String type and immutability in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code?

Consider the following C# code snippet. What will be printed to the console?

C Sharp (C#)

string s = "hello";
s = s.Replace('h', 'j');
Console.WriteLine(s);

A Hello

B jello

C hello

D jello\nhello

💡 Hint

Remember that strings in C# are immutable. The Replace method returns a new string but does not change the original.

The Replace method returns a new string with the replacements, but since the returned value is not assigned, the original string 's' remains unchanged. So, 'hello' is printed.

❓ Predict Output

intermediate

What is the output of this code?

What will this C# program print?

C Sharp (C#)

string a = "abc";
string b = a + "def";
Console.WriteLine(a);
Console.WriteLine(b);

A abcdef\nabc

B abcdef\nabcdef

C abc\nabc

D abc\nabcdef

💡 Hint

Adding strings creates a new string; the original string remains unchanged.

The variable 'a' remains 'abc'. The variable 'b' is a new string 'abcdef'. So the output is 'abc' then 'abcdef'.

🔧 Debug

advanced

Why does this code not change the string?

This code tries to change the first character of a string. Why does it fail?

C Sharp (C#)

string s = "test";
s[0] = 'b';
Console.WriteLine(s);

A Compilation error: strings are immutable and cannot be changed by index.

B Runtime error: index out of range.

C Output: test

D Output: best

💡 Hint

Think about whether you can assign to a character in a string by index in C#.

Strings in C# are immutable. You cannot assign to an index of a string. This causes a compilation error.

❓ Predict Output

advanced

What is the output of this code?

What will this program print?

C Sharp (C#)

string s1 = "hello";
string s2 = s1.ToUpper();
Console.WriteLine(s1);
Console.WriteLine(s2);

A HELLO\nHELLO

B hello\nHELLO

C hello\nhello

D HELLO\nhello

💡 Hint

ToUpper returns a new string; it does not change the original string.

s1 remains 'hello' because strings are immutable. s2 is a new string 'HELLO'.

🧠 Conceptual

expert

Why is string immutability important in C#?

Which of the following is NOT a reason why strings are immutable in C#?

A It allows direct modification of characters in the string to save memory.

B It improves performance by enabling string interning and caching.

C It allows strings to be safely shared across multiple threads without synchronization.

D It simplifies security by preventing unexpected changes to string data.

💡 Hint

Think about what immutability means and how it affects string modification.

Strings being immutable means you cannot modify characters directly. This is a design choice for safety and performance, not to allow direct modification.

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