Common string methods in C Sharp (C#) - Mini Project: Build & Apply

Common string methods

📖 Scenario: You are working on a simple text processing tool that helps users analyze and modify their input strings.

🎯 Goal: Build a program that uses common string methods to check, modify, and display information about a given string.

📋 What You'll Learn

Create a string variable with a specific sentence.

Create a variable to hold a substring to search for.

Use string methods to check if the substring is in the string and to convert the string to uppercase.

Print the results clearly.

💡 Why This Matters

🌍 Real World

String methods like these are used in text editors, search tools, and data validation to process and analyze text.

💼 Career

Knowing common string methods is essential for software developers, data analysts, and anyone working with text data.

1

Create the initial string

Create a string variable called sentence and set it to the exact value "Learning C# is fun and useful".

C Sharp (C#)

# Create the string variable 'sentence' with the given text
// Your code here

Hint

Use string sentence = "Learning C# is fun and useful"; to create the variable.

2

Add a substring to search for

Create a string variable called searchWord and set it to the exact value "fun".

C Sharp (C#)

string sentence = "Learning C# is fun and useful";
// Create the string variable 'searchWord' with the value "fun"
// Your code here

Hint

Use string searchWord = "fun"; to create the variable.

3

Check if the substring is in the string and convert to uppercase

Create a boolean variable called containsWord that uses sentence.Contains(searchWord) to check if searchWord is in sentence. Then create a string variable called upperSentence that stores sentence.ToUpper().

C Sharp (C#)

string sentence = "Learning C# is fun and useful";
string searchWord = "fun";
// Create 'containsWord' using sentence.Contains(searchWord)
// Create 'upperSentence' using sentence.ToUpper()
// Your code here

Hint

Use bool containsWord = sentence.Contains(searchWord); and string upperSentence = sentence.ToUpper();.

4

Print the results

Write two Console.WriteLine statements: one to print "Contains 'fun': " followed by containsWord, and another to print "Uppercase sentence: " followed by upperSentence.

C Sharp (C#)

string sentence = "Learning C# is fun and useful";
string searchWord = "fun";
bool containsWord = sentence.Contains(searchWord);
string upperSentence = sentence.ToUpper();
// Print if sentence contains the word
// Print the uppercase version of sentence
// Your code here

Hint

Use Console.WriteLine("Contains 'fun': " + containsWord); and Console.WriteLine("Uppercase sentence: " + upperSentence);.

Practice

1. Which C# string method returns a new string with all characters converted to uppercase?

easy

A. Substring()

B. Trim()

C. Contains()

D. ToUpper()

Solution

1. Step 1: Understand the purpose of ToUpper()

   The ToUpper() method converts all letters in a string to uppercase and returns a new string.

2. Step 2: Compare with other methods

   Trim() removes spaces, Contains() checks for substring presence, Substring() extracts part of the string.

3. Final Answer:

   ToUpper() -> Option D

4. Quick Check:

   Uppercase conversion = ToUpper() [OK]

Hint: Uppercase all letters? Use ToUpper() method [OK]

Common Mistakes:

• Confusing ToUpper() with ToLower()
• Using Trim() to change case
• Thinking Contains() changes text

2. Which of the following is the correct syntax to check if the string text contains the word "hello"?

easy

A. Contains(text, "hello")

B. text.Contains = "hello"

C. text.Contains("hello")

D. text.Has("hello")

Solution

1. Step 1: Recall the correct method call syntax

   In C#, to check if a string contains another, use the instance method with parentheses: text.Contains("hello").

2. Step 2: Identify incorrect syntax

   Assignments (=) or wrong method names like Has() are invalid for this check.

3. Final Answer:

   text.Contains("hello") -> Option C

4. Quick Check:

   Method call with parentheses = Contains() [OK]

Hint: Use text.Contains("word") with parentheses [OK]

Common Mistakes:

• Using assignment instead of method call
• Wrong method name like Has()
• Passing parameters incorrectly

3. What is the output of the following code?

string s = "  Hello World  ";
string result = s.Trim().Substring(0, 5);
Console.WriteLine(result);

medium

A. " Hel"

B. "Hello"

C. "Hello "

D. "Hello World"

Solution

1. Step 1: Apply Trim() method

   Trim() removes spaces at the start and end, so " Hello World " becomes "Hello World".

2. Step 2: Apply Substring(0, 5)

   Substring(0, 5) extracts characters from index 0 to 4, which is "Hello".

3. Final Answer:

   "Hello" -> Option B

4. Quick Check:

   Trim + Substring(0,5) = "Hello" [OK]

Hint: Trim removes spaces; Substring extracts exact part [OK]

Common Mistakes:

• Not trimming before substring
• Counting spaces in substring
• Confusing substring length

4. Identify the error in this code snippet:

string s = "Example";
if(s.Contains("ex"))
{
    Console.WriteLine("Found");
}

medium

A. Contains() is case-sensitive, so "ex" won't match "Ex"

B. Missing semicolon after if statement

C. Contains() method does not exist for strings

D. Console.WriteLine syntax is incorrect

Solution

1. Step 1: Check Contains() behavior

   Contains() is case-sensitive, so "ex" does not match "Ex" in "Example".

2. Step 2: Verify syntax correctness

   Semicolons and method calls are correct; no syntax errors present.

3. Final Answer:

   Contains() is case-sensitive, so "ex" won't match "Ex" -> Option A

4. Quick Check:

   Contains() case matters = true [OK]

Hint: Remember Contains() is case-sensitive by default [OK]

Common Mistakes:

• Assuming Contains() ignores case
• Looking for syntax errors that don't exist
• Confusing method availability

5. You want to extract the domain name from an email string email = "[e-mail]". Which code correctly extracts "example" using common string methods?

hard

A. string domain = email.Substring(email.IndexOf('@') + 1).Split('.')[0];

B. string domain = email.Trim().ToUpper();

C. string domain = email.Contains("@example");

D. string domain = email.Substring(0, email.IndexOf('.'));

Solution

1. Step 1: Find position after '@'

   email.IndexOf('@') finds the '@' position; adding 1 moves to start of domain.

2. Step 2: Extract substring from domain start and split by '.'

   Substring gets "example.com", then Split('.')[0] gets "example".

3. Step 3: Check other options

   B changes case, C returns bool, D extracts wrong part before '@'.

4. Final Answer:

   string domain = email.Substring(email.IndexOf('@') + 1).Split('.')[0]; -> Option A

5. Quick Check:

   Extract domain by substring + split = string domain = email.Substring(email.IndexOf('@') + 1).Split('.')[0]; [OK]

Hint: Use IndexOf('@') + Substring + Split('.') to get domain [OK]

Common Mistakes:

• Using Contains() instead of extracting
• Trimming or changing case wrongly
• Extracting before '@' instead of after