Common string methods in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of ToUpper and Trim methods

What is the output of this C# code snippet?

C Sharp (C#)

string text = "  Hello World  ";
string result = text.Trim().ToUpper();
Console.WriteLine(result);

A HELLO WORLD

B DLROW OLLEH

C hello world

D Hello World

💡 Hint

Remember that Trim removes spaces at the start and end, and ToUpper converts all letters to uppercase.

The Trim method removes spaces from both ends of the string, so " Hello World " becomes "Hello World". Then ToUpper converts all letters to uppercase, resulting in "HELLO WORLD".

❓ Predict Output

intermediate

Result of Replace and Substring methods

What will be printed by this C# code?

C Sharp (C#)

string s = "apple pie";
string newS = s.Replace("pie", "tart").Substring(0, 5);
Console.WriteLine(newS);

A tart

B applet

C apple pie

D apple

💡 Hint

Replace changes "pie" to "tart" and Substring(0,5) takes the first 5 characters.

Replace changes "apple pie" to "apple tart". Then Substring(0,5) extracts characters from index 0 to 4, which is "apple".

❓ Predict Output

advanced

Output of IndexOf and Contains methods

What is the output of this code?

C Sharp (C#)

string phrase = "Learning C# is fun!";
int index = phrase.IndexOf("C#");
bool hasFun = phrase.Contains("fun");
Console.WriteLine($"{index} {hasFun}");

A 9 True

B 9 False

C -1 True

D 10 True

💡 Hint

IndexOf returns the first position of the substring or -1 if not found. Contains returns true if substring exists.

The substring "C#" starts at index 9 in the phrase. The phrase contains "fun", so Contains returns true. Output is "9 True".

❓ Predict Output

advanced

Effect of ToLower and StartsWith methods

What does this code print?

C Sharp (C#)

string input = "OpenAI ChatGPT";
string lower = input.ToLower();
bool starts = lower.StartsWith("open");
Console.WriteLine(starts);

A False

B true

C True

D false

💡 Hint

ToLower converts all letters to lowercase. StartsWith checks if string begins with given substring.

After ToLower, the string is "openai chatgpt" which starts with "open". So StartsWith returns true, printed as "True".

❓ Predict Output

expert

Output of complex string manipulation

What is the output of this C# code?

C Sharp (C#)

string s = "  CSharp Programming  ";
string result = s.Trim().Replace(" ", "_").ToLower();
Console.WriteLine(result);

A CSharp_Programming

B csharp programming

C csharp_programming

D CSharp Programming

💡 Hint

Trim removes spaces at ends, Replace changes spaces to underscores, ToLower makes all letters lowercase.

Trim removes spaces at start and end, so "CSharp Programming" remains. Replace changes spaces to underscores: "CSharp_Programming". ToLower converts to "csharp_programming".

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