String type and immutability in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a string variable named greeting.

C Sharp (C#)

string greeting = [1];

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

AHello

B"Hello"

C'Hello'

Dnew String("Hello")

Attempts:

3 left

💡 Hint

Common Mistakes

Using single quotes instead of double quotes for strings.

Not using quotes at all around the text.

Strings in C# must be enclosed in double quotes to be valid string literals.

2fill in blank

medium

Complete the code to concatenate two strings greeting and name.

C Sharp (C#)

string message = greeting [1] name;

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication operators instead of plus.

Forgetting to add a space between words.

The plus sign (+) is used to join strings together in C#.

3fill in blank

hard

Fix the error in the code to correctly assign a new value to the string variable.

C Sharp (C#)

string name = "Alice";
name [1] "Bob";

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

A+=

B==

C=

D===

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of = for assignment.

Using += which appends instead of replacing.

To assign a new value to a string variable, use the single equals sign (=).

4fill in blank

hard

Fill both blanks to create a new string that replaces part of the original string.

C Sharp (C#)

string original = "Hello World";
string modified = original.[1]("World", [2]);

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

AReplace

B"C#"

C"CSharp"

DRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove instead of Replace.

Not putting the new text in quotes.

The Replace method changes part of the string. The second argument is the new text in quotes.

5fill in blank

hard

Fill all three blanks to create a new string with trimmed spaces and converted to uppercase.

C Sharp (C#)

string messy = "  hello world  ";
string clean = messy.[1]().[2]().[3]();

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

ATrim

BToUpper

CToLower

DReplace

Attempts:

3 left

💡 Hint

Common Mistakes

Using ToLower instead of ToUpper.

Not trimming spaces before changing case.

Trim() removes leading and trailing whitespace. ToUpper() converts the string to uppercase letters. The second ToUpper() call is redundant but completes the required three-method chain, resulting in "HELLO WORLD".

Practice

1. What does it mean that strings in C# are immutable?

easy

A. Once created, the string cannot be changed; any modification creates a new string.

B. Strings can be changed directly without creating a new string.

C. Strings are stored as numbers internally.

D. Strings can only contain letters and no numbers.

Solution

1. Step 1: Understand immutability concept

   Immutability means the value cannot be changed after creation.

2. Step 2: Apply to strings in C#

   In C#, strings cannot be changed once created; any change makes a new string.

3. Final Answer:

   Once created, the string cannot be changed; any modification creates a new string. -> Option A

4. Quick Check:

   String immutability = cannot change original string [OK]

Hint: Remember: strings never change, they create new copies [OK]

Common Mistakes:

• Thinking strings can be changed in place
• Confusing immutability with read-only variables
• Assuming strings only hold letters

2. Which of the following is the correct way to declare a string variable in C#?

easy

A. string name = "Alice";

B. String name = 'Alice';

C. string name = Alice;

D. String name = Alice;

Solution

1. Step 1: Check string declaration syntax

   In C#, strings use double quotes and the lowercase keyword 'string'.

2. Step 2: Identify correct syntax

   string name = "Alice"; uses lowercase 'string' and double quotes correctly.

3. Final Answer:

   string name = "Alice"; -> Option A

4. Quick Check:

   Use double quotes and string keyword [OK]

Hint: Use double quotes for strings and lowercase 'string' keyword [OK]

Common Mistakes:

• Using single quotes for strings
• Omitting quotes around text
• Confusing 'string' with 'String' casing

3. What will be the output of this C# code?

string s = "hello";
s = s.ToUpper();
Console.WriteLine(s);

medium

A. hello

B. HELLO

C. Hello

D. Error

Solution

1. Step 1: Understand ToUpper() method

   ToUpper() returns a new string with all letters uppercase.

2. Step 2: Assign new string to s and print

   Variable s is updated to the uppercase string, so printing s shows "HELLO".

3. Final Answer:

   HELLO -> Option B

4. Quick Check:

   ToUpper() creates uppercase string [OK]

Hint: ToUpper() returns new uppercase string; assign it to variable [OK]

Common Mistakes:

• Assuming original string changes without assignment
• Expecting lowercase output
• Thinking ToUpper() modifies string in place

4. Identify the error in this code snippet:

string s = "test";
s[0] = 'T';
Console.WriteLine(s);

medium

A. Strings cannot be indexed in C#.

B. Console.WriteLine requires parentheses.

C. Cannot assign to string index because strings are immutable.

D. Missing semicolon after string declaration.

Solution

1. Step 1: Check string indexing and assignment

   Strings can be indexed to read characters but cannot assign to indexes.

2. Step 2: Understand immutability prevents changing characters

   Because strings are immutable, s[0] = 'T' causes a compile error.

3. Final Answer:

   Cannot assign to string index because strings are immutable. -> Option C

4. Quick Check:

   Strings immutable = no character assignment [OK]

Hint: You can read chars by index but cannot assign to them [OK]

Common Mistakes:

• Trying to change string characters by index
• Confusing indexing with arrays
• Ignoring immutability rules

5. Given this code, what will be the output?

string original = "cat";
string changed = original.Replace('c', 'b');
Console.WriteLine(original);
Console.WriteLine(changed);

hard

A. Error at Replace method

B. bat
bat

C. cat
cat

D. cat
bat

Solution

1. Step 1: Understand Replace method on strings

   Replace returns a new string with specified characters replaced; original string stays unchanged.

2. Step 2: Analyze output of both Console.WriteLine calls

   original prints "cat" unchanged; changed prints "bat" with 'c' replaced by 'b'.

3. Final Answer:

   cat bat -> Option D

4. Quick Check:

   Replace returns new string, original unchanged [OK]

Hint: Replace returns new string; original string stays same [OK]

Common Mistakes:

• Assuming original string changes after Replace
• Expecting both outputs to be same
• Thinking Replace modifies string in place