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".