 = "Apple";
string sStr2 = "Banana";

int iResult = sStr1.CompareTo(sStr2);
// Returns: negative if str1 < str2
//          0 if str1 == str2
//          positive if str1 > str2

Console.WriteLine(iResult);  // Negative number (Apple comes before Banana)

Checking Empty or Null Strings

IsNullOrEmpty()

string sEmpty = "";
string sNull = null;
string sValid = "Hello";

bool bCheck1 = String.IsNullOrEmpty(sEmpty);  // true
bool bCheck2 = String.IsNullOrEmpty(sNull);   // true
bool bCheck3 = String.IsNullOrEmpty(sValid);  // false

Console.WriteLine(bCheck1);  // True
Console.WriteLine(bCheck3);  // False

IsNullOrWhiteSpace()

string sSpaces = "   ";
string sEmpty = "";
string sValid = "Hello";

bool bCheck1 = String.IsNullOrWhiteSpace(sSpaces);  // true
bool bCheck2 = String.IsNullOrWhiteSpace(sEmpty);   // true
bool bCheck3 = String.IsNullOrWhiteSpace(sValid);   // false

Console.WriteLine(bCheck1);  // True
Console.WriteLine(bCheck3);  // False

Practical Examples

Example 1: Email Validator

using System;

namespace EmailValidator
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter email address: ");
            string sEmail = Console.ReadLine();

            if (IsValidEmail(sEmail))
            {
                Console.WriteLine("Valid email address");
            }
            else
            {
                Console.WriteLine("Invalid email address");
            }

            Console.WriteLine("\nPress any key to exit...");
            Console.ReadKey();
        }

        static bool IsValidEmail(string sEmail)
        {
            if (String.IsNullOrWhiteSpace(sEmail))
            {
                return false;
            }

            if (!sEmail.Contains("@"))
            {
                return false;
            }

            if (!sEmail.Contains("."))
            {
                return false;
            }

            int iAtPos = sEmail.IndexOf('@');
            int iDotPos = sEmail.LastIndexOf('.');

            if (iAtPos > iDotPos)
            {
                return false;
            }

            return true;
        }
    }
}

Example 2: Word Counter

using System;

namespace WordCounter
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter a sentence: ");
            string sSentence = Console.ReadLine();

            int iWordCount = CountWords(sSentence);

            Console.WriteLine($"\nNumber of words: {iWordCount}");

            Console.WriteLine("\nPress any key to exit...");
            Console.ReadKey();
        }

        static int CountWords(string sText)
        {
            if (String.IsNullOrWhiteSpace(sText))
            {
                return 0;
            }

            string sTrimmed = sText.Trim();
            string[] sWords = sTrimmed.Split(' ');

            return sWords.Length;
        }
    }
}

Example 3: String Reverser

using System;

namespace StringReverser
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter a string: ");
            string sInput = Console.ReadLine();

            string sReversed = ReverseString(sInput);

            Console.WriteLine($"\nReversed: {sReversed}");

            Console.WriteLine("\nPress any key to exit...");
            Console.ReadKey();
        }

        static string ReverseString(string sText)
        {
            string sResult = "";

            for (int i = sText.Length - 1; i >= 0; i--)
            {
                sResult += sText[i];
            }

            return sResult;
        }
    }
}

Example 4: Palindrome Checker

using System;

namespace PalindromeChecker
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter a word: ");
            string sWord = Console.ReadLine().ToLower();

            if (IsPalindrome(sWord))
            {
                Console.WriteLine($"\n'{sWord}' is a palindrome");
            }
            else
            {
                Console.WriteLine($"\n'{sWord}' is not a palindrome");
            }

            Console.WriteLine("\nPress any key to exit...");
            Console.ReadKey();
        }

        static bool IsPalindrome(string sText)
        {
            string sReversed = "";

            for (int i = sText.Length - 1; i >= 0; i--)
            {
                sReversed += sText[i];
            }

            return sText == sReversed;
        }
    }
}

Example 5: Name Formatter

using System;

namespace NameFormatter
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter your full name: ");
            string sFullName = Console.ReadLine();

            string sFormatted = FormatName(sFullName);

