19. Strings in C#

Level: Beginner
Goal: Learn how to store, clean, check, join, and display text in C#.

ℹ️ What You'll Learn

• What a string is
• Clean text using Trim
• Change text using ToUpper and ToLower
• Check text using Contains, StartsWith, and EndsWith
• Split text into parts
• Build output using string interpolation
• Use StringBuilder for many lines

A string is text.

In a school application, these are strings:

• Student name: "Sahasra Kumar"
• Roll number: "STU-001"
• Class name: "10th"
• Email: "Sahasra@example.com"
• Fee status: "Paid"

Why Strings Matter

Most user input comes as text first.

Examples:

• A student enters their name in a form.
• A teacher enters exam remarks.
• An admin searches by roll number.
• A report card prints student details.

So before writing big programs, you must become comfortable with strings.

Quick Definitions

• String - Text data type (e.g., "Sahasra Kumar")
• Immutable - Cannot be changed after creation (creates new string when modified)
• Trim - Remove extra spaces at start and end
• Interpolation - Placing variables inside text using $"text {variable}"
• StringBuilder - Efficient class for building many text lines
• Split - Break text into parts using a separator
• Substring - Extract part of a string
• Contains - Check if text exists inside another text
• StartsWith/EndsWith - Check beginning or ending of string
• IsNullOrWhiteSpace - Check for empty, null, or only spaces

---

Create a String

string studentName = "Sahasra Kumar";
string className = "10th";
string rollNumber = "STU-001";

Console.WriteLine(studentName);
Console.WriteLine(className);
Console.WriteLine(rollNumber);

String Length

Length tells how many characters are inside the string.

string name = "Sahasra";

Console.WriteLine(name.Length); // 4

This is useful when validating input.

string password = "abc";

if (password.Length < 6)
{
    Console.WriteLine("Password is too short");
}

Common String Methods

Method	Meaning	Example
Trim()	Removes extra spaces at start and end	" Sahasra ".Trim()
ToUpper()	Converts text to uppercase	"Sahasra".ToUpper()
ToLower()	Converts text to lowercase	"Sahasra".ToLower()
Contains()	Checks if text exists inside	"Sahasra Kumar".Contains("Kumar")
StartsWith()	Checks starting text	"STU-001".StartsWith("STU")
EndsWith()	Checks ending text	"photo.jpg".EndsWith(".jpg")
Replace()	Replaces text	"10-A".Replace("-", "/")
Split()	Breaks text into parts	"Sahasra,10th,A".Split(',')

Clean User Input

Students may type extra spaces by mistake.

string input = "   Sahasra Kumar   ";

string cleanedName = input.Trim();

Console.WriteLine(cleanedName); // Sahasra Kumar

Important point: Trim() does not change the original string. It returns a new string.

string input = "   Sahasra   ";

input.Trim();

Console.WriteLine(input); // still has spaces

Correct:

string input = "   Sahasra   ";

input = input.Trim();

Console.WriteLine(input); // Sahasra

Check Empty Text

Use string.IsNullOrWhiteSpace() for required fields.

string name = "   ";

if (string.IsNullOrWhiteSpace(name))
{
    Console.WriteLine("Name is required");
}

This handles:

• null
• empty text ""
• only spaces " "

Join Text with Interpolation

String interpolation means placing variables inside text using $.

string name = "Sahasra";
string className = "10th";

Console.WriteLine($"Student {name} is studying in class {className}");

Output:

Student Sahasra is studying in class 10th

This is easier to read than using + many times.

Format Numbers Inside Strings

double percentage = 87.4567;
decimal fee = 25000;

Console.WriteLine($"Percentage: {percentage:F2}");
Console.WriteLine($"Fee: {fee:C}");

F2 means show 2 digits after decimal.

Split Text

Sometimes data comes as one line.

string line = "STU-001,Sahasra Kumar,10th,A";

string[] parts = line.Split(',');

Console.WriteLine(parts[0]); // STU-001
Console.WriteLine(parts[1]); // Sahasra Kumar
Console.WriteLine(parts[2]); // 10th
Console.WriteLine(parts[3]); // A

This is common when reading CSV files.

Strings Are Immutable

Immutable means once a string is created, it cannot be changed.

When you modify a string, C# creates a new string.

string name = "Sahasra";

string upperName = name.ToUpper();

Console.WriteLine(name);      // Sahasra
Console.WriteLine(upperName); // Sahasra

This is safe, but repeated changes in loops can become slow.

