Common string methods in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to convert the string to uppercase.

C Sharp (C#)

string text = "hello";
string result = text.[1]();

Drag options to blanks, or click blank then click option'

AToUpper

BToLower

CTrim

DReplace

Attempts:

3 left

💡 Hint

Common Mistakes

Using ToLower() which converts to lowercase instead.

Using Trim() which removes spaces but does not change case.

The ToUpper() method converts all characters in the string to uppercase.

2fill in blank

medium

Complete the code to check if the string starts with "Hi".

C Sharp (C#)

string greeting = "Hi there!";
bool startsWithHi = greeting.[1]("Hi");

Drag options to blanks, or click blank then click option'

AStartsWith

BIndexOf

CEndsWith

DContains

Attempts:

3 left

💡 Hint

Common Mistakes

Using Contains() which checks anywhere in the string.

Using EndsWith() which checks the end of the string.

The StartsWith() method checks if the string begins with the specified substring.

3fill in blank

hard

Fix the error in the code to find the position of 'a' in the string.

C Sharp (C#)

string word = "banana";
int position = word.[1]('a');

Drag options to blanks, or click blank then click option'

ALocate

BFind

CIndexOf

DSearch

Attempts:

3 left

💡 Hint

Common Mistakes

Using Find() which does not exist for strings in C#.

Using Locate() or Search() which are not valid string methods.

The IndexOf() method returns the position of the first occurrence of a character.

4fill in blank

hard

Fill both blanks to create a substring from index 2 with length 4.

C Sharp (C#)

string text = "Programming";
string part = text.[1]([2], 4);

Drag options to blanks, or click blank then click option'

ASubstring

BRemove

C2

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove() which deletes characters instead of extracting.

Using wrong starting index like 4.

The Substring method extracts part of the string starting at the given index. Here, index 2 means starting from the third character.

5fill in blank

hard

Fill all three blanks to replace 'cat' with 'dog' in the string.

C Sharp (C#)

string sentence = "The cat sat on the mat.";
string newSentence = sentence.[1]("[2]", "[3]");

Drag options to blanks, or click blank then click option'

AReplace

Bcat

Cdog

DRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove() which deletes text instead of replacing.

Swapping the order of 'cat' and 'dog'.

The Replace method replaces all occurrences of a substring with another substring.

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