            Console.WriteLine($"\nFormatted name: {sFormatted}");

            Console.WriteLine("\nPress any key to exit...");
            Console.ReadKey();
        }

        static string FormatName(string sName)
        {
            string sTrimmed = sName.Trim();
            string[] sParts = sTrimmed.Split(' ');
            string sResult = "";

            foreach (string sPart in sParts)
            {
                if (sPart.Length > 0)
                {
                    // Capitalize first letter, lowercase rest
                    string sFormatted = sPart.Substring(0, 1).ToUpper() + 
                                       sPart.Substring(1).ToLower();
                    sResult += sFormatted + " ";
                }
            }

            return sResult.Trim();
        }
    }
}

String Escape Sequences (Recap)

Sequence	Description	Example
`\n`	New line	`"Line1\nLine2"`
`\t`	Tab	`"Name\tAge"`
`\\`	Backslash	`"C:\\Files"`
`\"`	Double quote	`"He said \"Hi\""`
`\'`	Single quote	`'It\'s'`

Practice Exercises

Exercise 1: Count Vowels

Write a program that counts the number of vowels (a, e, i, o, u) in a string.

Exercise 2: Remove Spaces

Write a program that removes all spaces from a string.

Exercise 3: Find Longest Word

Write a program that finds the longest word in a sentence.

Exercise 4: Acronym Generator

Write a program that creates an acronym from a phrase. Example: "Central Processing Unit" → "CPU"

Exercise 5: Password Strength Checker

Check if password has: - At least 8 characters - Contains uppercase and lowercase - Contains a number

Common Mistakes to Avoid

❌ Modifying string directly (strings are immutable):

string sText = "Hello";
sText[0] = 'J';  // ERROR! Cannot modify

✅ Correct - Create new string:

string sText = "Hello";
sText = "J" + sText.Substring(1);  // "Jello"

❌ Forgetting string indices are zero-based:

string sText = "Hello";
char cLast = sText[5];  // ERROR! Index out of range

✅ Correct:

char cLast = sText[4];  // 'o'
// Or
char cLast = sText[sText.Length - 1];

❌ Not handling null strings:

string sName = null;
int iLength = sName.Length;  // CRASH!

✅ Correct:

if (sName != null)
{
    int iLength = sName.Length;
}
// Or
if (!String.IsNullOrEmpty(sName))
{
    int iLength = sName.Length;
}

Key Takeaways

✅ Strings are immutable (cannot be modified) ✅ Use .Length to get string length ✅ Strings use zero-based indexing ✅ .ToUpper() and .ToLower() for case conversion ✅ .Trim() removes whitespace ✅ .Substring() extracts part of string ✅ .Contains() checks if substring exists ✅ .Replace() replaces text ✅ .Split() divides string into array ✅ Use string interpolation $"" for formatting ✅ Use IsNullOrEmpty() or IsNullOrWhiteSpace() to check for empty strings

Next Topic: C# Course Summary and Best Practices

C# Crash Course

C# String Methods and Manipulation

What is a String?

String Properties

Length Property

Accessing Characters by Index

String Methods

ToUpper() and ToLower()

Trim(), TrimStart(), TrimEnd()

Substring()

Contains()

StartsWith() and EndsWith()

IndexOf() and LastIndexOf()

Replace()

Split()

Insert()

Remove()

String Concatenation

Using + Operator

Using String.Concat()

Using String Interpolation (Recommended)

Using StringBuilder (For Multiple Concatenations)

String Comparison

Using == Operator

Case-Insensitive Comparison

CompareTo()

Checking Empty or Null Strings

IsNullOrEmpty()

IsNullOrWhiteSpace()

Practical Examples

Example 1: Email Validator

Example 2: Word Counter

Example 3: String Reverser

Example 4: Palindrome Checker

Example 5: Name Formatter

String Escape Sequences (Recap)

Practice Exercises

Exercise 1: Count Vowels

Exercise 2: Remove Spaces

Exercise 3: Find Longest Word

Exercise 4: Acronym Generator

Exercise 5: Password Strength Checker

Common Mistakes to Avoid

Key Takeaways