Use StringBuilder for Many Lines

If you are building a report with many lines, use StringBuilder.

var report = new System.Text.StringBuilder();

report.AppendLine("School Report");
report.AppendLine("-------------");
report.AppendLine("Name: Sahasra Kumar");
report.AppendLine("Class: 10th");
report.AppendLine("Result: Pass");

Console.WriteLine(report.ToString());

School Management Example

Create a simple student summary.

💻 Try It — Console App

💡 Paste into Program.cs and run⌥ GitHub

string rollNumber = " STU-001 ";
string name = " Sahasra kumar ";
string className = "10th";
double percentage = 87.456;

rollNumber = rollNumber.Trim().ToUpper();
name = name.Trim();

string message = $"Roll: {rollNumber}\nName: {name}\nClass: {className}\nPercentage: {percentage:F2}%";

Console.WriteLine(message);

---

When You'll Use This in SMS

Every SMS feature uses strings.

Real string operations:

Student name validation: Trim(), Length check
Roll number format: StartsWith("STU-"), split by "-"
Email validation: Contains("@"), EndsWith(".com")
Password validation: Length >= 8
Report generation: Interpolation to build formatted text
CSV import: Split(',') to parse student data
Log messages: Interpolation for user-friendly errors

Real SMS code:

string name = studentInput.Trim();
if (name.Length < 2) throw new ArgumentException("Name too short");

string email = input.ToLower();
if (!email.Contains("@")) throw new ArgumentException("Invalid email");

string csvLine = $"{student.RollNumber},{student.Name},{student.Email}";

StringBuilder report = new StringBuilder();
report.AppendLine("=== Report ===");
report.AppendLine($"Generated: {DateTime.Now}");

Real impact: SMS processes 1000+ strings per session. Performance depends on string handling.

---

Try This Now

Run the student summary example above. Then experiment:

1. Add email field and validate Contains("@")
2. Validate roll number starts with "STU-"
3. Add more fields and use StringBuilder instead of string +
4. Read from CSV: split line by comma
5. Count students where name starts with "S"

See how string methods organize and validate student data.

---

ℹ️ Video Tutorial

Strings explained: Trim, ToUpper, ToLower, Contains, Split, Interpolation, StringBuilder, immutability. Video coming soon. Subscribe to NexCoding YouTube for updates.

---

Common Mistakes

Mistake 1: Forgetting to assign the result.

string name = " Sahasra ";
name.Trim(); // wrong

Correct:

name = name.Trim();

Mistake 2: Checking only empty text.

if (name == "")
{
    Console.WriteLine("Invalid");
}

Better:

if (string.IsNullOrWhiteSpace(name))
{
    Console.WriteLine("Invalid");
}

Mistake 3: Using + too much for reports.

string report = "";
report = report + "Line 1\n";
report = report + "Line 2\n";

Better:

var report = new System.Text.StringBuilder();
report.AppendLine("Line 1");
report.AppendLine("Line 2");

Best Practices

• Use Trim() before validating user input.
• Use IsNullOrWhiteSpace() for required text.
• Use string interpolation for readable output.
• Use StringBuilder for long reports or loops.
• Use ToUpper() or ToLower() when comparing text safely.
• Remember that string methods return a new string.

Practice Task

Create a program that asks for:

• Student name
• Roll number
• Class name

Then:

• Remove extra spaces
• Convert roll number to uppercase
• Print a clean student profile

Quick Revision

• string stores text.
• Trim() removes extra spaces.
• Contains() checks whether text exists inside another text.
• Split() breaks text into parts.
• Strings are immutable.
• StringBuilder is useful for reports and repeated text building.

🎯 Q1: What is a string in C#?

A string is a data type used to store text, like student name, email, roll number, or address.

🎯 Q2: Why should we use Trim()?

Trim() removes extra spaces from the beginning and end of user input. This helps validation work correctly.

🎯 Q3: What does immutable string mean?

It means the original string cannot be changed. Any string method returns a new string.

🎯 Q4: When should we use StringBuilder?

Use StringBuilder when creating long text, reports, or adding text repeatedly inside a loop.

🤖Use AI to Learn Faster

Use ChatGPT, Claude, or Copilot to go deeper on C# strings. Try these prompts:

• "Explain C# strings with student examples"
• "Give me 10 beginner exercises for C# string methods"
• "Explain StringBuilder in simple words"
• "How do I validate student name input in C#?"

💡 Tip: After reading this article, paste your own code into AI and ask "What could go wrong here and why?" — fastest way to find edge cases and deepen understanding.

Next Article

Tuples in C# ->