Boxing and unboxing execution in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this boxing and unboxing code?

Consider the following C# code snippet. What will it print when run?

C Sharp (C#)

int x = 10;
object obj = x; // boxing
int y = (int)obj; // unboxing
Console.WriteLine(y);

A 10

B 0

C Compilation error

D InvalidCastException at runtime

💡 Hint

Boxing stores the value type inside an object. Unboxing extracts it back.

The integer 10 is boxed into an object. Then it is unboxed back to int y. Printing y outputs 10.

❓ Predict Output

intermediate

What happens if unboxing to wrong type?

What will this C# code print or do when executed?

C Sharp (C#)

int x = 5;
object obj = x; // boxing
long y = (long)obj; // unboxing to wrong type
Console.WriteLine(y);

A 5

B InvalidCastException at runtime

C Compilation error

D 0

💡 Hint

Unboxing requires exact type match.

Unboxing to a different value type than originally boxed causes InvalidCastException at runtime.

🔧 Debug

advanced

Why does this code cause a runtime error?

Examine the code below. Why does it throw an exception at runtime?

C Sharp (C#)

object obj = 42; // boxing int
short s = (short)obj; // unboxing to short
Console.WriteLine(s);

A Because boxing failed due to missing cast

B Because Console.WriteLine cannot print short

C Because short cannot hold int values

D Because unboxing requires exact type, not convertible types

💡 Hint

Unboxing must match the original boxed type exactly.

The object holds a boxed int. Unboxing to short is invalid even if int fits in short. This causes InvalidCastException.

🧠 Conceptual

advanced

What is the main cost of boxing in C#?

Why is boxing considered costly in C# programs?

A Because it modifies the original variable's value

B Because it changes the value type to a reference type permanently

C Because it creates a new object on the heap and copies the value

D Because it requires special compiler directives

💡 Hint

Think about what happens when a value type is boxed.

Boxing creates a new object on the heap and copies the value inside it, which uses extra memory and CPU time.

❓ Predict Output

expert

What is the output of this mixed boxing and unboxing code?

Analyze the following C# code and determine its output:

C Sharp (C#)

object obj = 123;
int a = (int)obj;
obj = a + 1;
int b = (int)obj;
Console.WriteLine(a + "," + b);

A 123,124

B 124,124

C 123,123

D InvalidCastException at runtime

💡 Hint

Remember boxing creates a new object each time.

Initially obj boxes 123. a unboxes 123. Then obj is assigned a+1 (124) boxed. b unboxes 124. Output is "123,124".

Practice

1. What happens during boxing in C#?

easy

A. A value type is converted to an object type.

B. An object type is converted to a value type.

C. A reference type is converted to a value type.

D. A value type is converted to a string.

Solution

1. Step 1: Understand boxing concept

   Boxing means wrapping a value type (like int) inside an object so it can be treated as an object.

2. Step 2: Identify the conversion direction

   Boxing converts a value type to an object type, not the other way around.

3. Final Answer:

   A value type is converted to an object type. -> Option A

4. Quick Check:

   Boxing = value type to object [OK]

Hint: Boxing wraps value types as objects [OK]

Common Mistakes:

• Confusing boxing with unboxing
• Thinking boxing converts to string
• Mixing up value and reference types

2. Which of the following is the correct syntax to unbox an object to an int in C#?

easy

A. int x = obj.ToInt();

B. int x = obj as int;

C. int x = Convert.ToInt32(obj);

D. int x = (int)obj;

Solution

1. Step 1: Recall unboxing syntax

   Unboxing requires an explicit cast from object to the original value type.

2. Step 2: Check each option

   int x = (int)obj; uses explicit cast (int)obj which is correct. int x = obj as int; uses 'as' which works only for reference types. int x = Convert.ToInt32(obj); converts but is not unboxing. int x = obj.ToInt(); is invalid syntax.

3. Final Answer:

   int x = (int)obj; -> Option D

4. Quick Check:

   Unboxing = explicit cast (int)obj [OK]

Hint: Unboxing needs explicit cast (int)obj [OK]

Common Mistakes:

• Using 'as' keyword for unboxing
• Using Convert.ToInt32 instead of cast
• Calling non-existent methods like ToInt()

3. What is the output of the following C# code?

int a = 10;
object obj = a; // boxing
int b = (int)obj; // unboxing
b += 5;
Console.WriteLine(a);

medium

A. 15

B. 10

C. 5

D. Compilation error

Solution

1. Step 1: Understand boxing and unboxing here

   Variable 'a' is boxed into 'obj'. Then 'obj' is unboxed into 'b'. 'b' is a separate copy.

2. Step 2: Check value changes

   Incrementing 'b' by 5 does not affect 'a'. So 'a' remains 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Boxing copies value; original stays 10 [OK]

Hint: Boxing copies value; original unchanged [OK]

Common Mistakes:

• Assuming 'a' changes when 'b' changes
• Confusing reference and value types
• Thinking boxing links variables

4. Identify the error in this C# code snippet:

object obj = 123;
long l = (long)obj;
Console.WriteLine(l);

medium

A. Invalid cast exception at runtime

B. Compilation error due to missing cast

C. No error, prints 123

D. Null reference exception

Solution

1. Step 1: Analyze boxing and unboxing types

   Boxing stores an int (123) in obj. Unboxing must cast back to int, not long.

2. Step 2: Understand runtime behavior

   Trying to unbox as long causes InvalidCastException because obj holds int, not long.

3. Final Answer:

   Invalid cast exception at runtime -> Option A

4. Quick Check:

   Unboxing type mismatch causes runtime error [OK]

Hint: Unbox only to original value type [OK]

Common Mistakes:

• Casting to different value type than boxed
• Expecting compile-time error instead of runtime
• Ignoring boxing type stored

5. Given a list of integers boxed as objects, how can you sum their values correctly in C#?

List<object> list = new List<object> { 1, 2, 3, 4 };

Which code snippet correctly sums the integers?

hard

A. int sum = 0; foreach (int x in list) sum += x;

B. int sum = 0; foreach (object o in list) sum += (o as int);

C. int sum = 0; foreach (object o in list) sum += (int)o;

D. int sum = list.Sum();

Solution

1. Step 1: Understand list content and types

   List contains objects boxed from ints. To sum, unbox each object to int.

2. Step 2: Evaluate each option

   A: int sum = 0; foreach (int x in list) sum += x; compile error - cannot implicitly convert object to int.
   B: int sum = 0; foreach (object o in list) sum += (o as int); compile error - 'as' cannot be used with non-nullable value types.
   C: int sum = list.Sum(); fails because list is List<object>, no Sum() overload.
   D: int sum = 0; foreach (object o in list) sum += (int)o; unboxes correctly with (int)o.

3. Final Answer:

   int sum = 0; foreach (object o in list) sum += (int)o; -> Option C

4. Quick Check:

   Unbox each object with (int)o to sum [OK]

Hint: Unbox each object with (int)o before summing [OK]

Common Mistakes:

• Trying to foreach as int directly
• Using Sum() on List<object>
• Using 'as' operator for unboxing